Only about 1 out of 4 Americans gets enough vitamin D, and that number is going down, researchers say. A new study compared data from about 20 years ago with data from early in this decade. In the first period, nearly 1 out of 2 Americans had ideal levels of vitamin D. In recent years, that dropped to 1 out of 4. The number of people with very low levels (deficiency) increased from 2% to 6%. Among blacks, 29% had a deficiency. Very few blacks had ideal levels. The authors cited two causes for the trend: more use of sunscreen and less time spent outdoors. They said people should get about 1,000 international units of vitamin D daily. The study appeared in the Archives of Internal Medicine. HealthDay News wrote about it March 23.

By Robert H. Shmerling, M.D.
Harvard Medical School

What Is the Doctor's Reaction?

Vitamin D is getting a lot of attention lately -- and for good reason. Recent studies have linked low vitamin D levels in the blood with cancer, poor immune function, heart disease and a higher risk of death.

This is in addition to its well-known role in bone health. Kids with very low levels of vitamin D (deficiency) can develop rickets. This condition causes bones to form abnormally. Adults with low vitamin D may develop osteomalacia. The name means "soft bones." For people with this condition, bones fracture more easily than normal.

Despite these benefits, a new study suggests that most of us do not get enough vitamin D. And apparently it's getting worse.

The new report was just published in the Archives of Internal Medicine. Researchers studied the blood levels of vitamin D among more than 13,000 people between 2001 and 2004. They compared the numbers with nearly 19,000 people studied between 1988 and 1994.

They found that:

• The average blood level of vitamin D fell by 20% between the two time periods.
• The average blood level was lower than the amount considered ideal.
• The rate of adequate vitamin D levels decreased from 45% to 23%. If this reflects the population as a whole, more than 3 out of every 4 Americans have low vitamin D.
• The rate of severe vitamin D deficiency tripled, from 2% to 6%.
• Racial differences were striking. The rate of severe vitamin D deficiency among blacks increased from 9% to 29%. Only 3% of blacks had ideal levels.

Several trends could explain these findings. During the last 20 years, far more people have learned about the dangers of sun exposure (especially skin cancer). Limiting sun exposure can be one cause of low blood levels of vitamin D. That's because sun causes the skin to make vitamin D.

The average American also does not get enough of this important vitamin from food. And the dose of vitamin D in pills, such as multivitamins, may be too low. A common dosage is 400 international units (IU) daily. But some experts believe supplements should contain 1,000 IU. This may be especially important during the winter and in northern climates.

Vitamin D deficiency could be more common in blacks because of their skin color. Dark skin needs more sun than lighter skin in order to make vitamin D. Blacks also are more likely to have lactose intolerance. Therefore they may eat and drink fewer vitamin D-rich dairy foods.

This research should raise awareness about how widespread vitamin D deficiency is and how to prevent it. But I hope it won't lead to overreaction. Too much vitamin D and too much sun aren't healthy either. Too much vitamin D is defined as 2,000 IU or more each day. This much vitamin D can cause nausea, vomiting, bone or joint pain, constipation and loss of appetite. In severe cases, excess vitamin D can cause the blood calcium to rise. That can lead to depression, confusion or even coma.

What Changes Can I Make Now?

Think about how much vitamin D is in your diet. Some good food sources of this vitamin are:

• Fish
• Eggs
• Milk, cheese and other dairy products
• Fortified cereals

Read food labels. Try to increase your intake of dietary vitamin D. Or consider taking a supplement. The daily recommended amounts of vitamin D are:

• 200 units for people younger than 50
• 400 units for people ages 50 to 70
• 600 units for people older than 70

As mentioned, these recommendations may soon be increased.

Get some sun, but don't overdo it. About 10 to 15 minutes in the sun each day without sunscreen is probably enough. Moderation is important. Excessive sun exposure can damage skin and increase the risk of skin cancer.

Your risk of vitamin D deficiency may be particularly high if you are elderly, obese or have a digestive disease (such as Crohn's or celiac disease). As noted in this new study, blacks may be at increased risk as well.

What Can I Expect Looking to the Future?

You can expect to hear much more about the health benefits of vitamin D. We need careful research to show whether more vitamin D in the diet or supplements can improve health. You can expect future research to address these questions:

• Why are vitamin D levels falling? What is the most reliable way to reverse this trend?
• Why do blacks have a higher rate of vitamin D deficiency than other groups?
• What is the ideal amount of sun exposure? It should provide the health benefits of vitamin D without the health risks of too much sun.
• Should the current advice about vitamin D intake be changed?

Researchers keep learning more about the health benefits of vitamin D. Most of the studies suggest that we don't get enough. So I think there will be considerable buzz about vitamin D for quite some time.