Prevalence of Vitamin D Deficiency Among Indoor Cyclists in Taiwan and Supplementation Recommendations

This article is based on peer-reviewed research from international sports science journals, providing an in-depth analysis of the effects of “Vitamin D” on athletic performance, and integrating Taiwan’s local cycling and race scenarios to offer actionable training nutrition recommendations.

Vitamin D is not only crucial for bone health, but also affects muscle function, immunity, and performance. Although Taiwan enjoys abundant sunshine, indoor training and sun-protection culture have led to a surprisingly high prevalence of deficiency.

In Taiwan’s endurance sports community—whether it’s climbing enthusiasts tackling the Wuling ascent from the west, long-distance riders crossing the East Rift Valley from the east, or participants in Sun Moon Lake laps, Taroko Marathon, or 226 km Ironman triathlons—the topic of “Vitamin D” matters because it directly determines whether you can maintain your pace in the latter stages of a race, avoid cramps and hitting the wall, and recover effectively between consecutive training days. Many amateur athletes pour all their effort into power training and equipment upgrades while overlooking this “free margin for improvement” in nutrition. In fact, when training volume and equipment are comparable, the quality of nutritional strategy is often the key variable distinguishing finishing from dropping out, and personal bests from collapse. This article will walk you through the complete context—from cellular molecular mechanisms and randomized controlled trial evidence to dose-response curves and practical applications—dispelling long-standing myths so that your fueling strategy is truly built on science.

Academic Research Review

Regarding the scientific exploration of “Vitamin D,” top international journals have accumulated rich and rigorous evidence. Below are several representative studies selected for their value in methodological design, sample populations, and strength of conclusions, collectively forming our current understanding of this topic:

1. Owens DJ et al. (2018, Sports Medicine) reviewed the effects of vitamin D on athletic performance and muscle.

2. Close GL et al. (2013, J Sports Sci) showed that vitamin D supplementation improved muscle strength in deficient athletes.

3. Larson-Meyer DE and Willis KS (2010, Curr Sports Med Rep) reviewed vitamin D status in athletes.

4. Owens DJ et al. (2015, AJP-Endo) explored vitamin D and muscle repair.

Taken together, these studies reveal that the scientific picture of “Vitamin D” is not a single fixed conclusion, but rather one that is continually refined and deepened as research methods advance. Early studies predominantly used laboratory-controlled time trials or exhaustion tests; subsequent research gradually incorporated stable isotope tracing, muscle biopsies, functional magnetic resonance imaging (fMRI), and molecular biomarkers, allowing us to move from “observing phenomena” to “explaining mechanisms.” Notably, most high-quality studies employed randomized crossover designs, where each subject serves as both experimental and control, greatly reducing noise from individual differences. However, extrapolating research conclusions requires caution: the responses of well-trained laboratory subjects may not fully apply to general amateur athletes, and the effects of a single acute intervention may not equate to long-term chronic adaptation. When reading the “effect sizes” and “statistical significance” of these studies, we must also distinguish between “statistically significant” and “practically meaningful”—a 1% improvement might decide medal placement in elite competition, but its significance is relatively limited for recreational riders.

Core Mechanisms

Vitamin D is synthesized in the skin via ultraviolet exposure; once activated, it acts on nuclear receptors in muscle cells, regulating calcium metabolism, muscle fiber function, protein synthesis, and mitochondrial efficiency. Deficiency leads to decreased muscle strength, increased fatigue, and impaired immunity. In Taiwan, indoor trainers, nighttime exercisers, and those who consistently use sun protection often have serum 25(OH)D levels below 50 nmol/L.

