The Complete Guide to Carbohydrate Periodization: Aligning Carbs with Training Demands, Benefits, Scheduling, and Red Lines You Must Never Cross

健康與醫學

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2026年7月28日

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You may have seen this scenario before: in the same team, Rider A follows a long-term low-carb diet, avoiding white rice at every meal, claiming it “improves fat-burning efficiency”; Rider B, on the other hand, eats to his heart’s content no matter what today’s workout is, reasoning that “if you train, you can afford to eat.” On the day of the long Wuling climb, Rider A completely runs out of steam on the final stretch and finishes with cramps; Rider B, meanwhile, gains weight year after year, and his power-to-weight ratio on climbs keeps getting worse.

Carbohydrate periodization aims to solve precisely the problem between these two extremes. Its core concept is often distilled into one phrase: fuel for the work required—deciding “how much fuel to add today” based on “the work to be done today.” This is not a diet method, but a way of thinking that ties nutrition scheduling to training scheduling.

This article will break down the full picture: what it actually means, the physiological logic behind it, the potential benefits and the caveats that must be acknowledged, how to actually schedule it (including a one-week example table), how to adjust it for Taiwan’s cycling and running contexts, and finally—the most important section on risks and red lines—because the most common harm in this topic is not “scheduling that isn’t precise enough,” but “accidentally starving yourself into long-term low energy availability.”

1. First, Let’s Define It Clearly: Periodization Is Not Low-Carb

Three Things Often Confused With Each Other

Many arguments never end because three completely different concepts are being talked about as if they were the same thing:

Long-term low-carb diet: Regardless of training content, carbs are always kept low. Overall carbohydrate intake stays low long-term, and the body relies primarily on fat as its fuel source over time.
Always high-carb: Eating high-carb every day, whether it’s a rest day or an interval day. The advantage is that glycogen is almost always full; the disadvantage is that on low-training-volume days, it’s easy to consume excess energy.
Carbohydrate periodization: Total carbs are not necessarily low, nor necessarily high, but the distribution fluctuates according to training demands. On hard workout days, carbs are raised; on easy days and rest days, they are dialed down.

In other words, periodization works on “when to eat” and “how much to eat on which day,” rather than being a yes/no question about “whether to eat carbs at all.” A well-executed periodization plan might even have weekly total carbohydrate intake similar to a “moderate-carb-every-day” approach; the difference lies in concentrating carbs into the periods where they are most needed.

What “Fuel for the Work Required” Means

The spirit of this phrase is simple: the amount of fuel should correspond to the amount and intensity of the work.

Today is a two-hour high-intensity interval session with significant anaerobic contribution → glycogen needs to be plentiful, so carbs are raised to a relatively high level.
Today is a 90-minute easy recovery ride, with power held in a very low zone → fat oxidation alone is more than sufficient, and carbs don’t need to be raised to the same level as an interval day.
Today is a complete rest day → there’s no training demand to support, so carbs can be the lowest of the week (but still not zero, and total calories should not be cut to unreasonable levels).

This principle is appealing because it addresses two common contradictions at once: not eating enough on hard workout days, leading to poor training quality; and eating too much on easy days, leading to long-term caloric surplus and weight gain.

It Does Not Guarantee You’ll Get Faster

This must be stated clearly upfront: carbohydrate periodization has been discussed in endurance sports circles for many years, and it is considered a reasonable strategy with a physiological basis—but it is not a magic guarantee of getting faster. The consensus that is currently most widely accepted in the field is:

For high-intensity training and racing, adequate carbohydrate intake has a relatively clear benefit to performance.
Deliberately restricting carbs during some low-intensity training sessions is thought to potentially amplify certain training adaptation signals, but whether this translates into “better race results” varies greatly between individuals, and not everyone needs it.
Execution difficulty and psychological risk are the real barriers—if you get the scheduling wrong, the cost can be long-term low energy availability and health problems.

So the stance of this article is: treat it as a tool, not a belief system.

2. The Physiological Logic: Why Intensity Determines Fuel

To be able to schedule your own training rather than memorizing tables by rote, you need to understand the following three things.

Glycogen Is the Primary Fuel for High Intensity

The body has two main categories of usable fuel: carbohydrates (stored as glycogen in muscle and liver) and fat. The key difference between them lies in the “rate at which energy can be produced.”

Fat stores are very large—even in athletes with relatively low body fat, the total energy available from fat far exceeds glycogen; however, the upper limit of fat oxidation rate is relatively low, and it cannot supply large amounts of energy in a short time. Glycogen is the opposite: total storage is limited, but it can be mobilized quickly, and it can continue producing energy even when oxygen supply is insufficient (anaerobic glycolysis).

This is why:

When you ride for two hours at conversational intensity on the riverside bike path, fat is the dominant fuel source, and glycogen depletion is relatively slow.
When you sprint hard up a steep section on Fengguizui, those few minutes are almost entirely supported by glycogen.
On the final stretch of Wuling, when you’re exhausted, cold, and still trying to hold power, if your glycogen is already bottomed out, it’s not a matter of willpower—it’s a matter of fuel—you’ll find that you simply can’t push the power, which is what’s commonly called “hitting the wall” or “bonking.”

