A Deep Dive into Vitamin D, Athletic Performance, and Health: Field Notes from a Sports Science Consultant

It Starts with a Blood Test Report

A few years ago, I coached a very dedicated road cyclist—let’s call him Ah-Hsiang. Ah-Hsiang was the kind of hardcore rider who logged three to four hundred kilometers a week and even headed to Wuling for hill repeats on weekends. His skin was tanned dark and glossy—the very picture of a “sunshine athlete.” One year, he kept complaining about easily fatigued thighs and slower recovery, and he picked up two suspected overuse injuries in the same season. I asked him to get a full sports blood panel, which included serum 25-hydroxyvitamin D (25(OH)D).

When the results came back, I still remember his expression. A guy that tan, and his 25(OH)D was only 18 ng/mL—squarely in the “deficient” range. His first words were: “Coach, I’m out in the sun every day. How can I possibly be deficient in vitamin D?”

I’ve heard that question countless times over the past decade-plus. In this article, I want to explain vitamin D—a nutrient most people seriously underestimate—from the perspective of coaching athletes: what it actually does in the body, why so many people are still deficient even in a sunny place like Taiwan, and most importantly—if you’re an athlete, how to practically get it back up.

For those in a hurry, here’s the bottom line: Vitamin D isn’t just about “bone health.” It’s more like a hormone precursor that acts throughout the body, linked to muscle function, immunity, and inflammation regulation. And even in sunny Taiwan, the rate of deficiency among athletes is shockingly high. But supplementing isn’t a case of “more is better”—there are plenty of nuances.

Conceptual Foundation: Vitamin D Is Actually More Like a Hormone

It’s Not Just a “Vitamin”

We’re used to classifying vitamin D as a vitamin, but physiologically, it’s closer to a steroid hormone. When skin is exposed to UVB in sunlight, a cholesterol derivative is converted into vitamin D3, which then undergoes two processing steps in the liver and kidneys before becoming the active “1,25-dihydroxyvitamin D.” This active form enters the cell nucleus, binds to the vitamin D receptor (VDR), and regulates the on/off switches of a huge number of genes.

Here’s the key point: Vitamin D receptors are found in nearly every tissue in the body, including bone, the gut, and kidneys—but also skeletal muscle, immune cells, and the cardiovascular system. That’s why its effects are so widespread, not just the textbook “helps calcium absorption and prevents rickets.”

How to Read Blood Markers

Clinically, vitamin D status is assessed by measuring serum 25(OH)D, which reflects the body’s storage level. The cutoffs vary somewhat between organizations, but the common reference ranges are roughly as follows. I’ve organized them into a table so you can compare against your own report:

Serum 25(OH)D (ng/mL)	Approximate Conversion (nmol/L)	General Classification	My Practical Interpretation
< 20	< 50	Deficient	Needs active intervention—don’t delay
20–30	50–75	Insufficient	Where most athletes are stuck
30–50	75–125	Sufficient	A reasonable target range for general health
> 50	> 125	Some literature considers this ideal for athletes	Higher isn’t better; too high carries risks

According to the Endocrine Society’s recommendations, serum 25(OH)D ≥ 30 ng/mL (≥ 75 nmol/L) is considered sufficient; some sports-specific literature pushes the ideal range to around 50 ng/mL. But I want to emphasize: These numbers are ranges and references, not precise pass/fail lines. Whether your report says 29 or 31 doesn’t mean a world of difference—what matters is the trend, along with your overall symptoms and training status.

(Sources listed in the references at the end)

How Widespread Is Deficiency, Really?

Many people assume vitamin D deficiency is exclusive to “Nordic people or those who never go outside in winter” and has nothing to do with sun-drenched Taiwan. That’s a major misconception.

Research shows that even athletes who should be outdoors regularly have surprisingly high rates of insufficiency. Among NCAA Division I athletes in the US, for example, one study found that about one-third (33.6%) had 25(OH)D values in the abnormal range. In higher-latitude regions, it’s even worse—data from German elite athletes shows more than half (about 55.5%) had serum 25(OH)D below 30 ng/mL, with roughly 16% even below 20 ng/mL. Among young soccer players at high northern latitudes, insufficiency plus deficiency combined approached two-thirds.