To truly understand how “Vitamin D” affects athletic performance, we must return to physiology at the cellular and systemic levels. Athletic performance is the result of multi-system coordination: the cardiovascular system handles oxygen and fuel delivery, muscle cells manage energy conversion and mechanical contraction, the central nervous system regulates motor unit recruitment and fatigue perception, and the gut and liver form the hub of nutrient absorption and metabolism. The reason the aforementioned mechanisms translate into measurable performance differences is precisely because they act on one (or more) critical links in this chain. The table below summarizes the key effects of this topic at different physiological levels, helping you build a complete mechanistic picture:

| Level of Action | Key Mechanisms | Implications for Athletic Performance |

|—|—|—|

| Cellular/Molecular | Affects mitochondrial efficiency, enzyme activity, and signal transduction | Determines the efficiency of energy conversion and the direction of adaptation |

| Muscle Tissue | Regulates substrate utilization, buffering capacity, and contractile function | Influences sustainable power output and the onset of fatigue |

| Systemic Integration | Alters blood flow distribution, thermoregulation, and hormonal environment | Determines stability and safety during prolonged exercise |

| Central Nervous System | Regulates fatigue perception, drive, and motor unit recruitment | Affects “how hard it feels” and the ability to persevere |

Two dimensions deserve particular emphasis: “dose-response” and “temporal dynamics.” The same nutritional intervention, at different dosages and different timing, can produce vastly different—even opposite—effects. This is precisely why many popular recommendations are one-sided. Only by understanding the mechanisms can we judge “when to use it, how much to use, and when to take it,” rather than blindly following trends.

Going further, the limiting factors of athletic performance shift dynamically with exercise intensity and duration: in short, high-intensity bursts, limitations often stem from the accumulation of phosphagen system byproducts and glycolytic metabolites; in multi-hour endurance events, limitations shift toward the combined effects of glycogen depletion, rising core temperature, fluid and electrolyte imbalance, and central fatigue. The reason “Vitamin D” deserves in-depth discussion is precisely because it can specifically influence certain of these limiting factors. This also reminds us that no nutritional strategy can be evaluated in isolation from the “exercise context”—a fueling rhythm suited to a 40-minute criterium may not apply to a 6-hour mountain climb, and vice versa. The more thoroughly you understand the mechanisms, the more flexibly you can adjust across different race formats, rather than rigidly adhering to a single fixed formula. This ability to “adapt to context” is precisely the dividing line between amateur athletes and those who truly understand sports science.

Dose-Response Relationship

In sports nutrition, “the dose determines both the toxicity and the benefit.” Too low a dose fails to reach the physiological threshold and is futile; too high a dose may trigger side effects, gastrointestinal discomfort, or even interfere with training adaptations. The table below summarizes the dose-response correspondence for “Vitamin D” and serves as the most important quantitative reference for developing a personal supplementation plan:

| Dose / Condition | Effect Description |

|—|—|

| 25(OH)D > 75 nmol/L | Ideal for athletes |

| 50–75 | Insufficient |

| < 50 nmol/L | Deficient, supplementation needed |

| 1000–2000 IU/day | Common maintenance dose |

As shown in the table above, benefits often follow an “inverted U-shaped” or “threshold-plateau” curve: effects increase with dose until the effective threshold is reached, but beyond a certain plateau, there are not only no additional benefits—marginal costs (side effects, gastrointestinal burden, financial expense) rise sharply instead. This means that “finding your own optimal dose” matters far more than “taking as much as possible.” It is recommended to progressively test different doses during training (not on race day), recording subjective feelings, gastrointestinal responses, and power data to build your own dose profile. Remember: the laboratory average is a starting point, not the endpoint; each individual’s body weight, metabolic rate, gut tolerance, and genetic background will cause the optimal dose to shift in a personalized way.