A very practical takeaway: the higher the intensity and the longer the duration of a workout, the greater the carbohydrate demand; and the demand is the product of “intensity” and “duration,” not just one of them alone.

Fat Oxidation and Its Relationship With Intensity

As intensity rises, the body’s fuel usage gradually shifts from predominantly fat to predominantly carbohydrate. At low intensity, the fat contribution ratio is high; as intensity rises into a certain range, carbohydrate contribution exceeds fat; and at even higher intensities, it’s almost entirely carbohydrate.

There are several commonly misunderstood points worth clarifying:

“Highest fat-burning ratio” does not equal “best fat-loss results.” At low intensity, the fat percentage is high, but total energy expenditure is low; at high intensity, the fat percentage is low, but total expenditure is high. Weight loss is a matter of total energy balance, not just looking at the fuel ratio at a single moment.
This crossover point is trainable. Long-term consistent aerobic training is generally believed to allow people to rely more on fat and use less glycogen at the same absolute intensity—in other words, metabolic flexibility improves. This is one of the directions carbohydrate periodization aims to reinforce.
Individual variation is enormous. At the same power output, some people have clearly higher fat contribution, while others have clearly lower; diet habits, training background, and muscle fiber composition all play a role. Don’t apply someone else’s numbers to yourself.

Training Adaptation Signaling: Why Some People Deliberately Train With Low Glycogen

Training makes you stronger because exercise generates a cascade of “signals” at the cellular level, telling the body to increase mitochondria, increase related enzymes, improve fat metabolism capacity, and so on.

A widely discussed concept in the field is: when training with muscle glycogen in a relatively low state, certain signals related to aerobic adaptation may be amplified. In plain terms, the same easy ride, performed with less-than-full glycogen, may send a stronger message to the body that “I need to become better at using fat and producing energy.”

This is the theoretical basis for “train low” (low-glycogen training) approaches. Common practical implementations include:

Fasted morning rides: Wake up and do a low-intensity ride first, then eat afterward.
Sleep low: Do a glycogen-depleting workout in the evening, deliberately keep carbs low at dinner afterward, then do another low-intensity session the next morning before replenishing carbs.
Incomplete replenishment between two sessions: Train twice a day; after the first session, deliberately avoid fully replenishing carbs so the second session is performed in a lower-glycogen state.

The Most Critical Distinction: Amplified Signals ≠ Better Performance

This is where the whole topic gets most dangerous. When you train in a low-glycogen state, your performance will definitely suffer—power won’t go up, heart rate runs relatively high, perceived exertion feels harder, and focus declines.

So a sensible schedule must always be:

Low glycogen should only be used on workouts that “don’t require performance anyway” (low intensity, recovery, base aerobic work),
high-intensity workouts and races should always be done well-fueled.

If you apply train low to interval days, the result will be: reduced training quality → less high-intensity stimulus actually accumulated → adaptations actually get worse → plus added fatigue and injury risk. That’s a completely negative trade.

3. Potential Benefits, and the Caveats That Must Be Acknowledged

1. Metabolic Flexibility

The most frequently cited benefit. Over the long term, getting the body accustomed to working in different fuel states is widely considered to help improve the ability to “spare glycogen at the same intensity.” For long-duration events (rides lasting several hours, full marathons, long-distance triathlon), this means: your glycogen lasts longer, and the risk of bonking decreases.

Caveat: This does not mean you can eat less on race day. You still need to fuel on race day; metabolic flexibility just gives you a larger buffer.

2. Body Weight and Body Composition Management

For many Taiwanese cyclists, this is the most practical motivation. Climbing Wuling or Beiyi, power-to-weight ratio is a hard metric. The most direct benefit of carb cycling is: it gives you a logical method to reduce unnecessary energy intake without sacrificing the quality of key workouts.

The traditional approach to weight loss is “eat a little less every day,” which results in not eating enough every day and a slight drop in training quality every day. The periodized approach is “eat well on the days you should, and rein it in on the days you don’t need to,” concentrating the calorie deficit on low-training-volume days.

Caveat: The rate of weight loss must be reasonable. If you lose weight too quickly, it likely includes muscle, and it directly increases the risk of relative energy deficiency (see Chapter 9).

3. Avoiding Unnecessary Energy Intake

Many people have a “training day mentality” habit: as long as they ride, they feel they can eat freely. But a relaxed 90-minute riverside ride likely burns far fewer calories than you think, and a big meal after the ride often easily exceeds that expenditure. Periodization provides a simple check: does today’s workout deserve this food?

Caveat: Don’t swing to the other extreme and develop a “I can’t eat unless I train hard” mindset. That mentality is a precursor to disordered eating.

