Vitamin D is essential for bone health and serves other important functions in the body, including modulation of cell growth, nerve and muscle function, and immune-system regulation. Vitamin D is necessary for effective absorption of calcium by the gastrointestinal tract. Without vitamin D, only about 10-15 percent of calcium in the diet is absorbed. Vitamin D helps to maintain a healthy calcium concentration in the blood and influences the function of the cells responsible for bone remodeling. Vitamin D deficiency in children causes rickets, in which the bones become soft and bent. Inadequate vitamin D can cause osteomalacia (soft bones) in young adults and greatly increases risk of osteoporosis and broken bones in middle-age and older adults.
The source of vitamin D is unique. Vitamin D is produced in the skin when it is exposed to sunlight. Most people get at least some vitamin D this way, but today’s appropriate emphasis on using sunscreens and avoiding sun exposure has the side effect of limiting vitamin D availability for many people. Vitamin D is found naturally in very few foods (primarily fish like salmon and mackerel). The main source of dietary vitamin D is from foods that are fortified. Almost all milk in the U.S. is fortified with vitamin D , and vitamin D is also sometimes added to breakfast cereals, orange juice and yogurt.
Because vitamin D from sunlight and diet is limited and variable, the level of vitamin D in the blood is the most reliable measure of adequacy. A Vitamin D test can be ordered for people who are at risk, such as those who have had a broken bone, who have medical conditions or who take medications that can cause bone fragility. Vitamin D deficiency is quite common, especially among older adults, those with limited sun exposure and those who have intestinal malabsorption or history of gastric bypass.
Vitamin D supplementation is important to protect bone strength and prevent fractures. The amount required is controversial, as too much vitamin D can be toxic. The Endocrine Society recommends that all adults age 50–70 require 600 IU (international units) of vitamin D daily, and adults over 70 require 800 IU to maximize bone health and muscle function. Of course, more may be needed to correct deficiency. The goal of treatment is to bring blood levels of vitamin D to the desirable level, so a blood test for vitamin D (called 25-hydroxyvitamin D) is important.
Two excellent websites on the role of Vitamin D in bone health are:
- Office of Dietary Supplements at the National Institutes of Health
- FamilyDoctor.org (American Academy of Family Physicians)