Vitamin D is one of the few nutrients your body can produce on its own, provided your skin gets sufficient exposure to sunlight. For anyone who trains indoors for most of the year, often after sunset, that condition is rarely met.
This article explains what vitamin D does in the body, why combat athletes face a higher risk of deficiency, and what the consequences can be for strength, recovery and immune function.
What is vitamin D? A vitamin that functions like a hormone
Vitamin D is classified as a fat-soluble vitamin, but it functions differently from most nutrients in the body. After absorption, vitamin D is converted into calcitriol, its active form, which behaves as a steroid hormone. Where a standard nutrient serves as a building block, a hormone acts as a messenger that instructs cells.
Vitamin D receptors are present in virtually every tissue in the body, which explains why it is involved in such a wide range of processes.
The recognised functions of vitamin D include:
- Contributing to the maintenance of normal bones
- Contributing to normal muscle function
- Contributing to the normal absorption and utilisation of calcium and phosphorus
- Contributing to the normal function of the immune system
- Playing a role in the process of cell division
Because vitamin D acts on so many systems at once, a deficiency rarely presents as a single isolated complaint.
Why combat athletes face a higher risk
Three factors apply specifically to combat athletes.
Training environment and timing
BJJ, MMA, boxing, wrestling, judo and kickboxing are almost exclusively practised indoors. For most athletes, training also takes place in the evening, after the working day.
At Belgian latitudes, UVB radiation from October through March is too weak to produce meaningful amounts of vitamin D in the skin. Throughout those six months, the contribution of sunlight is negligible regardless of how much time someone spends outdoors. During the summer months, when radiation is strong enough, training hours typically coincide with the exact window in which that exposure would need to occur.
Increased load on the skeletal system
Vitamin D contributes to the maintenance of normal bones and to the normal absorption of calcium. Research links deficiency to an increased risk of stress fractures, in both military and athletic populations.
Combat sports involve structurally higher impact loading than most other disciplines. Falls, throws, strikes and repeated joint compression form part of regular training rather than competition alone. The skeletal component of vitamin D therefore carries more weight for this group.
Sustained pressure on the immune system
Vitamin D contributes to the normal function of the immune system. In combat athletes, that system already operates under elevated pressure from high training volumes and from extensive skin contact on the mat.
That pressure increases further during a weight cut, when energy intake and micronutrient intake decline simultaneously. This period coincides with the final weeks before competition, precisely when illness has the greatest impact on preparation.
What research shows about vitamin D deficiency in athletes
Research in athletic populations associates vitamin D deficiency with reduced muscle strength, lower aerobic capacity and anaerobic power, an increased risk of stress fractures and bone injury, and impaired immune function. Biochemical comparisons between supplemented and non-supplemented athletes have also identified a possible association between serum 25(OH)D levels and total testosterone.
In one controlled study, athletes supplemented daily with vitamin D3 for twelve weeks showed significant improvements in squat, deadlift and vertical jump performance compared with the control group. Fat-free body mass increased by 13.6 percent over the same period, while remaining unchanged in the control group.
One nuance matters when interpreting results of this kind. The observed improvements occur primarily in participants who began from a deficient state. Supplementation in individuals with already optimal status generally offers no additional benefit. The effect lies in correcting a limitation rather than raising capacity beyond it.
Practical considerations for supplementation
Have your levels measured. A 25(OH)D blood test through your GP establishes your starting position. Without that measurement, supplementation is an estimate.
Take vitamin D with a meal containing fat. As a fat-soluble vitamin, absorption is considerably lower when taken on an empty stomach.
Combine with vitamin K2. Vitamin D increases calcium absorption from the diet, while vitamin K2 plays a role in directing that calcium toward bone tissue. For this reason, the two are frequently dosed together.
Choose cholecalciferol over ergocalciferol. Vitamin D3 is the same form the skin produces under UVB exposure, and raises serum levels more effectively than the D2 form.
Vitamin D in SHIDŌ Foundation
SHIDŌ Foundation provides 75 µg of vitamin D3 as cholecalciferol per daily dose of three capsules, combined with 90 µg of vitamin K2 as menaquinone-7. The dosage reflects the increased prevalence of suboptimal levels among athletes who train predominantly indoors at northern latitudes.
The complete composition, including the form and dose of every ingredient, is listed on the label.