4. Adaptation for Long-Duration Aerobic Work

If your goal is long-distance events (Wuling, cycling around Taiwan, ultramarathon, long-distance triathlon), the focus of periodization falls on the “quality and metabolic adaptation of base aerobic workouts.” During the base phase, moderately scheduling some low-intensity workouts in a low-carbohydrate state is a relatively common practice.

Caveat: The closer you get to race day, the lower the proportion of this practice should be. The focus in the pre-race phase is practicing your race-day fueling strategy, not continuing to train your fasted capacity.

5. Who Benefits the Most

Generally speaking, this approach is more suitable for:

Those who already have a stable training foundation and can schedule workouts regularly
Those whose training volume is large enough that daily needs vary significantly (e.g., one hour on weekdays, five hours on weekends)
Those with clear body weight management needs who are currently in a base phase
Those with a healthy relationship with food who are less prone to falling into compulsive control

Conversely, if your training volume is not that large to begin with and your days are fairly similar, the marginal benefit of periodization is actually limited. Maintaining a balanced diet and increasing your training volume will yield far greater benefits.

4. Practical Scheduling (Part 1): Classify Your Training Days First

The first step in any implementation is to split your weekly schedule into four types of days. The table below uses relative high/low designations, not fixed numbers—because actual intake depends on body weight, body fat, training volume, age, sex, and health status. Individual variation is enormous, and if needed, a sports nutrition professional should set targets based on your specific situation.

Table 1: Training Day Classification and Nutrition Principles

Day Type	Typical Workout	Relative Carb Amount	Protein	Fat	Key Principles
A: High-Intensity / Interval Day	Threshold intervals, hill repeats, race simulation, high-intensity group ride	Highest	Sufficient and consistent	Low before/after training, normal at other times	Fuel up before, refuel during, replenish promptly after; no low-carb experiments on this day
B: Long-Endurance Day	3+ hour rides, Wuling, long LSD, long runs	High (highest total, due to longer duration)	Sufficient and consistent	Moderate	Focus on continuous fueling during the workout, not loading everything into the pre-workout meal
C: Moderate-Low Intensity / Recovery Day	1–2 hour easy rides, recovery runs, skills sessions	Moderate to Low	Do not reduce	Can be slightly increased to maintain satiety	Low-intensity workouts in a low-carb state can be scheduled, but intensity must be genuinely low
D: Complete Rest Day	No training, stretching, light activity	Lowest	Do not reduce	Moderate	Reduce carbs but do not eliminate them; total energy must still support basal metabolism and recovery

Three Principles You Must Remember

Principle 1: Protein is not periodized. This is the most commonly botched aspect. Carbs can be adjusted up or down based on training demands, but protein should remain consistent and sufficient every day, and it must not be cut on recovery or rest days. Rest days are precisely when muscle repair and remodeling are most active; cutting protein then is like dismantling your own recovery.

Principle 2: A low-carb day is not a low-calorie day. If you cut carbs but don’t partially compensate with protein and fat, that’s a calorie deficit stacked on top of a calorie deficit. The original intent of carb cycling is “redistribution,” not “eat less every day.” Whether to create an overall deficit (for weight loss) is a separate decision that needs to be designed independently and carefully.

Principle 3: Fiber, micronutrients, and hydration must not be sacrificed to periodization. The easiest collateral damage when reducing carbs is fruits, vegetables, and whole grains, resulting in insufficient fiber, insufficient micronutrients, and worsened gut health. On low-carb days, the priority should be cutting refined sugar and extra starch portions—not vegetables.

5. Practical Scheduling (Part 2): Timing Within a Day

For the same total daily carbs, placing them at different times produces completely different effects. This is the layer of periodization that is finer than day-level classification and delivers more immediate, noticeable results.

Pre-Workout

Before high-intensity or long-duration workouts: You need a meal that is carb-dominant, relatively low in fat and fiber, with enough time allowed for digestion. Eating too much fat or fiber too close to the start of exercise is a common cause of gastrointestinal discomfort.
When time is tight (heading straight to the trainer after work): Rather than forcing down a full meal, have an easily digestible carb snack before starting and move the main meal to after the workout.
Before low-intensity workouts: This is the only time slot suitable for scheduling “no fuel” or “minimal fuel.” A fasted morning ride, or an easy morning session after a low-carb evening the night before, falls into this category.

During the Workout

This is the aspect most often overlooked by Taiwanese cyclists. For workouts over two hours, or at higher intensity, fueling during the session is almost essential, especially in the hot, humid summer environment.

Principles:

The longer the duration and the higher the intensity, the greater the need for carbs during the workout.
Fuel in multiple, regular portions; don’t wait until you’re hungry—by the time you feel hungry, it’s usually already too late.
There is no standard answer for the form of fuel (energy gels, energy bars, rice balls, bread, sports drinks). The best fuel is whatever you can get down without discomfort.
Gut tolerance can be trained, but it must be practiced in advance. Do not try new fuel products or a new fueling frequency for the first time on the day of Wuling.

