'Fine' and 'optimal' are very different things
The standard sufficient threshold for vitamin D — 25-hydroxyvitamin D, or 25(OH)D — is typically around 50 nmol/L. Below this, you're officially deficient. Above this, you're officially fine. But 'fine' and 'optimal' are very different things. Research on vitamin D and athletic performance consistently shows benefits extending well beyond the deficiency threshold. Studies have linked vitamin D levels above 100 nmol/L with improved muscle protein synthesis, reduced injury rates, better immune function, lower inflammation markers, improved testosterone levels, and better mood and cognitive function. The optimal range for active individuals, based on the weight of current evidence, is approximately 100–150 nmol/L. At 55 nmol/L, your standard report says you're fine. Your body disagrees.
Why athletes are particularly at risk
Indoor training is the biggest factor. An athlete who spends most of their time in a gym, office, or home has minimal UVB exposure — and UVB is the mechanism by which the skin produces vitamin D. Even athletes who train outside often do so early morning or late evening, when UVB intensity is insufficient for meaningful vitamin D synthesis. Sunscreen blocks UVB synthesis almost entirely. Darker skin tones require longer sun exposure to produce the same amount of vitamin D. Athletes in northern latitudes get minimal meaningful UVB from October to April regardless of time outdoors.
Correction is simple
Most people with levels below 75 nmol/L respond well to 3,000–5,000 IU of vitamin D3 daily, taken with a fat-containing meal. Vitamin K2 (100–200mcg) taken alongside D3 helps direct calcium to bones rather than soft tissue. Retest after 12 weeks to confirm you've reached the target range — individual response varies significantly. Don't guess. Test, correct, retest.