Differences Across Populations

The benefits of vitamin D are not equal for everyone. Age, sex, training status, body size, and genetic background all significantly modulate individual responses. Applying a one-size-fits-all recommendation while ignoring these differences is one of the most common mistakes in sports nutrition.

| Population Factor | Response Characteristics | Practical Recommendations |

|—|—|—|

| Beginners vs. Advanced Athletes | Advanced athletes have more mature physiological adaptations, with responses often more stable but with smaller marginal gains | Beginners should start conservatively with low doses to build tolerance first |

| Men vs. Women | Differences in body weight, hormonal cycles, and sweat composition affect dosage and requirements | Women should individualize based on body weight and pay attention to iron and energy availability |

| Young vs. Older Athletes | Older individuals often experience reduced absorption efficiency and anabolic resistance | Older individuals may require higher doses or better timing |

| Body Size Differences | Body weight directly affects the absolute amount calculated per mg/kg or g/kg | Always convert to a dose corresponding to individual body weight rather than copying general guidelines |

When interpreting “individual differences,” one must also be wary of a common statistical pitfall: studies mostly report “group average responses,” but beneath the average often lies enormous individual variability. In the same intervention, some may be strong responders, some non-responders, and some even negative responders. This is why even when a study shows “average effectiveness,” you still need to confirm through your own experimentation which category you fall into. The recommended approach is to conduct personalized A/B testing: across two training sessions with conditions as similar as possible, apply and withhold the strategy respectively, compare power, heart rate, and subjective feelings, and repeat several times before drawing conclusions. This empirical spirit of “using yourself as the sample” is the essential path from translating group science into personal prescriptions.

Taking Taiwan’s common amateur endurance population as an example, many are middle-aged riders over 35 who train in their spare time around work. This group simultaneously faces declining recovery rates, insufficient sleep, and time pressure, so the “return on investment” of nutritional strategies is often higher than for young elites—meaning that correct nutritional interventions can yield relatively greater room for improvement. Female athletes, in particular, need to pay close attention to the effects of the menstrual cycle on metabolism and requirements, as well as whether energy availability is sufficient, to avoid falling into the low energy availability (LEA) trap while pursuing lighter body weight. After understanding population differences, you will realize: truly professional nutritional advice is always an individualized prescription that varies from person to person, not a one-size-fits-all slogan.

Practical Training Application

Theory must ultimately be implemented into training plans and race day. Below is a practical framework for translating vitamin D into concrete training and competition operations:

• Pre-race testing principle: All nutritional strategies must first be rehearsed in training; “never try anything new on race day” is an iron rule. Gastrointestinal tolerance to new fueling products takes time to build.

• Periodization mindset: Align nutritional strategies with training cycles—the base phase can emphasize adaptation-oriented strategies, while the pre-season shifts to performance-oriented fueling optimization.

• Progressive introduction: Start with low doses and low frequency, adjust gradually based on bodily responses, and build a personalized dosage and timing profile.

• Data tracking: Combine records from power meters, heart rate, subjective fatigue (RPE), and gastrointestinal comfort to objectively evaluate whether an intervention is truly effective.

• Holistic context: Nutrition is one component of training, sleep, recovery, and psychology; no single supplement can compensate for sleep deprivation or flawed training design.

Using a one-week training schedule as an example, it is recommended to rehearse different fueling scenarios during key midweek high-intensity sessions (such as threshold intervals or repeated climbs) and weekend long-distance rides: high-intensity days focus on rapid energy supply and central nervous system activation, while long-distance days focus on sustained energy delivery, gastrointestinal tolerance, and recovery. Through repeated rehearsal, your body can execute the optimal fueling rhythm “automatically” on race day, leaving mental resources for pacing and tactical decisions. Remember, the goal of a nutritional strategy is not to pursue theoretical perfection, but to remain stable and reliable under the fatigue, heat, and pressure of a real race course.

The most common mistake many people make when executing a nutrition plan is “being serious only on race day while eating carelessly during regular training.” This is precisely putting the cart before the horse: regular training is the ideal laboratory for building gut tolerance, testing dosages, and cultivating fueling rhythm. If you want to execute 80 grams of carbohydrates per hour on race day, you must rehearse it repeatedly in training until your body adapts; if you want to rely on a certain supplement, you must confirm in training that it is genuinely effective for you and free of side effects. It is recommended to integrate a nutrition log with your training log, recording fueling content, intake timing, gastrointestinal responses, and performance data for each key session. After weeks to months of accumulation, the value of this personalized database will far exceed any generic nutrition guide. Additionally, do not overlook the often-underestimated aspect of “post-training recovery fueling”—the quality of recovery between consecutive training days often determines whether you can steadily accumulate training volume without injury, and training volume is the most fundamental engine of long-term progress. Treat nutrition as a serious part of training rather than a last-minute accessory before race day, and your improvement curve will be noticeably different.