Post-Workout

If you have a second session later that day, or a hard workout the next day: The timing of post-workout carb replenishment matters most. The sooner you start, the better, and pair it with protein.
If the day is done and tomorrow is a rest day: Timing is less urgent. A normal meal is sufficient; there’s no need to deliberately load up on high sugar.
Protein is recommended after any workout, regardless of circumstances.

Before Bed and Daily Distribution

If you have an important workout the next morning, don’t push carbs too low the night before.
If you’re using advanced strategies like “sleep low” (depleting glycogen in the evening, a low-carb dinner, then refueling only after a low-intensity morning session the next day), you must confirm that the next morning’s workout is genuinely low intensity, and it’s not recommended to do this on consecutive days.
Those with sleep issues should note: very low-carb dinners can sometimes affect falling asleep and sleep quality. In such cases, you should dial it back—sleep is worth far more than this marginal adaptation.

6. Practical Scheduling (Part 3): Weekly Example

The two examples below are structural demonstrations, not prescriptions. Treat them as “templates for the scheduling logic,” then adjust based on your own workout schedule, body weight, work hours, and physical feedback.

Table 2: Weekly Commuter Version (Weekday Evening Trainer Focus)

Day	Training Content	Carb Level	Pre-Training	During Training	Post-Training	Notes
Mon	Complete rest	Lowest	—	—	—	Protein intake as usual, vegetable portions unchanged
Tue	High-intensity intervals on trainer (~1 hour)	High	Afternoon snack with carbs, allow digestion time	Sports drink based on sweat loss	Normal dinner with carbs + protein	No low-carb experiments on this day
Wed	Recovery ride or complete rest	Low-ish	—	—	Normal dinner	Do not reduce protein
Thu	Threshold workout on trainer (~1 hour)	High	Same as Tuesday	Same as Tuesday	Normal carbs at dinner	Workout quality takes priority
Fri	Easy ride or complete rest	Medium	—	—	Normal dinner	Slightly top up glycogen for the weekend long ride
Sat	Long endurance ride (3–5 hours)	Highest	Carb-focused breakfast, low fiber, low fat	Consistent fueling throughout	Replenish carbs + protein as soon as possible after returning	Fueling during the ride is the key
Sun	Medium-length aerobic ride or recovery run	Medium	Depends on recovery status from the previous day	Still need to fuel if over two hours	Normal meal	If you lost too much the day before, eat it back today

Table 3: Weekend Heavy-Volume Version (Wuling / Beiyi Long-Climb Week)

Day	Training Content	Carb Level	Key Reminders
Mon	Complete rest	Lowest	Maintain protein and vegetables; don’t eat very little overall just because you “didn’t train”
Tue	Short intervals (high intensity)	High	Eat enough before training; prioritize workout quality over sacrificing the session for low-carb
Wed	Low-intensity aerobic (fasted morning ride optional)	Low	Intensity must be genuinely low; stop immediately and take sugar if dizziness, heart palpitations, or shaking occur
Thu	Threshold workout	High	If you went low-carb the day before, make sure to replenish today
Fri	Easy ride or rest	Medium-high	Start accumulating glycogen for the weekend long climbs
Sat	Wuling or Beiyi long climb (heavy volume)	Highest	Carb-focused dinner the night before and breakfast that morning; consistent fueling throughout without interruption
Sun	Recovery ride (very easy)	Medium	Recovery takes priority; given the huge expenditure the day before, this is not the day to push low-carb

How to Adapt the Examples to Your Own Schedule

Write down your weekly training plan first, and mark which days are “performance days.”
Set performance days as high-carb first; don’t touch those.
For the remaining days, rank carb levels from high to low based on training volume.
Draw a horizontal line for protein — keep it the same across all seven days.
Check the weekly total: if you’ve trimmed every single day, that’s not periodization, that’s dieting.
Execute for two to three weeks, then review using the monitoring items from Chapter 11, and fine-tune accordingly.

VII. Local Taiwan Context: How This Approach Applies Here

Taiwan’s training environment has several distinctive characteristics that directly affect how carb periodization is scheduled.

Weekday Evening Trainer Sessions

For most office workers, the weekday schedule is “one hour on the trainer after work.” This creates three practical issues:

Whether to eat before training: The gap between finishing work and starting the session is often very short. Instead of forcing a full meal, design your lunch and afternoon snack so your glycogen isn’t empty when you get on the bike.
Eating too late after training: High-intensity sessions often end close to midnight, and eating a large meal at that point can affect sleep. It’s better to have an easily digestible carb + protein snack right after training, and keep the main meal relatively modest.
Intensity tends to be overestimated: One hour on the trainer is usually much higher intensity than the same time outdoors. If you treat trainer interval days as “just one hour” and cut carbs, the fatigue the next day will be very noticeable.

Weekend Wuling Long Climbs

Wuling is the highest point on Taiwan’s road network (approximately 3,275 meters above sea level, on Provincial Highway 14甲). What makes this route unique is its long duration, massive elevation gain, high altitude in the latter half, and extreme temperature swings.

