Vitamin D and calcium are both important for bone health.

While calcium builds bone, vitamin D is needed for the absorption of calcium and phosphorus. Vitamin D also regulates blood levels of calcium to support healthy bone mineralization (bone building) and to prevent calcium from dipping to dangerously low levels. Vitamin D and calcium work together to protect against osteoporosis and other conditions related to weak or brittle bones.

The body makes vitamin D.

When sunlight hits the skin, the body produces vitamin D. The National Institutes of Health report that the process requires 10-15 minutes of direct sunlight to the face, arms, back, or legs at least three times per week. Darker skin requires more time in the sun and lighter skin less time. Although sunscreens interfere with this process, you should avoid spending more than a few minutes in the sun without it due to increased risks for skin cancer.

Some people are at greater risk for vitamin D deficiency.

The forms of vitamin D made through sunlight and consumed in food and supplements must be converted to an active form to be used in the body. This process takes place in the liver and the kidneys. As you age, not only is the body less efficient at making vitamin D from sunlight, the kidneys also become less effective at converting the vitamin to its active form. This makes older adults at greater risk for vitamin D deficiencies. Obesity also interferes with vitamin D function. Research shows that excess body fat may bind to vitamin D reducing blood levels and the amount available to support bone health. Those who live in places with more cloud cover, shorter days, or smog may also be at risk for deficiency due to reduced exposure to sunlight.

Food sources for vitamin D are limited.

Vitamin D is found naturally in few foods. The best sources are fatty fish such as salmon, tuna, and mackerel, and fish liver oils. Small amounts are also found in eggs, cheese, and mushrooms, specifically mushrooms that have been treated with ultraviolet light. Some foods are fortified with vitamin D such as orange juice, milk, yogurt, and breakfast cereals. The recommended intake for vitamin D in adults 70 and under is 600 International Units, and 800 International Units for adults over 70. Vitamin D is only listed on a nutrition label if the food has been fortified with it, making it difficult to measure exact intake.

Too much vitamin D from supplements is dangerous.

Due to a limited supply of natural vitamin D in food and limited sun exposure, many people turn to supplements. Supplements can be effective at raising blood levels of vitamin D, but it is important to monitor your intake. Vitamin D is a fat-soluble vitamin and excess is stored in the body making large doses toxic. Over time, toxicity can lead to weak bones and muscles and kidney damage. The safe upper limit for adults is set at 4000 International Units per day. If you take a multi-vitamin that contains vitamin D, eat fortified foods, and take a high-dose vitamin D supplement, it’s possible to consume too much.

Sources

• Harvard School of Public Health - Vitamin D and Health
• National Institutes of Health - Vitamin D Quick Facts
• MedlinePlus - Vitamin D

Lori Rice, M.S., is a nutritional scientist and author with a passion for healthy cooking, exercise physiology, and food photography.