Interestingly, studies have also observed that power athletes tend to have higher deficiency rates than endurance athletes—related to training venue (indoor vs. outdoor), training time of day, and sun-protection habits. (Sources listed in the references at the end)

My own experience coaching teams in Taiwan aligns closely with these international findings. Every time I run a group blood panel for a new team, the 25(OH)D column is often the “worst-hit” item—in a squad of athletes who look as strong as oxen, nearly half are stuck in the insufficient or even deficient range. At first, I thought the lab had made an error. After seeing it enough times, I realized: this is the norm, not the exception. So if your levels come back low, don’t feel particularly unlucky—you’ve just joined a very large club.

Season and Latitude: An Easily Overlooked Variable

Vitamin D synthesis is highly dependent on UVB, and UVB intensity varies dramatically with season, latitude, and time of day. Even in Taiwan, the sun angle is lower and daylight shorter in winter, so the skin’s ability to synthesize vitamin D is considerably less efficient than in summer. This means many people’s vitamin D follows a “high in summer, low in winter” seasonal pattern: they get plenty of sun in summer and their levels look fine, but come autumn and winter, levels quietly drop.

When coaching athletes, I specifically remind them to watch out for the combination of “winter + training moved indoors.” This is also why I don’t recommend testing once in summer and assuming you’re set for the year—what you measure might be your annual peak, and it won’t carry you through to next spring.

Why Taiwanese People “Get Plenty of Sun but Are Still Deficient”

Back to Ah-Hsiang’s question. Taiwan has abundant sunshine, so why are so many people still deficient? I’ve summarized several reasons that are especially common in Taiwan:

• Full-body coverage while cycling and running: Long-sleeve jerseys, arm warmers, leg warmers, head buffs, gloves—everything that can be covered is covered. Plus, Taiwan’s summers are so hot and the sun so harsh that people avoid sun exposure whenever possible to prevent tanning and sun spots. If the skin isn’t hit by UVB, it can’t synthesize vitamin D.
• Liberal sunscreen application: High-SPF sunscreens significantly block UVB. Taiwanese people prioritize skin whitening and sun protection, which is great for the skin—but one of the trade-offs is reduced vitamin D synthesis.
• Training times avoid midday: Taiwan’s summer midday heat is brutal, so most cyclists and runners train in the early morning or evening—precisely when UVB is weakest. You can soak up the 5 a.m. sun all you want, but vitamin D synthesis efficiency is very limited.
• A rising share of indoor training: Spinning, gyms, treadmills, indoor climbing—a lot of training has moved indoors.
• Dietary gaps for those who eat out: Natural food sources of vitamin D are limited to begin with—mainly fatty fish, egg yolks, and some fortified foods. For Taiwanese people who eat most meals out, these items are often underrepresented on the plate.
• Skin tone and individual differences: Skin with more melanin synthesizes vitamin D less efficiently. This is why “being tanned” and “having enough vitamin D” are two completely different things.

So Ah-Hsiang being tanned actually showed that his skin had been constantly fighting UV radiation—but that doesn’t mean his body had stored enough vitamin D. Skin tone reflects melanin; serum levels reflect storage. The two cannot be equated.

Vitamin D’s Multiple Roles in the Athletic Arena

In this section, I want to clarify exactly how vitamin D affects athletic performance and health. I’ll honestly distinguish between “stronger evidence” and “still under observation,” without exaggerating to sell supplements.

1. Bone Health and Stress Fractures

This is the most solidly evidenced area. Vitamin D helps the gut absorb calcium and maintains bone mineralization. For runners and ball-sport athletes who endure chronic impact, vitamin D plus adequate calcium is the foundation for preventing stress (fatigue) fractures. Clinically, when endurance athletes present with recurrent stress fractures, insufficient vitamin D and inadequate energy intake are often the underlying combination.

2. Muscle Function and Strength

Vitamin D receptors exist in skeletal muscle, so theoretically it’s related to muscle strength and repair. But here I have to be honest: the scientific evidence is mixed, not one-sided.