For carb planning, note the following:

Total expenditure is far higher than a typical long ride, so carbs the day before and at breakfast that morning cannot be skimped on.
Appetite often drops at high altitude in the latter half — the further you go, the less you can eat, so start fueling consistently from the early and middle sections; don’t wait until the end to think about it.
Cold temperatures make people forget to drink, and dehydration impairs gut absorption and reduces fueling efficiency.
This kind of day is absolutely not a day for low-carb experiments.

Beiyi, Fengguizui, and Yangjin P-Series Mountain Roads

These are the everyday routes for northern riders. Their common feature is large swings in intensity — high power on climbs, almost no effort on descents. This “intermittent” style of riding depletes glycogen more than you’d expect, because you often exceed threshold without realizing it on the climbs.

Practical advice: classify these routes as “Category B long-endurance days” or even “Category A high-intensity days,” rather than “casual recovery rides.” Many people don’t eat enough because they underestimate these routes as “just going for a ride in the hills.”

Summer Heat and Humidity

Taiwan’s summer humidity and heat are a difficult environment recognized by endurance athletes worldwide. It affects fueling in three ways:

Sweat rate increases dramatically, and water and electrolytes are lost quickly, so fueling must account for both water and electrolytes.
In high heat, the body relies more heavily on carbs for energy — meaning the same intensity burns more glycogen on a hot day than on a cool day.
Heat suppresses appetite and impairs gut absorption; solid food may be hard to get down, so liquid or semi-solid fueling forms are often easier to tolerate.

Conclusion: summer is not a good time to push low-carb, especially for long outdoor sessions. If you want to schedule a low-carb, low-intensity workout, a more reasonable window is an early morning session when it’s cool, and keep it short with controlled distance.

Riverside Bike Path Long Rides

The riverside paths are characterized by flat terrain, intensity that tends to run low, time that tends to stretch long, and unevenly distributed fueling points. These sessions are essentially Category B (long endurance), but with low intensity, so:

Don’t skip fueling entirely just because “intensity is low” — long duration will still empty your glycogen.
When the wind picks up along the riverside (especially the northeast monsoon in winter), the actual intensity into a headwind is far higher than it feels.
In summer, the riverside lacks shade, and the risk of heatstroke and dehydration is high — shade, water, and electrolytes take priority over carbs.

Building Your Plan with Taiwanese Foods

You don’t need imported specialty foods to do periodization. Everyday Taiwanese eating is actually very easy to work with:

High-carb days: white rice, noodles, sweet potatoes, steamed buns, bananas, convenience store rice balls and onigiri, sports drinks.
Fueling during rides: besides energy gels and bars, rice balls, bread, bananas, and yokan (sweet red bean jelly) are long-standing choices many riders use.
Low-carb days: reduce staple portions, while maintaining or even increasing protein (chicken breast, soy products, eggs, fish) and vegetables.
Watch out for hidden carbs: the sugar in hand-shaken drinks, bread, and sauces often breaks your “low-carb day” without you realizing it.

VIII. The Eight Most Common Mistakes

Mistake 1: Turning Periodization into Chronic Energy Deficiency

This is the most serious and the most common. Trim a little every day, and over time it becomes overall under-eating. The way to tell is simple: if your low-carb days are never compensated by high-carb days, what you’re doing isn’t periodization — it’s dieting.

Mistake 2: Cutting Protein on Recovery Days Too

“Didn’t train today, so eat less” is an intuitive thought, but recovery days are exactly when repair happens. Carbs can go down; protein cannot.

Mistake 3: Using Train-Low on High-Intensity Days

Already detailed in Chapter 2. Low-glycogen training should only be used on sessions that don’t require performance anyway. Using it on interval days is self-sabotage.

Mistake 4: Experimenting Randomly During Race Week

The week before a race should be about confirmation, not exploration. Changing your diet structure, trying new fueling products, or attempting carb loading for the first time during race week are all high-risk behaviors. Every fueling strategy must be tested during training first.

Mistake 5: Confusing “Train Low” with “Low Energy Availability”

This is a critically important distinction:

Train low: Short-term, specific workouts, deliberately keeping muscle glycogen low, but overall energy intake is sufficient.
Low energy availability: After subtracting exercise expenditure from total energy intake, the energy remaining to support basic physiological functions is insufficient. This is a pathological state that directly harms health.

Many people think they are doing the former when they are actually doing the latter. This is the most important sentence in this article.

Mistake 6: Only Looking at Carbohydrates, Ignoring Total Energy and Micronutrients

When carbohydrates are reduced, if other components are not adjusted, iron, calcium, vitamin D, and fiber can easily drop as well. Over the long term, anemia, bone problems, and gut issues may all emerge.

Mistake 7: Neglecting Gut Training

The success or failure of long-distance races is often not determined by the legs—it’s the stomach. The amount and form of in-race fueling must be practiced repeatedly during training so the gut adapts to absorbing during exercise. Suddenly consuming large amounts on race day carries a high risk of gastrointestinal distress.

