Low Glycemic Index Diet and Endurance Sports: What GI Is, How to Eat Before, During, and After Races, and Common Misconceptions Clarified

Medical and Dietary Disclaimer (Please Read First)

This article is a general educational summary of sports nutrition, not a personalized dietary prescription, and it cannot replace the professional assessment and dietary plan provided by a nutritionist or physician for individual health conditions (such as diabetes, metabolic syndrome, history of eating disorders, gastrointestinal diseases, etc.). If you have type 1 or type 2 diabetes, gestational diabetes, kidney disease, or other chronic conditions that require strict blood sugar or dietary management, any dietary adjustments must be discussed with your medical team—do not rely solely on this article to change your medication-associated dietary patterns or insulin dosages on your own. All principles mentioned in this article are general descriptions, and individual variation is enormous. Whether you can tolerate them and how they should be applied requires repeated testing and adjustment during your own training, rather than adopting them wholesale. If you notice that you have abnormal anxiety or compulsive control tendencies regarding food, body weight, or eating, please pay attention to the warning signs list at the end of the article and seek professional help.

In the endurance sports community, “low-GI diet” has almost become a buzzword—eating oatmeal before a race and avoiding white toast seems like common knowledge. But if you ask further questions like “What is GI?” “What’s the difference between GI and GL?” or “Should we aim for high GI or low GI before and after training?” not many people can actually explain it clearly. This article aims to break down the concept of GI from the ground up, focus on how the GI concept should be practically applied in the specific context of endurance sports, and also point out several common misconceptions in the sports community.

What Is GI? Understanding the Basic Definition First

GI (Glycemic Index) is a relative numerical concept used to describe “the speed and magnitude of blood sugar rise after consuming a carbohydrate-containing food.” The basic logic is: subjects consume a fixed amount (usually 50 grams) of digestible carbohydrates, the area under the curve of blood glucose concentration over time is observed, and this value is then compared to the same amount of a reference food (usually glucose or white bread) to derive a relative index. The higher the index, the faster the carbohydrates in that food are digested, absorbed, and converted into blood sugar; the lower the index, the more gradual the blood sugar rise.

Foods are generally divided into three rough ranges: low GI (roughly below 55), medium GI (between 56 and 69), and high GI (70 and above). This is a conventional industry classification, and actual values may vary depending on measurement methods, samples, food varieties, and cooking methods. Readers do not need to memorize a food’s GI value as an absolute fixed number, but rather understand its relative position and trend.

Common Misunderstandings About GI Values

The first common misconception is that “GI value equals the sugar content of that food.” This is wrong. GI measures the speed at which carbohydrates are absorbed and converted into blood sugar, not the total carbohydrate content in the food. For example, watermelon’s GI value is generally classified as high, but watermelon has high water content, and the actual total carbohydrates consumed per serving are not that much. This leads to the next, more complete concept—Glycemic Load (GL).

GL (Glycemic Load) = GI value × actual carbohydrate content of that food ÷ 100. GL takes both “the speed of blood sugar rise” and “how many carbohydrates you actually consume in that serving of food” into consideration. Therefore, when evaluating “the actual impact of this meal or this food on my blood sugar,” GL is usually closer to reality than looking at GI alone. This is also why some foods appear to have high GI values (such as watermelon and pumpkin), but because the total carbohydrate content in typical serving sizes is low, their GL is actually low, and their actual impact on blood sugar is limited.

The second common misconception is that “low GI equals healthy food, and high GI equals unhealthy food.” This is also an oversimplification. GI values only reflect the single dimension of blood sugar response speed and do not cover other important aspects such as overall nutritional value, fiber content, micronutrients, or degree of processing. Certain highly processed foods with high fat content may measure a low GI because fat delays gastric emptying, but that does not make them “healthy foods.” Conversely, certain natural foods with higher GI values (such as certain root vegetables or ripe fruits) are still rich in fiber and micronutrients and should not be labeled “unhealthy” and completely excluded from the diet.

Third, GI values can vary considerably due to cooking methods, ripeness, food combinations, and individual digestive absorption rates. For the same ingredient, the softer it is cooked and the higher the degree of starch gelatinization, the higher the GI value tends to be. If the same carbohydrates are consumed together with protein, fat, and fiber, the overall glycemic response of that meal is usually more gradual than consuming the carbohydrates alone. Therefore, GI values should be understood in the context of “overall dietary combinations,” rather than calculating the GI of individual ingredients one by one.

Why Endurance Athletes Care Particularly About the GI Concept

Endurance sports (long-distance cycling, road running, triathlon) rely heavily on a stable supply of glycogen (the form of carbohydrates stored in the liver and muscles) and blood sugar for energy. Throughout a training session or race, the body’s demand for and utilization patterns of carbohydrates actually differ significantly across three phases—before, during, and after exercise—and each phase has its own optimal strategy. This is precisely why the GI concept is particularly practical in endurance sports.

