A study in mice suggests that having low levels of vitamin D may harm athletes and exercisers by limiting how long they can exercise (Aging, June 2018). Six-month old mice were put on a low vitamin D diet for one year. A six-month-old mouse is equivalent to a 25 year old human and one mouse-year equals 25 human-years. After two weeks, the mice on the low vitamin D diet developed blood levels of vitamin D under 15 ng/ml, (equivalent to low levels in humans), where they remained for the entire study. They did not lose strength, but they lost a lot of their endurance (their ability to sustain exercise). The authors believe that this implies a decrease in their ability to take in and use oxygen. The mice also lost some muscle size. This study may be important for humans because vitamin D deficiency disease is defined as less than 12 ng/ml and vitamin D insufficiency borderline disease is less than 20 ng/ml. More than 35 percent of North Americans have these low levels, particularly in the winter.

Lack of Vitamin D Associated with Muscle and Tendon Injuries
During my competitive running career, I spent more time injured than training because of wintertime muscle injuries in Boston and Washington, D.C. Many years later, I found the probable cause: my wintertime vitamin D3 blood level was below 20 ng/ml (normal for the general population is >20 ng/ml and for athletes may be >30). Since I moved to Florida nine years ago, I have not had even one wear-and-tear sports injury. 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 competitive athletes are vitamin D deficient even if they live in the sunbelt.

Vitamin D deficiency is associated with:
• decreased athletic performance (Molecular Aspects of Medicine, June 2005;26(3):203-219),
• muscle weakness (Scandinavia Journal of Medicine & Science in Sports, Oct 2009), and
• increased risk for athletic injuries (Current Opinion in Clinical Nutrition & Metabolic Care, November 2009; Molecular Aspects of Medicine, December 2008; Arthroscopy, Dec 21, 2017).

Genetics and Vitamin D Deficiency
A report from the University of Toronto showed that genetic factors cause some people to develop signs and symptoms of severe vitamin D deficiency (Clinical Biochemistry, July 2009). People who are genetically at increased risk for vitamin D deficiency are at increased risk for injuries when they try to exercise vigorously in the winter. Lack of vitamin D can weaken muscles (Lancet, Mar 1976;20;1(7960):626-9) and taking that vitamin can correct that weakness (Aging-Clinical and Experimental Research, December 2000;12(6):455–460). Vitamin D acts directly on specific receptors in muscles to make them stronger and to help prevent injuries (Scandinavian Journal of Medicine & Science in Sports, April 2010;20(2):182-90).

How Lack of Vitamin D is More Harmful in Older People
As people age, they become increasingly susceptible to muscle weakness and falls caused by lack of vitamin D (Molecular Aspects of Medicine, June 2005). 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 helps to maintain the function of the fast twitch strength fibers (Calcif Tissue Int, 2013;92:151–162). With aging, you lose muscle fiber; for example, the vastus medialis muscle in the front of the upper leg typically has about 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).

My Recommendations
You probably do not need to take vitamin D pills if your blood level of hydroxy vitamin D is above 30 ng/ml unless you have a condition that your doctor feels puts you at increased risk for the signs and symptoms of a deficiency. You can take up to 1000 IU/day of vitamin D pills if your blood levels are below 30 ng/ml. Most researchers and clinicians now feel that, with few exceptions, high doses of vitamin D are not beneficial and are potentially harmful. People who could possibly benefit from treating low blood levels of hydroxy vitamin D include those who:
• are inactive and do not go outdoors
• suffer from generalized muscle and joint pain
• are athletes with recurrent injuries and decreased performance
• have weak bones (osteoporosis)
• are diabetic, particularly if LDL cholesterol is over 100
• have an auto-immune disease
• are critically ill or debilitated
• suffer from muscle pain from taking statin drugs
I believe that sunlight offers benefits that cannot be obtained just by taking vitamin D pills, but take proper precautions to avoid skin cancer.

Checked 4/20/23