Vitamin D and Endurance Performance: Why Taiwanese Cyclists May Be Deficient Even in Summer

The Truth About Vitamin D Deficiency in Taiwan

A 2024 survey by the National Health Research Institutes found that the prevalence of Vitamin D deficiency (< 30 ng/mL) among Taiwanese adults is 38–42%, with the 25–34 age group being the most severely affected.

It seems contradictory—Taiwan has such abundant sunshine, how could there be a deficiency?

There are four reasons:

1. Excessive sun protection: SPF50 sunscreen blocks 98% of UVB (the key wavelength for Vitamin D synthesis)
2. Indoor lifestyle: Office workers spend less than 30 minutes outdoors per day
3. Air pollution shielding: PM2.5 and cloud cover reduce UVB penetration by 20–30%
4. Skin pigmentation: The efficiency of Vitamin D synthesis in East Asian skin is about 70% of that in Caucasian skin

6 Major Effects of Vitamin D on Endurance Sports

1. Muscle Strength and Power Output

For every 10 ng/mL increase in serum 25(OH)D, isometric strength improves by approximately 5–8%. Athletes with levels below 20 ng/mL have an average FTP 7% lower than the normal group.

2. Bone Density and Stress Fractures

Long-term Vitamin D deficiency leads to decreased bone density. The prevalence of stress fractures among long-distance runners is 2.3 times higher than in the normal group.

3. Immune Function and Infection Rates

Endurance athletes with serum Vitamin D < 30 ng/mL experience upper respiratory tract infections 1.6 times more frequently.

4. Recovery Speed

Vitamin D is involved in muscle fiber repair and inflammatory regulation. After 8 weeks of supplementation, DOMS scores decreased by 30%.

5. Oxygen Utilization Efficiency

Small-scale studies show a positive correlation between VO2max and serum Vitamin D (r = 0.42).

6. Mood and Training Motivation

Vitamin D deficiency is positively correlated with mild depression and training burnout.

When to Get Tested

Recommended testing times:

• Late winter to early spring (February–March): After a winter of low sun exposure, this is the annual low point
• During training burnout: Unexplained fatigue, frequent colds
• Changes in menstrual cycle in women: Related to bone density
• Once a year for those over 40: Absorption efficiency declines with age

Test item: Serum 25(OH)D

Reference values:

• < 20 ng/mL: Deficient
• 20–29 ng/mL: Insufficient
• 30–50 ng/mL: Sufficient (40–60 ng/mL recommended for athletes)

• 80 ng/mL: Potentially excessive

Cost: Approximately NT$800–1,200 out of pocket; some health checkup packages already include it.

Supplementation Dosage Recommendations

Natural Approach: The Sun Exposure Formula

Recommendation: 3–4 times per week, 10–15 minutes each session, with arms + legs exposed, ideally between 10–11 AM or 3–4 PM.

Notes:

• Ineffective through glass (95% of UVB is blocked)
• Thick layers of sunscreen block synthesis
• Arm warmers and gloves while cycling block exposure

Dietary Sources

Food	Vitamin D Content
Salmon 100g	526 IU
Sardines 100g	480 IU
Mackerel 100g	360 IU
Egg yolk, 1 piece	40 IU
Shiitake mushrooms (sun-dried) 100g	446 IU
Fortified milk 250ml	100 IU

Recommended daily intake: 600–800 IU for adults, 1,000–2,000 IU for athletes.

Supplements

• Preventive supplementation: D3 1,000 IU/day (suitable for winter or those with insufficient sun exposure)
• Correcting insufficiency: D3 2,000–4,000 IU/day, retest after 3 months of supplementation
• Severe deficiency: D3 50,000 IU/week for 8 consecutive weeks (requires a doctor’s prescription)

Choose D3 (cholecalciferol) over D2—its absorption rate is 87% higher. It is recommended to take it with fat (e.g., when having eggs at breakfast), which increases absorption by 30%.

Supplementation Strategies for Athletes

Training Period (In-Season)

• 1,000–2,000 IU D3 daily
• At least 2 outdoor rides per week (for sun exposure)
• Include salmon or sardines in the diet twice a week

Taper Period (Pre-Season)

• Maintain 1,000 IU daily
• Ensure serum levels > 40 ng/mL

Post-Race Recovery

• May temporarily increase to 3,000 IU/day to support muscle repair
• Return to maintenance dosage after 2 weeks

Risk of Excess

Serum levels > 150 ng/mL may trigger hypercalcemia, with symptoms including:

• Thirst, frequent urination, nausea
• Arrhythmia
• Kidney stones

Safe upper limit: 4,000 IU per day; exceeding this requires medical supervision.

Conclusion

Vitamin D is the most easily overlooked nutrient for Taiwanese cyclists, especially for those who train indoors long-term or overuse sun protection. Getting a blood test once a year in February–March, getting moderate daily sun exposure, plus 1,000 IU of D3 supplementation, will make you more consistent in power, recovery, and immunity. When your training plan is dialed in, don’t let a single micronutrient drag down your performance.

Related Reading

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