Before Exercise: The Trade-off of Blood Sugar Stability

Before exercise, especially for the pre-race meal, carbohydrate source selection generally leans toward low-to-medium GI foods, and the reason is related to a phenomenon called “reactive hypoglycemia.” When we consume a large amount of high-GI carbohydrates before exercise, the body secretes more insulin in response to the rapid blood sugar rise. But if we don’t start exercising immediately after eating and instead wait more than half an hour before starting activity, the insulin effect may cause blood sugar to drop rapidly from its peak, resulting in hypoglycemic discomfort (dizziness, trembling hands, inability to summon energy) at the start of exercise. In contrast, low-GI foods produce a more gradual blood sugar rise and a milder insulin response, making the “high-then-low” blood sugar swing less likely, and blood sugar status is relatively stable when exercise begins.

However, there is one often-overlooked exception: if carbohydrates are consumed within a very short time before exercise begins (e.g., within five to ten minutes before the start), or even during the initial phase of exercise, the exercise itself stimulates adrenaline secretion, and adrenaline has an insulin-suppressing effect. In this “critical pre-race timing window,” consuming high-GI carbohydrates is actually less likely to trigger the aforementioned reactive hypoglycemia. This is why some athletes choose to take a simple high-GI carbohydrate supplement right at the starting line rather than eating an hour earlier. The timing of this varies from person to person, and it is recommended to test repeatedly during regular training to find out how your body responds to different intake timings. Never try a new fueling timing or a new food type for the first time on race day.

During Exercise: High GI Is Actually the Norm

Interestingly, once you are in the middle of prolonged exercise, the demand completely reverses—during exercise, immediate energy replenishment typically leans toward medium-to-high GI carbohydrate sources, such as energy gels, sports drinks, bananas, and white toast—foods that are digested and absorbed relatively quickly. This is because during exercise, the body needs to quickly convert consumed carbohydrates into usable blood sugar to supply continuously working muscles. At this point, “fast absorption” is an advantage rather than a disadvantage, which is a different logic from pursuing blood sugar stability before exercise. At the same time, prolonged exercise itself depletes large amounts of muscle and liver glycogen, and carbohydrate replenishment during exercise also helps delay the onset of “glycogen depletion” (commonly known as hitting the wall, or “bonking”).

After Exercise: Balancing Recovery Speed and Overall Diet Quality

The goal of carbohydrate replenishment after exercise is to initiate muscle and liver glycogen resynthesis as quickly as possible. This is especially important if there is another training session or race the next day or within a short period (e.g., multi-day challenge events or training camps), where recovery speed becomes even more critical. At this stage, medium-to-high GI carbohydrate sources also have their role, as they can raise blood sugar and insulin levels more quickly and promote glycogen synthase activity. However, if there is ample recovery time between training sessions (e.g., more than a day), the flexibility in choosing carbohydrate sources for the post-exercise meal is much greater. There is no need to deliberately restrict yourself to high-GI foods; instead, you can shift the focus back to overall dietary balance and quality, pairing with adequate protein to support muscle repair.

Summary of Three-Phase GI Application

Clarifying Common Misconceptions

Misconception 1: “You should always eat low-GI foods before and after training.” As mentioned earlier, this only applies to the “pre-race meal eaten well in advance” scenario. During exercise and for immediate post-exercise fueling, the goal is often relatively rapid absorption. Overly insisting on “low GI at all times” may make the absorption of in-race fuel too slow to support the body’s needs promptly, and could even increase the risk of hitting the wall in long-distance events.

Misconception 2: “A low-GI diet can replace overall calorie and nutrition planning.” GI only describes one aspect of blood sugar response speed. Whether a dietary plan suits your training needs also depends on total calorie intake, macronutrient ratios, micronutrients, hydration, and electrolytes, among other factors. Focusing only on GI values when choosing foods may overlook the overall balance of the diet.

Misconception 3: “High-GI foods should be completely avoided.” As explained earlier, high-GI foods play a functional role in in-race fueling, very short-term pre-race intake, and rapid post-exercise recovery. They should not be indiscriminately excluded from the diet. Simply dividing foods into “good low-GI” and “bad high-GI” while ignoring the context is the most common misuse of this concept.

Misconception 4: “The GI value is a fixed, absolute number.” As mentioned earlier, GI values can vary considerably due to cooking methods, ripeness, food combinations, and individual differences. Treating a food’s GI value as an absolute, black-and-white standard ignores the complexity of real-world dietary contexts. Rather than memorizing specific GI numbers, it’s better to understand trend-based principles like “the more refined, the less fiber, and the softer the cooking, the faster the blood sugar response usually is.”