Local Application in Taiwan

Taiwan’s unique climate, terrain, and race culture add localized considerations to the application of vitamin D. Taiwan’s summer is hot and humid, with perceived temperatures often exceeding 35°C, and sweat rates and fluid/electrolyte losses far exceeding the research contexts of temperate countries. This means that hydration and fueling recommendations from foreign literature often need to be “revised upward.” In events like the Westbound Wuling climb, which ascends from sea level to 3,275 meters, the appetite suppression of high altitude, low temperatures, and prolonged exercise pose a severe test for energy planning.

Regarding local fueling options, Taiwan’s abundant bananas, sweet potatoes, pineapples, sports drinks, and convenience store ready-to-eat foods can all be incorporated into fueling strategies; the extremely high density of convenience stores also makes mid-ride refueling on long-distance rides relatively easy. It is recommended that Taiwanese riders planning classic routes such as Sun Moon Lake, Wuling, Beiyi, Buyan Ting, and the East Coast Crossing inventory fueling points along the way in advance, and strengthen sodium and fluid intake in response to Taiwan’s humid and hot environment. Athletes in the Taroko Marathon, Taipei Marathon, and triathlon events across the island should likewise incorporate the aforementioned local climate factors into their individualized nutrition plans to perform at their best under subtropical conditions.

Debunking Common Myths

Myth: The myth is that “Taiwan has abundant sunshine, so vitamin D deficiency is unlikely.” In fact, sunscreen use, indoor training, and nighttime exercise make deficiency quite common; testing and supplementation are recommended.

Such myths spread widely often because they “sound reasonable,” are easily passed by word of mouth, or are amplified by marketing rhetoric. Yet the value of science lies precisely in testing intuition with rigorous evidence: many seemingly obvious notions fail to hold up under randomized controlled trials. The field of sports nutrition is especially rife with oversimplified “panacea” marketing that compresses complex dosages, timing, and individual differences into a single slogan. The next time you hear a categorical nutritional claim, it is worth asking: “What is the level of evidence for this claim? Who is the target population? Are the dosage and timing clearly specified?” Cultivating this evidence-based critical thinking is more valuable than memorizing any single conclusion, and it is a key step for amateur athletes moving toward scientific training.

Conclusion

Vitamin D is a topic in sports nutrition with both theoretical depth and practical value. From the academic evidence reviewed in this article, its benefits are real, but it is by no means an unconditional panacea—the key lies in correct dosage, appropriate timing, individualized adjustment, and synergy with overall training, recovery, and sleep. For endurance sports enthusiasts in Taiwan, while mastering the scientific principles, it is equally important to combine local climate, terrain, and race characteristics to transform general guidelines into personalized prescriptions suited to oneself. May every rider sweating on Wuling, in the rift valley, or on the round-island route break through their limits through science-based nutritional strategies and enjoy the pure joy that sport brings. Before you next step onto the race course, do not forget—your fuel bottle contains not just water and sugar, but an entire validated system of sports science.

Related Reading

• Vitamin D for Athletes: The Relationship Between Sunlight, Supplementation, and Athletic Performance
• Vitamin D and Endurance: The Impact of the Sunshine Hormone on Bone, Muscle, and Immunity
• Vitamin D Supplementation Strategies for Athletes: A Complete Guide to Prevalence of Deficiency, Dosage Management, and Seasonal Adjustment
• An In-Depth Analysis of Vitamin D and Athletic Performance and Health: Practical Notes from a Sports Science Consultant