Mistake 8: Turning It into a Compulsive Numbers Game

Some people, once they start periodization, end up calculating every bite, feeling anxious about eating out, and unable to join social gatherings. This is not progress—this is a warning sign. Please skip directly to Chapter 9.

9. Risks and Red Lines: Please Read This Chapter Thoroughly

The biggest risk of carbohydrate periodization is not scheduling the wrong workouts—it’s that it can easily slide into energy deficiency. Here are the things you must understand.

Relative Energy Deficiency in Sport (RED-S)

Relative Energy Deficiency in Sport (RED-S) refers to a state where an athlete’s energy intake chronically fails to support training expenditure plus the energy required for basic physiological functions. It does not only affect women—men can experience it too, and it is not necessarily accompanied by obvious weight loss.

RED-S affects a wide range of systems and is generally understood to potentially involve:

Endocrine and hormonal function
Bone health and bone density
Immune function
Menstrual function (in women)
Reduced metabolic rate
Mental health
Growth and development (in adolescents)
Training adaptation capacity and performance

Common early warning signs (if several are present, they should be taken seriously):

Long-term stagnation or decline in training performance, with no improvement even after increasing volume
Persistent fatigue, slower recovery, constantly feeling “tired no matter how I train”
Feeling unusually cold, cold hands and feet
Decreased libido
Low mood, irritability, anxiety
Poorer sleep quality
Frequent minor illnesses, recurring colds
Repeated bone or tendon injuries
Changes in or loss of menstrual cycle (in women)
Deteriorating hair, skin, and nail condition

Menstrual Cycle and Amenorrhea Warning Signs

Menstruation is a highly sensitive indicator of energy status. For athletes who menstruate, if the following occur after starting a dietary adjustment, you should stop the current dietary strategy and seek medical evaluation:

Menstrual cycle becomes longer or irregular
Menstrual flow noticeably decreases
No period for several consecutive months
Previously regular, now unpredictable

Do not treat “training until your period disappears” as a badge of dedication. In sports medicine, this is considered a health issue that needs to be addressed, not an achievement. Long-term amenorrhea is associated with decreased bone density and an increased risk of stress fractures.

For those taking hormonal contraceptives, note: bleeding induced by medication cannot be used as an indicator of adequate energy status. In such cases, other warning signs and professional assessment are even more necessary.

Bone Density and Stress Fractures

The relationship between chronic energy deficiency and bone health is the most irreversible aspect of this topic. Bone density loss is often asymptomatic and is only discovered when a stress fracture occurs—and bone that has already been lost may not be fully recoverable.

High-risk groups include: those with chronic energy deficiency, those who are amenorrheic, those with chronically low body weight, and those whose primary sport is running (high impact loading). If you experience recurring tibial, metatarsal, or femoral pain, do not self-diagnose it as “muscle soreness”—seek medical attention.

Immune Function

Chronic energy and carbohydrate deficiency is generally understood to be associated with an increased frequency of upper respiratory infections. If you notice that after adjusting your diet you become “especially prone to colds” or “colds take a long time to recover,” this is your body sending a signal.

Warning Signs of Eating Disorders (Very Important)

Any diet with “rules” can become an entry point to an eating disorder for certain individuals. If any of the following appear, professional help (physician, clinical psychologist, registered dietitian) is recommended:

Strong guilt about food, feeling anxious, self-blaming after eating something “not allowed,” and wanting to “offset” it with exercise
Inability to eat normally in social settings, avoiding gatherings, family meals, and travel because food content cannot be controlled
Compulsive counting, unable to eat anything without calculating, with counting consuming significant mental energy
Food list keeps narrowing, with the range of acceptable foods gradually shrinking
Frequent checking of weight/body shape (weighing multiple times a day, repeatedly looking in the mirror, repeatedly measuring circumferences)
Abandoning social activities, affecting work or relationships because of dietary rules
Alternating between bingeing and extreme restriction
Refusing to adjust diet even when the body shows warning signs (amenorrhea, frequent injuries)

Eating disorders are treatable, and the earlier the intervention, the better the prognosis. If you see these signs in yourself or someone around you, please do not dismiss them with “they’re just very disciplined.”

Not Suitable for Adolescents and Older Adults

Adolescents: Still in the growth and development stage, with high energy and nutritional demands. Any form of energy restriction can affect growth, bone development, and endocrine function. Carbohydrate periodization or any restrictive diet is not recommended for adolescent athletes, unless there is a medical need and it is conducted under professional supervision.
Older adults: Muscle mass and bone density naturally decline with age, and protein utilization efficiency also decreases. The nutritional focus for older adults is adequate protein and adequate energy to maintain muscle and bone, not restriction. Those with chronic diseases or on medication require professional evaluation.

Diabetes and Those Using Insulin or Blood Sugar-Lowering Medications

This is an absolute red line. Large fluctuations in carbohydrate intake directly affect blood sugar. For those using insulin or oral blood sugar-lowering medications, this can cause severe hypoglycemia, which is an immediately dangerous situation.