Recent systematic reviews and meta-analyses have produced divergent results—some find that vitamin D supplementation significantly helps “lower-limb strength,” but not upper-limb strength or power; other reviews find the overall effect size is small and the evidence limited. A more consistent observation is: those who are genuinely deficient are more likely to see performance improvements after being brought up to sufficient levels; those who are already sufficient typically won’t get stronger by adding more. This point is crucial, and I’ll emphasize it again below. (Sources listed in the references at the end)

3. Immunity and Respiratory Health

Vitamin D participates in immune regulation. For athletes with high training loads, immunity tends to be suppressed during intense periods, leading to more colds and upper respiratory infections. Maintaining sufficient vitamin D is thought to help stabilize immune function. But again, I need to be conservative: this doesn’t mean taking vitamin D will “prevent colds.” It’s more like shoring up the foundation, not a miracle cure.

4. Inflammation, Recovery, and Overall Health

Vitamin D is associated with inflammation regulation, cardiovascular health, and metabolic health. For the general athletic population, maintaining sufficiency is more of a “long-term health investment” than a stimulant that lets you ride five more kilometers tomorrow.

I often use this analogy with athletes: Vitamin D isn’t a turbocharger that lets you set a new PB. It’s more like engine oil. If the oil is low, the engine still runs—but it wears out faster and is more prone to problems. Getting it right is about being able to train consistently for the long haul.

5. Overall Positioning for Athletes

Putting the four pieces above together, here’s how I position vitamin D’s role for athletes: it’s a “defensive” nutrient, not an “offensive” one. In other words, getting it to sufficient levels mainly helps you protect the baseline of your bones, immunity, and recovery, so you don’t suffer repeated injuries, repeated illness, or sluggish recovery because the foundation is crumbling. But it’s unlikely to directly push your power or pace upward the way certain supplements might.

This positioning matters because it avoids two extremes: one is completely ignoring it, thinking “I get sun anyway, so I don’t need to worry,” while the foundation quietly erodes; the other is over-glorifying it, treating it as a panacea and supplementing heavily, taking on unnecessary risk of excess. Many nutrients in sports science work this way—it’s not a binary of effective vs. ineffective. When you’re deficient, correcting it matters a lot; once you’re sufficient, adding more has limited value. Vitamin D is a textbook example.

So whenever an athlete asks me, “Coach, is vitamin D really that important?” my answer is always the same: Yes, it’s important—but in a different way than you might think. It matters in “don’t let yourself become deficient,” not “the more you take, the stronger you get.”

Practical Approach: How to Get Vitamin D Right

Step 1: Test First, Don’t Blindly Supplement

My first words to every athlete are: Get bloodwork done. Don’t just swallow pills based on a hunch. Testing 25(OH)D is easy in Taiwan—it’s included in many health checkups or available as a reasonably priced self-pay item. Knowing your starting point tells you how much to take and for how long.

If you can’t test right now, but you match multiple high-risk criteria (primarily indoor training, heavy sun protection, early-morning/evening training, eating out often, rarely eating deep-sea fish), you can assume you’re “probably insufficient” and start with a conservative dose, then verify when you get the chance.

I also want to flag a common interpretation trap: Don’t let a single number hold you hostage. The same person can get different results at different times, in different seasons, or even from different labs. Rather than obsessing over “am I 28 or 31,” look at the overall trend—are you moving steadily from deficient toward sufficient after supplementing? Interpret it alongside your training recovery, injury frequency, and overall mental state. Numbers are tools, not report cards to be anxious over.