Misconception 5: “All endurance athletes should adopt the same low-GI dietary strategy.” Individual differences are particularly pronounced on this topic—the same food can produce vastly different blood sugar response curves in different people, related to gastrointestinal digestion and absorption rates, insulin sensitivity, current training status, and even sleep and stress. Rather than applying a one-size-fits-all “standard answer,” a more practical approach is to repeatedly test different food and timing combinations during training (not on race day), identify your body’s actual response patterns, and gradually build your own fueling strategy.

Misconception 6: “Experiencing reactive hypoglycemia symptoms means you have diabetes or another metabolic disease.” Reactive hypoglycemia in the context of exercise is a different discussion from clinical hypoglycemia or diabetes. You should not self-diagnose based on this. If you frequently experience symptoms such as heart palpitations, cold sweats, or confusion in daily life (outside of exercise), this should be evaluated by a physician to rule out underlying endocrine or metabolic conditions, rather than being interpreted solely through exercise nutrition strategies.

How Food Combinations Change the Overall Glycemic Response

Beyond the GI value of a single food, what often matters more in practice is the combined effect of the “whole meal,” which is particularly useful for endurance athletes planning their pre-race meals. The following mechanisms are worth understanding:

Fat delays gastric emptying: Fat prolongs the time food stays in the stomach, indirectly slowing the absorption of carbohydrates into the small intestine, so the overall glycemic response of a meal is more gradual than consuming the same carbohydrates alone. But this is a double-edged sword—if the pre-race meal has too high a fat content, gastric emptying is prolonged, and the gastrointestinal tract may still be under digestive load when exercise begins, easily causing bloating, nausea, and other discomfort. This is why pre-race meals are generally recommended to avoid excessive fat.

The buffering effect of protein: Protein also slows overall digestion and flattens the glycemic curve. Additionally, protein itself stimulates some insulin secretion, though typically to a lesser extent than an equivalent amount of refined carbohydrates. Pairing a pre-race meal with moderate protein (e.g., eggs, dairy, soy products) is a common practice to smooth blood sugar fluctuations.

The role of dietary fiber: Soluble fiber forms a gel-like substance in the gut, physically slowing the breakdown and absorption of carbohydrates by digestive enzymes. This is why carbohydrate sources with higher fiber content, such as whole grains, legumes, and root vegetables, generally produce a more gradual glycemic response than refined grains with the bran and germ removed (such as white rice or white bread). However, note that consuming excessive high-fiber foods before exercise may also increase the risk of gastrointestinal discomfort, bloating, or even the need for an unscheduled bathroom stop during exercise. This is why many endurance athletes deliberately choose food combinations that are “relatively easy to digest with moderate fiber” rather than “maximizing fiber” for their pre-race meals—a balance must be struck.

Direction of Impact of Food Combinations on Overall Glycemic Response

Building Your Own Fueling Test Protocol

Because individual differences are enormous, rather than directly adopting someone else’s food combinations, a more practical approach is to build your own testing protocol—and be sure to conduct these tests during training periods, not on race day:

1. Establish a baseline first: During a training session with intensity and duration close to your target event, record your usual pre-race diet and in-race fueling rhythm, and subjectively log how you feel (mental state, gastrointestinal condition, any signs of hypoglycemia, overall performance).
2. Change only one variable at a time: For example, adjust only the carbohydrate source of the pre-race meal (switching to a low-GI version) while keeping all other fueling rhythms unchanged. This allows you to determine the actual difference caused by that change, avoiding the inability to attribute results when too many variables are changed at once.
3. Record the perceived differences of consuming carbohydrates at different time points: For example, test eating the same high-GI food “one hour before the race” versus “ten minutes before the race,” and record whether your perceived state differs during the first twenty to thirty minutes after exercise begins.
4. Treat long-distance simulation training as a full dress rehearsal: If your goal is a long climbing challenge like Wuling, or a long-duration event like a full marathon or triathlon, schedule at least one or two training sessions with duration and intensity close to the actual event, fully rehearsing the entire fueling strategy for pre-race, in-race, and post-race, rather than only testing fragments in short training sessions.
5. Build your own “safe list” and “landmine list”: Record the food and timing combinations that your body responded well to during testing as priority options for race day. Also note combinations that have caused discomfort, and firmly avoid them on race day—do not make impromptu attempts at new things due to race-day atmosphere or aid station limitations.

This protocol has no shortcuts; it requires gradual accumulation through multiple training sessions. But in the long run, compared to chasing the “optimal GI diet formula” circulating online, a fueling strategy built on your own body’s actual responses is generally far more reliable.