If you have type 1 or type 2 diabetes, or are taking any medication that affects blood sugar:

Do not self-adjust your carbohydrate intake pattern
Any dietary adjustment must be planned together with your primary care physician and diabetes educator/dietitian
Exercise itself also affects blood sugar; combining exercise with dietary changes carries even higher risk
Carry a fast-acting source of sugar at all times, and let those around you know your condition and how to respond

Kidney Disease

When carbohydrates are reduced, the protein proportion in the diet often rises. For those with chronic kidney disease, protein intake must be strictly controlled—self-increasing it may place additional burden on the kidneys. The diet of anyone with kidney disease must be planned by a nephrologist and dietitian, and the general principles in this article do not apply.

Pregnancy and Lactation

Energy and nutritional needs change significantly during pregnancy and lactation, directly affecting the fetus and infant. During this period, no energy restriction or carbohydrate periodization aimed at weight management or training adaptation should be undertaken. All dietary arrangements should be evaluated by an obstetrician and dietitian.

Other Situations Requiring Professional Evaluation

Cardiovascular disease, liver disease, thyroid disease, or gastrointestinal diseases (such as inflammatory bowel disease or celiac disease)
Taking medications that may interact with diet
A history of eating disorders (this is a fairly clear contraindication)
Recent major surgery or illness
A history of recurrent stress fractures or known low bone density

Gut Tolerance Must Be Tested in Advance

One final practical safety reminder: any new supplement, new fueling amount, or new fueling frequency must be tested during training first. Start with small amounts, increase gradually, and observe gastrointestinal reactions. Wuling, race day, and long-distance challenge day are not the time for experiments.

10. Who Should Not Do Carbohydrate Periodization

Even without the aforementioned medical conditions, the following groups are also advised to hold off for now:

1. Beginners

If you haven’t been training consistently for a considerable period, your room for improvement comes primarily from “consistent training” itself, not from fine-tuning your nutrition. At this stage, focusing your energy on building a stable training habit and a balanced diet offers a much higher return on investment. Beginners also find it harder to distinguish between “fatigue caused by low carbohydrate intake” and “fatigue caused by training volume.”

2. Those actively losing weight, or those already at a low body weight

Weight loss itself is an energy deficit. Stacking carbohydrate restriction on top of a deficit significantly increases the risk of energy deficiency. If you are already losing weight, the role of periodization should be to “allocate the deficit more intelligently,” not to “cut even more.” For those already at a low body weight or body fat percentage, this approach is not recommended.

3. Peak competitive season

During the racing period, the top priority is performance and recovery, not adaptation. At this time, you should keep carbohydrate intake high and stick to a fixed fueling strategy—no experiments.

4. Sleep deprivation or high-stress periods

Sleep deprivation and chronic stress themselves elevate cortisol and impair appetite regulation and recovery. Adding dietary restrictions on top of this state can easily harm both recovery and mental health. If you’ve been under heavy work stress or sleeping poorly lately, fix your sleep first—don’t touch your diet.

5. Those with low training volume and little day-to-day variation

If you train three times a week for one hour each session, the differences in daily needs are actually quite small, and the benefits of periodization are very limited. Just maintain a balanced diet and stay consistent with your training.

6. Those with a history of eating disorders

Already listed as an explicit contraindication in Chapter 9; this is reiterated here.

11. How to Know If You’ve Overdone It: Monitoring Methods

Periodization must be paired with monitoring, otherwise you’re just flying blind. Below are relatively practical approaches that don’t require expensive equipment.

Table 4: Daily Monitoring Items and Red-Flag Criteria

Monitoring Item	How to Assess	Signals Requiring Caution
Body weight trend	Look at the weekly average, not single-day readings	Weight loss too rapid, or unexplained weight loss persisting for several weeks
Morning resting heart rate	Measure under the same conditions every day upon waking; look at the trend	Clearly deviating from personal baseline for several consecutive days (too high or abnormally low)
Heart rate variability (HRV)	Same as above; look at trends only, not single-day values	Consistently below your normal personal range for several consecutive days
Training quality	Completion rate and subjective feel of the same workout	Familiar workouts become difficult, power drops, unable to complete sessions
Subjective recovery and mood	Record daily with a simple scale	Persistently low mood, irritability, loss of motivation
Sleep	Time to fall asleep, number of nighttime awakenings, how you feel upon waking	Difficulty falling asleep, never feeling rested
Menstruation (for those who menstruate)	Track cycle regularity	Cycle lengthens, becomes irregular, or stops
Illness and injury frequency	Track colds and injuries	Recurrent colds, recurring minor injuries
Appetite and food thoughts	Whether you’re constantly thinking about food	Excessive fixation on food, or abnormally suppressed appetite

A Few Principles for Use

Look at trends, not single data points. Any single day’s body weight, heart rate, or HRV can be affected by sleep, salt intake, stress, or the female menstrual cycle. What matters is the direction over consecutive days.
Establish your own baseline. These numbers have no universal standard values; what matters is “relative to how you normally are.” So first record data for two to three weeks under normal eating and normal training conditions before making any adjustments.
Heart rate and HRV should only be used qualitatively. They are signals that “remind you to check in with yourself,” not diagnostic tools. Don’t use wearable device numbers to self-diagnose any medical condition.
Laboratory tests should be interpreted by a physician. Blood tests (such as iron, vitamin D, hormones, thyroid function, etc.) may be helpful when energy deficiency is suspected, but they should be ordered and interpreted by a physician based on your overall condition—don’t go to a health check center, grab a bunch of numbers, and compare them online. The same applies to bone density scans.
Set a stop-loss point. Before you begin, write down in advance: “If X, Y, or Z occurs, I will stop and return to normal eating.” Don’t wait until the situation arises and rely on willpower to decide.