Know Your Food Sources: See How Much You Can Get from Your Plate

Before discussing supplements, I usually ask athletes to audit their diet first. Natural food sources of vitamin D are limited to begin with, and the amounts vary widely. I’ve compiled the common sources into a reference table (values are approximate ranges; actual amounts vary by origin, season, and cooking method—use for comparison only):

Food Source	Vitamin D Content (Relative Level)	Coach’s Notes
Fatty deep-sea fish (salmon, Pacific saury, sardines, mackerel)	High	The most practical natural source for Taiwanese athletes; schedule a few servings per week
Egg yolks	Medium-low	Easy to get, but hard to reach meaningful amounts
Sun-dried shiitake mushrooms, some other mushrooms	Medium-low	One of the few plant sources for vegetarians; sun-dried ones have more than non-sun-dried
Vitamin D-fortified dairy, soy milk, cereals	Depends on fortification level	Read the nutrition label—some are fortified, some aren’t
Regular meat, vegetables, fruit	Very low to nearly none	Don’t count on these for vitamin D

You’ll notice that relying on natural food alone to “fill the gap” in vitamin D is quite difficult, especially for those who eat out. That’s why sun exposure and (when necessary) supplements are so important—diet can usually only serve as the foundation; it can’t single-handedly cover the entire deficit.

Step 2: Three-Pronged Approach—Sun, Diet, Supplements

I break the supplementation strategy into three layers, organized in a table:

Channel	Specific Approach	Taiwan-Specific Reminders
Sunlight	Expose arms and lower legs; get moderate sun	Avoid the harshest midday hours; choose gentler morning sun; don’t get sunburned just for vitamin D—the skin damage from sunburn far outweighs any benefit
Diet	Fatty fish like salmon, Pacific saury, sardines; egg yolks; mushrooms; fortified dairy	If you eat out often, deliberately schedule a few deep-sea fish meals per week; sun-dried shiitake mushrooms are also a good source
Supplements	Choose D3 (cholecalciferol), which has better absorption and utilization; take with a meal containing fat	Available at pharmacies and big-box stores; check the dosage unit (IU); don’t buy an excessively high dose and take it daily

Step 3: How to Determine Dosage

This is the most common question from athletes. I’ve organized the common dosage ranges below, but please remember: These are general references. Actual dosages should be individualized based on your blood values and medical professional advice, especially for those with severe deficiency who need professional evaluation.

Scenario	Common Maintenance/Supplementation Range (Daily Concept)	Notes
General adult maintenance	Lower daily maintenance dose	Goal is “don’t drop”; suitable for those already sufficient
Athletes, suspected insufficiency	Moderate maintenance dose	Accounts for training demands; usually needs a bit more than the general population
Confirmed deficiency (< 20)	Higher supplementation dose, short-term concentrated	Usually requires professional guidance, with a follow-up test after several weeks
After reaching target	Return to maintenance dose	Reaching sufficiency isn’t the finish line; you need a maintenance dose to hold it

I’m deliberately not writing specific IU numbers here, because recommended upper limits vary widely across sources, and everyone’s absorption, body weight, sun exposure, and diet differ. Rather than copying an internet number and swallowing it daily, get your blood tested and have a physician or dietitian set your dose. Especially if you’re considering a higher dose, professional oversight is essential.

Step 4: Pair It with Good Partners

Vitamin D doesn’t work alone. In practice, I remind athletes:

• Calcium needs to keep up: Vitamin D helps absorb calcium, but you can’t cook rice without rice in the pot. Bone health is a combination move of vitamin D plus calcium.
• Take it with fat: Vitamin D is fat-soluble. Taking it with a meal that contains fat gives better absorption than on an empty stomach.
• Magnesium and vitamin K2 are often discussed together: They’re related to vitamin D metabolism and where calcium goes. The evidence here is still developing—don’t treat them as must-take miracle pills—but a balanced diet should cover these minerals anyway.

Step 5: Re-Test and Fine-Tune

Supplementation isn’t “set and forget.” I usually ask athletes to re-test a few weeks to a few months after adjusting their protocol, to see whether the numbers are moving toward the target range, then decide whether to increase, decrease, or maintain. Your body changes, the seasons change, training volume changes—your vitamin D protocol should adjust dynamically too.

Common Mistakes and Corrections

In all my years of coaching, the mistakes I’ve seen can be grouped into a few categories. Check whether any apply to you:

Mistake 1: “I Get Plenty of Sun, So I Can’t Be Deficient”

This is Ah-Hsiang’s misconception. As discussed, deep skin tone doesn’t equal adequate storage. Sun protection, time of day, and full-body coverage all disconnect “outdoor time” from “actual synthesis.” Correction: Don’t judge by feeling—judge by bloodwork.

