Most people on this planet have too little 25-hydroxy vitamin D in their blood, and as a result their muscle quality is below optimum. Radiologists at the University of Southern California in Los Angeles discovered that the muscles of young women contain up to 20 percent more fat if their vitamin D levels are too low.
Countless people have less vitamin D in their blood than is good for them. Initially researchers recorded this vitamin deficiency in people living in northern countries, where the sun hardly shines, and where much of the population works indoors. Well, that used to be the story.
More recently research has shown that, even in sun-drenched countries like Turkey, Lebanon, the United Arab Emirates, India and Australia, 30-50 percent of the population is low on vitamin D. [N Engl J Med. 2007 Jul 19; 357(3): 266-81.] In Brazil the figure is 60 percent among young adults. [Ann Nutr Metab. 2009; 54(1): 15-21.]
Sufficient vitamin D is necessary for strong muscles. One theory is that vitamin D is responsible for getting nutrients like calcium and phosphorus into the muscles. As a result muscles can contract better, and growth and fuel burning processes are also enhanced.
Animal studies have shown that rats put on a fattening diet develop more muscle and less fat when given extra vitamin D. In human studies, test subjects burn more calories after eating a breakfast that contains 0.5 g calcium and 9 mcg vitamin D. Elderly people who take vitamin D supplements are physically stronger, as are athletes who have high levels of vitamin D in their blood.
The researchers in this study made scans of the muscle and body composition of 90 young women aged between 16 and 22. Of these, 37 had sufficient vitamin D in their blood and 53 had too little. The more vitamin D the women had in their blood, the less fat they had in their muscles. Fatty muscle tissue is less strong and less sensitive to insulin. By the way, none of the women complained of having weak muscles.
Low levels of vitamin D in the blood were also associated with more subcutaneous fat [SF] and more abdominal [or visceral] fat [VF] Women with high levels of vitamin D also had more leg muscle, although the difference was not statistically significant.
Vitamin D is not the most exciting nutritional supplement in the world. Pills containing vitamin D don't sound as interesting as exotic plant extracts or high-dose amino acids. But it looks as though lots of athletes could be that little bit stronger, muscular and slimmer if they increased their vitamin D intake.
J Clin Endocrinol Metab. 2010 Apr; 95(4): 1595-601.