Practical Recommendations for the Taiwanese Context

When applying the GI concept, endurance-sports enthusiasts in Taiwan should keep several localised practical scenarios in mind:

• Long-distance cycling or road running in the hot, humid summer, where heavy sweating causes fluid and electrolyte loss that often becomes a performance-limiting factor before carbohydrate strategy does. Your fuelling plan must address both carbohydrates and fluid/electrolytes—don’t focus only on GI and neglect hydration.
• Long, high-intensity climbing challenges such as Wuling, where exercise duration tends to be very long, making fuelling rhythm (small amounts, frequent intake) just as important as GI selection. It’s advisable to test your tolerance for different fuels under high-intensity, long-duration conditions during shorter training climbs first.
• Races with early-morning starts such as the Taipei Marathon or Wan Jin Shi Marathon, where the time between your pre-race meal and the start must account for commuting and check-in time. Plan accordingly to avoid being pressed for time and eating an unsuitable food combination at the wrong moment.
• Races with widely spaced aid stations on rural courses, such as the Tanaka Marathon, where knowing in advance what foods are offered at the stations (bananas, sports drinks, etc., mostly medium-to-high GI) and simulating a similar fuelling rhythm in training is far more reliable than improvising on race day.

Important Reminder on Weight Loss and Eating Disorders

Low-GI diets are sometimes overextended into the context of “weight loss.” It must be clearly stated here: a low-GI diet is not a low-calorie diet, nor is it a risk-free weight-loss panacea. If your goal is to manage body weight by adjusting GI intake, you still need to consider overall energy balance and avoid falling into an extreme eating pattern that “only looks at GI values while ignoring total calories and nutritional balance.” Long-term over-restriction of specific food categories, intense guilt or anxiety about eating, deliberately cutting food drastically before races to feel “light,” or repeatedly engaging in extreme compensatory behaviours after bingeing (such as excessive exercise, self-induced vomiting, or laxative abuse) can all be warning signs of an eating disorder. This is not uncommon in the endurance-sports community, yet it is often mistaken for mere “self-discipline” or “being particular,” and deserves serious attention.

Checklist of Warning Signs for Seeking Medical or Professional Advice

If any of the following occurs, it is recommended that you seek professional help from a physician or dietitian rather than relying solely on self-adjustment or online information:

• Recurrent palpitations, cold sweats, confusion, or blurred vision during or after exercise that resemble hypoglycaemia, with a frequency or severity beyond your past experience.
• A history of diabetes, prediabetes, gestational diabetes, or other endocrine/metabolic conditions, and you plan to adjust your eating pattern to match your training.
• Unintentional rapid weight loss or gain, menstrual irregularities (for readers who menstruate), chronic fatigue, or a significant unexplained decline in athletic performance.
• Compulsive tendencies around food control, intense guilt after eating, avoidance of social eating situations, or a recurring cycle of dieting and bingeing.
• Children, adolescents, pregnant women, or breastfeeding women considering specific dietary strategies to accompany sports training. These groups have nutritional needs different from those of typical adult endurance athletes and must be evaluated by professionals.

Conclusion and Action Checklist

GI and GL are practical tools for understanding how carbohydrates affect blood glucose, but they are tools, not beliefs. They should be applied within the two contexts of “different phases—before, during, and after exercise” and “individual variation,” rather than as a single rigid rule. Here is an action checklist you can review right away:

• [ ] Understand that GI measures the speed of blood-glucose response, while GL more closely reflects the actual glycaemic impact of the amount consumed.
• [ ] For the pre-race meal (well in advance), favour low-to-medium GI options to avoid reactive hypoglycaemia; for fuelling in the very short window before the start, you may personally test high-GI options.
• [ ] During exercise, focus on rapid fuelling with medium-to-high GI foods; there is no need to insist on low GI.
• [ ] After exercise, decide based on the recovery window whether you need to emphasise high-GI foods to accelerate glycogen resynthesis.
• [ ] Test all fuelling strategies repeatedly in regular training; never try new combinations on race day.
• [ ] Do not treat GI as the sole criterion for health; maintain overall dietary balance and energy equilibrium.
• [ ] Watch for the medical and professional-consultation warning signs listed above, and seek help early, especially when chronic conditions or suspected eating-disorder tendencies are involved.

Use the GI concept in the right context, and you’ll find that it is not a rigid dietary dogma but a practical thinking framework that helps you arrange your fuelling rhythm more intelligently.

Related Reading

• Low-GI Pre-Ride Meals: 5 Practical Combinations for Stable Blood Sugar and Avoiding Hunger
• 5 Types of Low-GI Staples for Fat-Loss Rotation: An Energy-Curve Test of Switching White Rice for Quinoa
• A Complete Guide to Stable Blood-Sugar Eating for Athletes: GI/GL, Meal Timing, and Real-World Fuelling
• The Pre-Race Meal: Personalised Solutions for Timing, Portion Size, and “Reactive Hypoglycaemia”