Mistake 2: Blindly Taking Very High Doses, More Is Better

Vitamin D is fat-soluble and accumulates in the body. Long-term excess can cause hypercalcemia, with symptoms like nausea, thirst, and frequent urination—and in severe cases, kidney damage. This isn’t meant to scare you; it’s because fat-soluble vitamins are fundamentally different from water-soluble ones (which are excreted when you take too much). Correction: Aim for “reaching the target,” not “maxing out.” High doses require professional monitoring and follow-up testing.

Mistake 3: Supplementing D Only, Ignoring Calcium and Overall Diet

Some athletes think that swallowing vitamin D makes their bones bulletproof, while their diet is chronically low in calcium and energy intake. Correction: Vitamin D is one member of the team, not a lone hero. Getting overall nutrition and calorie intake right is the real foundation.

Mistake 4: Treating Vitamin D as a Performance Enhancer

Some people hear that vitamin D is linked to strength and expect to get instantly stronger after supplementing. As discussed, only those who were genuinely deficient are likely to see improvement after being brought to sufficient levels; those already sufficient typically get no extra benefit from more. Correction: Adjust your expectations. Position it as a health foundation, not a stimulant.

Mistake 5: Testing Once and Never Again

Your numbers might look great in summer, but drop in winter or after training moves indoors. Correction: Treat re-testing as routine maintenance—check at least once or twice a year, especially when seasons or training patterns change significantly.

Mistake 6: Ignoring Underlying Diseases and Medications

Certain digestive, liver, kidney, or metabolic conditions, as well as some medications, can affect vitamin D absorption and metabolism. If you have a chronic condition or are taking medication, discuss supplementation with your physician first. Correction: If you have a medical history or are on medication, your supplementation strategy must be individualized—don’t just grab a dosage off the internet.

Action Recommendations for Different Reader Levels

Everyone starts from a different point. I’ve split the recommendations by group—find where you fit.

For Beginners New to Sports

• Don’t rush to buy supplements. Add “test 25(OH)D once a year” to your health habits.
• Start with the free things in daily life: outside of training, get moderate sun on your arms and lower legs during gentler times of day; schedule a few deep-sea fish meals per week.
• If you do test as insufficient, start with a conservative maintenance dose, take it with a meal containing fat, and re-test after a few months.
• Remember the key point: Building a solid foundation matters more than chasing pretty numbers.

For Intermediate Enthusiasts with Regular Training

• Your training volume has gone up, so vitamin D’s significance for recovery and bones is more pronounced. Add it to your regular bloodwork list alongside iron and calcium.
• Check your “hidden gaps”: Do you train only in early morning/evening? Slather on sunscreen? Train mostly indoors? Rarely eat deep-sea fish? The more items you match, the more proactive you should be.
• Use a “reach the target” mindset for supplementation—get to the sufficient range, don’t chase ultra-high numbers.
• Schedule re-testing into your annual training plan, such as during season changes or major training block transitions.

For Advanced/Competitive Athletes

• You repeatedly endure high training loads. Vitamin D, along with overall energy and nutritional status, is a key piece of injury prevention. If you have recurrent stress fractures or abnormal recovery, make sure vitamin D is part of the workup.
• Supplementation protocols should be developed in collaboration with a sports dietitian or sports medicine physician—individualized and regularly monitored, not copied from internet dosages.
• Don’t treat vitamin D as a performance lifeline—it shores up the foundation so you can handle more training volume, not directly make you faster.

For Those with Chronic Conditions or Special Circumstances

• If you have diabetes, hypertension, heart disease, kidney or liver issues, or are taking multiple medications, discuss any supplementation—including vitamin D—with your physician first. Taiwan’s NHI system makes seeing a doctor convenient; make good use of that resource.
• Supplementation for these groups needs to be more conservative and more individualized; blanket statements about dosage don’t apply.
• Older athletes should also pay attention: as we age, the skin’s vitamin D synthesis efficiency declines and the risk of falls and fractures rises, making vitamin D and bone health well worth adding to your regular watch list.

