Vitamin D for fighters: four small trials found wins on some lab tests, no trial looked at a single bout

Vitamin D has a good reputation in gyms. It matters for bone and muscle, athletes who train indoors or through northern winters often run low, and the step from "low vitamin D is bad" to "more vitamin D makes you stronger" is an easy one to take. A team at the University of Nicosia in Cyprus wanted to see how well that step holds up for combat sports, where the job is repeated bursts of power, recovery in between, and technique under fatigue.

They set a strict bar. Only trials that randomly gave athletes oral vitamin D or a placebo (or nothing) at the same time counted, and only objective performance tests. That ruled out studies where both groups got vitamin D, ones comparing two doses, and one that gave everyone a placebo first and the supplement after. From 142 database records and extra searches through 1 September 2026, four trials were left: 108 athletes randomised, 99 analysed. National-level judoka got one 150,000 IU dose and were tested 8 days later. Collegiate taekwondo athletes, all low in vitamin D at the start, took 5,000 IU a day for four winter weeks. And two trials from one group of researchers gave recreational and sub-elite fighters from mixed martial arts, Brazilian jiu-jitsu, boxing and karate 50,000 IU a week for six weeks and two weeks, with an olive-oil placebo and the same training for both groups.

Each trial found something. The judoka on vitamin D gained 13% in average muscle strength on an isokinetic machine against about 3% on placebo. The taekwondo group improved more in Wingate peak power and fast knee-extension torque. In the six-week trial, lower-body peak oxygen uptake rose 8.2% on vitamin D and fell 0.7% on placebo. In the two-week trial, handgrip in both hands, a held press-up and best repeated-sprint power favoured vitamin D.

The review's point is everything sitting next to those results. In the same trials, Wingate mean power, slower knee torque, jump height, sit-ups, agility, the 20 m shuttle run, upper-body oxygen uptake, time to exhaustion and most repeated-sprint measures showed no difference between the groups. The judo groups didn't start level: the vitamin D group's blood level was lower at baseline (13.2 against 16.3 ng/mL). The two Marley trials never measured blood vitamin D, so nobody knows who started low or whether the capsules were taken, and the authors couldn't rule out the same athletes appearing in both. Only one trial included women. Confidence intervals for the group differences were rarely reported. On the GRADE scale every outcome came out at very low certainty, and not one trial tested fight performance.

The authors' conclusion is a careful split. Finding and correcting low vitamin D is worth doing for health, especially in athletes who train indoors or get little sun. Taking it in the hope of punching harder isn't supported yet. They also flag the doses: 50,000 IU a week works out to about 7,140 IU a day, above the European Food Safety Authority's upper limit of 4,000 IU a day for adults, and they say such protocols shouldn't be copied without professional supervision. A blood test first, then a dose based on it, is the order they suggest.