Because of injuries in the springtime, I missed six Boston Marathons back in the 1960s. It wasn't until 40 years later that I found the cause: my vitamin D3 blood level was 20 nmol/l (normal is greater than 75 nmol/L, equal to 30 ng/ml). Recently I moved to Florida and have been relatively injury free for the first time in my life. I now know that people genetically susceptible to vitamin D deficiency are the ones most likely to suffer muscle weakness, injuries and poor athletic performance. Many exercisers and even competitive athletes are vitamin D deficient even if they live in the sunbelt. I believe that sunlight offers benefits that cannot be obtained just by taking vitamin D pills.

Vitamin D acts directly on specific receptors in muscles to make them stronger and prevent injury (Scandinavian Journal of Medicine & Science in Sports, April 2010). As people age, they become increasingly susceptible to muscle weakness and falls caused by lack of vitamin D. Muscles are made of thousands of individual fibers that are classified into two types: slow twitch fibers that govern endurance, and fast twitch fibers that govern primarily strength and speed. Vitamin D specifically maintains the function of the fast twitch strength fibers. A review of the world's literature showed that lack of vitamin D is associated with muscle weakness in older people (Molecular Aspects of Medicine, June 2005). With aging, you lose muscle fibers. For example, the vastus medialis muscle in the front of the upper leg has 800,000 fibers in a 20 year old, but only 250,000 in a 60 year old. Vitamin D slows this loss of muscle fibers, preserves muscle strength and helps to prevent falls, while lack of vitamin D increases loss of fibers, muscle weakness and falls (Pediatric Clinics of North America, June 2010).

If you suffer muscle weakness, pain or injuries:
· Check your vitamin D3 level. That is the only available dependable test. If it is below 75 nmol/L (30 ng/ml), you are deficient.
· You can try taking vitamin D3 at a dose of at least 2000 IU/day for a month.
· If that does not bring your D3 level to normal, you can check with your doctor about taking higher doses.
· A certain percentage of people will have their vitamin D3 levels go above a normal 75 nmol/L and still suffer from muscle weakness, fatigue, pain and injuries.
· These people may benefit from exposure to sunlight.
· Since skin cancer is caused by cumulative exposure to sunlight over a lifetime, you should restrict exposure to sunlight on your head, face, top of ears, arms and hands.
· Try exposing your legs and bathing trunk areas. Be careful to avoid sunburn.
· Start at low exposures of less than a couple of minutes and work up gradually. You cannot tell that you have suffered a sunburn until the next day when your skin will burn, itch and perhaps blister.