Another Case Study: The Indoor Cycling Instructor’s Misconception

Besides Ah-Hsiang, let me share a very different case to break another misconception.

There was a senior indoor cycling instructor—let’s call her Hsiao-Mei—who taught many classes a week. She was incredibly fit with impressive leg strength. She was always proud of “working out so much, so I must be very healthy.” But she spent nearly all day indoors at the studio, and her commute was driving into a parking garage. In a full day, she probably got less than ten minutes of sun.

When she came to me for a consultation, her chief complaints were “unexplained fatigue” and “particularly noticeable mood swings and soreness around her menstrual cycle.” I asked her to get a full blood panel. Her 25(OH)D came back at 16 ng/mL—even lower than the deeply tanned Ah-Hsiang. She was stunned: “I work out so much. How is this possible?”

I told her: Training volume and sun exposure are two different things. No matter how intense your training, if you barely see daylight, your skin can’t synthesize vitamin D. Indoor athletes are actually one of the high-risk groups I worry about most—their fitness is so good that it lulls you into complacency, yet they’re precisely the ones getting the least sun.

The subsequent approach was similar to Ah-Hsiang’s: physician evaluation first, individualized supplementation, dietary adjustments, and squeezing some gentle sunlight into daily life (like walking part of the commute to get some sun). I want to honestly reiterate: Everyone’s body is different. These approaches aren’t a universal prescription—you must adjust based on your own testing and professional advice. The point of Hsiao-Mei’s story isn’t “how much to supplement”—it’s this: Don’t infer your vitamin D status from your training volume. The two have no necessary connection.

Coach’s Most Frequently Asked Questions (FAQ)

These are the questions I get asked to death when coaching athletes. Here they are, all in one place.

Q1: Can I rely solely on sun exposure and skip supplements?

It’s possible, but not easy given modern Taiwanese lifestyles. If you can regularly get sun at appropriate times, with enough skin exposed, and without overdoing sun protection, you might maintain adequate levels through sunlight alone. But the reality is—most athletes get severely insufficient sun exposure because of sun protection, avoiding heat, and training schedules. Sun exposure is great if you can manage it, but don’t confuse “theoretically possible” with “I can definitely do it.” A blood test will tell you whether your approach is working.

Q2: Does more vitamin D mean faster recovery and fewer injuries?

No. As I’ve said repeatedly: Getting to “sufficient” is enough. Excess has no extra benefit and carries potential risk. For those already sufficient, adding more typically won’t speed up recovery or reduce injuries. Set your goal at “reaching the target,” not “maxing out.”

Q3: Do I need to test “active vitamin D” (the 1,25 form)?

General assessment uses 25(OH)D (reflecting storage), not the active 1,25 form. Testing the active form is typically reserved for specific clinical situations, determined by a physician. For health checkups or self-pay testing, just look for the 25(OH)D item.

Q4: Is the vitamin D in a multivitamin enough?

It depends on the dose. The vitamin D content in some multivitamins is quite low and may not be enough to raise levels in someone who’s already deficient. Don’t assume “taking something means it’s enough”—use follow-up testing to confirm actual effectiveness.

Q5: What about vegetarians?

Vegetarians have even more limited natural vitamin D sources (sun-dried mushrooms, some fortified foods). There are also vitamin D3 supplements derived from lichen, or vitamin D2 options, available on the market. Vegetarians need to be even more proactive and consult a dietitian when necessary, because the dietary gap is harder to fill with natural foods.

Q6: If my levels come back low, does that mean something is seriously wrong with me?

In most cases, low levels reflect lifestyle factors (insufficient sun exposure, dietary gaps), not some serious disease. But if you’ve been supplementing for a while and your levels just won’t budge, or if other abnormal symptoms accompany it, then it’s worth having a physician evaluate whether there’s an absorption or metabolism issue. Don’t scare yourself, and don’t self-diagnose—leave it to the professionals.

