A UK-based study team recruited a dozen people clinically diagnosed with severely low D levels. For two weeks, each person received a 20,000-IU megadose of vitamin D3 (a form of vitamin D) every other day. The researchers recorded each participant’s muscle function and general fatigue levels at the start and end of the study period. Here’s what they discovered: After the super shot of D, two different measures of cellular muscle function improved by roughly 20%. Every one of the study participants also reported feeling less fatigued at the end of the two weeks, says study co-author Akash Sinha, MBBS, a clinical research fellow at Newcastle University. For your muscles to work properly—during both aerobic exercise and resistance training—they require an energy-rich molecule called ATP, which the mitochondria (cellular powerhouses) in your muscle fibers constantly replenish as you exercise, Sinha says. Some past research has indicated too little vitamin D may limit the mitochondria’s ability to hold onto the neurotransmitters than stimulate ATP replenishment, which may explain the vitamin D-muscle fatigue connection. How do you know if you’re vitamin D deficient? Apart from muscle fatigue or pain, cramps, sore bones, and feeling worn out are all signs of low D, Sinha says. High BMI and dark skin also up your risk for low D, according to the NIH. And a Norwegian study found D production also drops at latitudes north of 40 degrees (think Boston or Minneapolis) because of the angle at which the sun hits the Earth during certain periods of the year. Sinha’s advice? Women should aim for 600 to 800 IUs of vitamin D a day, especially those older than 70, who tend to be low on D. You could get sufficient D from sunlight and diet alone—salmon and swordfish are especially loaded with the vitamin. But during those months when sun-supplied D is scarce, most people don’t eat the type of seafood-heavy diets that supply adequate vitamin D, Sinha says. Look for supplements containing vitamin D3, or animal-sourced vitamin D, Sinha advises. It’s up to five times more potent than plant-sourced vitamin D2, explains James Dowd, MD, CEO of the Arthritis Institute of Michigan and author of The Vitamin D Cure. For more on getting the right amount—and not too much—check out How To Get Enough Vitamin D.