A Simple Self-Check

Every two to three weeks, ask yourself these five questions:

Has the quality of my hard workouts been maintained?
Has my sleep gotten worse?
Am I thinking about food more and more often, or feeling guilty about eating?
Is my rate of weight loss beyond a reasonable range?
(For those who menstruate) Is my cycle normal?

If the answers to two or more questions make you uneasy, return to normal eating first and consider seeking professional help.

12. Medical Attention Warning Signs

If any of the following occurs, seek professional medical help as soon as possible—do not continue adjusting your diet on your own:

Dizziness, blacking out of vision, cold sweats, trembling, or altered consciousness during or after exercise (may indicate hypoglycemia or another acute condition; seek immediate medical attention if consciousness is altered)
Chest tightness, chest pain, palpitations, or unexplained shortness of breath
No menstruation for three consecutive months or more, or a significant change in your menstrual cycle
Recurrent bone pain, especially localized tenderness in the tibia, metatarsals, hip, or femur
Unexplained continuous weight loss
Extreme fatigue lasting for several weeks that does not improve with rest
Frequent infections (recurrent colds, slower wound healing)
Intense anxiety or guilt about food, alternating binge eating and extreme dieting, or avoiding social eating situations
Persistently low mood, feelings of hopelessness, or loss of interest in things you once enjoyed (seek immediate help if you have thoughts of self-harm)
Unstable blood sugar or disease control in those with diabetes or other chronic conditions
Abnormal weight changes or discomfort during pregnancy or breastfeeding

Professionals you can consult include: family medicine, endocrinology, obstetrics/gynecology (for menstrual issues), sports medicine, registered dietitians (especially those with a sports nutrition background), and clinical psychologists (for eating- and emotion-related issues).

13. Necessary Reminders

This article is general health education and training information sharing and cannot replace professional medical evaluation, diagnosis, or treatment. All principles in this article are general guidelines in the field, and individual variation is enormous—the same strategy can produce completely different results in different people. Your body weight, body fat, training volume, training history, age, sex, existing medical conditions, medication use, sleep, and stress levels all change what approach is appropriate.

Any dietary adjustment should be made gradually, and you should stop if discomfort arises. If you have any chronic disease, are taking medication, are pregnant or breastfeeding, are an adolescent or elderly, or have a history of eating disorders, please be sure to consult a physician and a registered dietitian before starting.

One more reminder: When riding outdoors, obey traffic rules, pay attention to road conditions and weather, and do not race on open roads; avoid prolonged sun exposure during peak summer heat and always watch for signs of heatstroke.

14. Action Checklist: Ten Things You Can Start Doing Today

Write down your training schedule for the next four weeks first, marking which days require peak performance and which days don’t. This is the foundation of all planning.
Before adjusting anything, record a two-to-three-week baseline: weight trends, morning heart rate, sleep quality, training quality, and (for those who menstruate) menstrual cycle.
Treat protein as a horizontal line—unchanged across seven days, not reduced on rest days either. This is the step least likely to go wrong and the most consistent in its benefits.
Adjust only carbohydrates, starting with the simplest approach: reduce staple food portions on complete rest days, increase them on hard training days. There’s no need to dive into complicated sleep-low protocols from the start.
Check for hidden carbohydrates: bubble tea, sauces, bread. For many people, the problem isn’t eating too many staples—it’s these.
Don’t cut vegetables and fiber along with everything else. On low-carb days, what should be cut is refined sugar and extra starch, not vegetables.
Treat mid-ride fueling as a required course: fuel regularly on sessions lasting two hours or more, test the type and amount of fuel during training, and build up your own gastrointestinal tolerance.
Always eat enough on high-intensity days and race days—no experiments. All experiments belong on low-intensity days.
Write down your stop-loss conditions (for example: two consecutive weeks of declining training quality, weight loss happening too fast, menstrual irregularities, feelings of guilt around food), and actually follow through on them.
When in doubt, seek professional help. A single consultation with a sports nutritionist or physician is far more effective—and far safer—than comparing ten different opinions online.

And back to that core principle: fuel for the work required. The point has never been about “eating less,” but about “eating just enough, eating at the right time.” If you find yourself getting hungrier, more fatigued, and more anxious as you execute this, that’s not a lack of discipline—that’s the strategy telling you: it’s time to add carbohydrates back in.

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