A Practical Self-Checklist

Here’s a “risk self-check” I often give athletes. The more boxes you tick, the more strongly I recommend getting tested:

• [ ] I train almost exclusively in early morning or evening, avoiding midday
• [ ] I cover up fully when exercising (arm warmers, leg warmers, head buffs, etc.)
• [ ] I take sun protection seriously and apply high-SPF sunscreen liberally
• [ ] Most of my training is indoors (spinning, gym, treadmill)
• [ ] I eat out often and rarely eat deep-sea fish, egg yolks, or mushrooms
• [ ] I’ve had a stress fracture, or my recovery has always been slow
• [ ] My skin tone is on the darker side
• [ ] I haven’t tested 25(OH)D in over a year

This checklist isn’t meant to scare you—it’s meant to turn “feelings” into “action.” Ah-Hsiang ticked at least five or six items back then. No wonder he was deficient despite being so tan.

What Happened to Ah-Hsiang

Back to Ah-Hsiang. We didn’t have him mega-dose overnight. Instead, we started with a physician evaluation, set up an individualized supplementation protocol, adjusted his diet (adding deep-sea fish), arranged gentle morning sun exposure, and made sure his calcium was adequate. A few months later, his follow-up test showed 25(OH)D gradually climbing into the sufficient range.

More importantly, his recovery felt better, and he didn’t have those nagging minor injuries in the second half of the season. I’ll be honest: Vitamin D deserves some credit, but so do his overall improvements in nutrition, sleep, and training structure. It’s hard to attribute the credit to any single factor. That’s exactly the core message I want to convey—vitamin D is an important piece of the puzzle, but it’s never the only piece.

Conclusion: Shore Up the Foundation, and You Can Ride for the Long Haul

Vitamin D isn’t a star nutrient. It won’t make your power curve spike overnight, and it won’t suddenly speed up your pace. But it quietly influences your bones, muscles, immunity, and recovery—the kind of thing where “you don’t notice it when you have it, but you know how serious it is when you don’t.”

In sun-drenched yet broadly deficient Taiwan, my advice is simple: Test first, then supplement. Don’t blindly supplement. Do re-test. Get sun, eat the right foods, and use D3 supplements to reach the target when necessary. Shore up this foundation, and you’ll be able to run long and ride far—healthily—in cycling, road running, or whatever sport you love.

If, after reading this, you decide to dig out that dusty health checkup report to look at the 25(OH)D column, or even schedule a blood draw—then this article was worth it. After all, the best training plan is the one that lets you go far, healthily. And getting vitamin D right is a small but essential basic skill on that long road.

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This article is educational content and cannot replace individual diagnosis and treatment advice from a physician, physical therapist, or dietitian. If you have a chronic condition, are taking medication, or are considering higher-dose supplements, please consult a qualified medical professional first for an assessment based on your individual situation.

References

• Prevalence of Abnormal Vitamin D Levels Among Division I NCAA Athletes — https://pmc.ncbi.nlm.nih.gov/articles/PMC4065560/
• Vitamin D status and its determinants in German elite athletes — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12174178/
• Prevalence of Vitamin D Deficiency Among Young Elite Soccer Players Living Above 55 Degrees North Latitude — https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12008557/
• Effects of vitamin D3 supplementation on strength of lower and upper extremities in athletes: an updated systematic review and meta-analysis of RCTs — https://pmc.ncbi.nlm.nih.gov/articles/PMC11163122/
• Effects of vitamin D supplementation on maximal strength and power in athletes: a systematic review and meta-analysis of RCTs — https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2023.1163313/full
• Vitamin D in athletes: focus on physical performance and musculoskeletal injuries — https://pubmed.ncbi.nlm.nih.gov/34315203/

Related Reading

• Vitamin D Supplementation Strategies for Athletes: A Complete Guide to Deficiency Prevalence, Dosing, and Seasonal Adjustments
• Vitamin D for Athletes: Sunlight, Supplements, and Athletic Performance
• Vitamin D and Endurance: The Sunshine Hormone’s Impact on Bone, Muscle, and Immunity
• Prevalence of Vitamin D Deficiency Among Indoor Trainers in Taiwan and Supplementation Recommendations