Vitamin D is a vital nutrient that helps your body absorb calcium to strengthen your bones. It's produced naturally in your skin when it's exposed to sunlight, in fortified conventional foods and in a few unfortified foods or supplements.

People with darker skin can make less vitamin D in the sun than those with light-colored skin. Taking too much vitamin D may lead to excess levels in your blood and cause health problems.

Excess Effects

The surge in vitamin D popularity has been driven by a host of health benefits — from bone strength and reduced risk of fractures to improved mood and decreased risks of heart disease and cancer. As a result, people are stocking up on the nutrient in pill form and seeking screening tests to measure their blood levels.

It’s important to get vitamin D from a healthy diet, which includes foods like fatty fish, fortified dairy products, and orange juice, as well as moderate sun exposure. Most adults need no more than 4,000 international units (IU) a day. However, a healthcare professional may recommend you take larger doses for certain health conditions, and they will monitor your blood levels over time to make sure you don’t reach toxic levels.

Rates of vitamin D deficiency vary by race and ethnicity. People with darker skin have more difficulty making vitamin D from sunlight because the melanin in their skin limits the rate of skin synthesis. In addition, older adults who live in nursing homes and other institutions are less likely to be able to get enough vitamin D from daily sun exposure. People with malabsorption diseases like celiac disease and inflammatory bowel syndrome have higher rates of deficiency, as do infants who are only fed breast milk.

Health Risks

Before vitamin D can be used by the body, it must first be changed in the liver into a form called 25 hydroxyvitamin D or 25(OH)D. Most vitamin D blood tests measure the level of 25(OH)D because it's the best way to determine if you have enough of the nutrient in your system.

Getting too much vitamin D can cause a high calcium level in the blood (hypercalcemia). This can lead to bone problems and other health issues such as weakness, vomiting, and heart rhythm changes. The condition is rare and usually results from taking large doses of vitamin D supplements or from eating foods that are fortified with too much vitamin D.

There is ongoing debate about how much vitamin D is needed for good health. Some experts say people who aren't getting enough of the nutrient may have higher rates of cancer and heart disease, while others warn that excess levels can increase the risk for heart disease, osteoporosis, and other health conditions. For now, the Institute of Medicine recommends a daily intake of 600 international units, or IU, for adults under age 70 and 800 IU per day for those over that age. (Most healthy people only need 400 IU per day to maintain optimal blood levels of vitamin D.)

Expert Tips

Vitamin D is vital to bone health and is linked to a healthy immune system, but too much can be dangerous. People who take over-the-counter or prescription vitamin D supplements, or receive vitamin D injections, can easily get high levels of calcium in the blood (hypercalcaemia).

The vitamin is produced naturally by the sun's ultraviolet-B rays and also can be found in some foods, like fatty fish, mushrooms exposed to UV light, and fortified dairy products. Your liver changes the vitamin D you eat or absorb into a form that your body can use, called 25-hydroxyvitamin D (25(OH)D). Most vitamin D tests measure this form.

The best way to prevent excess vitamin D is to eat a balanced diet that includes plenty of fresh and fortified foods. Experts say the average person needs no more than 4,000 international units (IU) of vitamin D per day from food or supplements. Some healthcare professionals prescribe doses higher than this, but they only give the higher amounts for a short period of time and check your blood levels of vitamin D and calcium to make sure you don't get too much. It is especially important to use sunblock when going outside to prevent excessive vitamin D and skin cancer. This is especially true for people of darker skin who cannot generate enough vitamin D by sunlight.