(Health.com) -- People with heart disease and similar conditions who don't have enough vitamin D are more likely to be depressed than their counterparts with adequate levels of the "sunshine vitamin," according to a study presented at the annual meeting of the American Heart Association in Orlando. This link seems to be even stronger in the winter.
Vitamin D is called the sunshine vitamin because the human body produces it only when exposed to sunlight -- although it takes just 10 to 15 minutes a day to make an adequate amount. Vitamin D, which helps the bones better absorb calcium, is also added to multivitamins and milk, and occurs naturally in fish.
A second study by the same team of researchers found that people age 50 or older who lack vitamin D are at a higher risk for heart disease and stroke, and are more likely to die earlier than people the same age who get adequate amounts of the vitamin.
At the American Heart Association's annual Scientific Sessions meeting, more than 20,000 cardiologists and other physicians from around the country give presentations on new research and on advances in the diagnosis and treatment of heart disease and stroke.
These studies add to the mounting evidence about the dangers of vitamin D deficiency and may also shed light on the connection between depression and cardiovascular disease (which includes any disease caused by clogged arteries, including heart disease).
Depression and diseases of the arteries -- both have been associated with vitamin D deficiency in the past -- tend to occur together, says Heidi May, Ph.D., an epidemiologist with the Intermountain Medical Center at the University of Utah, in Murray, who participated in both studies.
"It is known that during the last century, the prevalence of depression has increased, and, more recently, that vitamin D deficiency has increased," May says. "It is well-known that depression is associated with cardiovascular disease and events."
This research, she adds, "is trying to elucidate whether vitamin D deficiency is associated with depression and may be contributing to this increase in cardiovascular disease and events."
In the first study, May and her colleagues measured blood levels of vitamin D in 8,680 people age 50 or older who had been diagnosed with heart disease, stroke, or another type of cardiovascular disease. vitamin D levels above 30 nanograms per milliliter of blood (ng/mL) were considered normal, levels between 15 and 30 ng/mL were low, and those 15 ng/mL and below were deemed very low.
Among those with very low levels of vitamin D, 32 percent were depressed, as were 25 percent of the people with low levels, and 21 percent of those with normal levels. This trend was seen even among individuals with no history of depression.
Winter seemed to make the association even more pronounced. Seasonal depression, which typically occurs in winter, may be linked to lack of sunshine.
In the second study, which looked at 27,686 people age 50 or older with no history of cardiovascular disease, May and her colleagues found that, compared to individuals with normal levels of the vitamin, people with very low levels of vitamin D were 77 percent more likely to die, 45 percent more likely to develop heart disease, and 78 percent more likely to have a stroke during the study, which lasted for more than a year. They also had double the risk of heart failure.
These findings didn't surprise Michael F. Holick, M.D., Ph.D., a professor of medicine, physiology, and biophysics at Boston University School of Medicine who has studied the dangers of low vitamin D levels for years.
"It is estimated that many of the genes that regulate cardiovascular health are directly or indirectly regulated by vitamin D," says Holick, author of the forthcoming book The Vitamin D Solution. "We know that vitamin D regulates blood pressure, and it is very important for maintaining good heart health."
Vitamin D is also essential to the brain, which contains receptors for the vitamin, says Holick. "The new research is confirming previous observations that vitamin D deficiency increases the risk of depression," he explains.
In addition to heart disease and depression, vitamin D deficiency has been linked to a host of chronic diseases, including some types of cancer, osteoporosis, diabetes, Alzheimer's disease, schizophrenia, and some autoimmune disorders.
Although further research is needed to confirm that a shortage of vitamin D in fact links depression to heart disease and stroke, May says that more aggressive monitoring of vitamin D levels could help improve disease outcomes in the meantime.
"Even a moderate deficiency of vitamin D is associated with cardiovascular disease and death; therefore, screening and treatment is very important," says May. "Since both are simple and relatively inexpensive, prevention of vitamin D deficiency should be cost-effective and may ultimately be able to save more lives."
Likewise, she adds, "patients with depression may need to be more aggressively screened and treated for vitamin D deficiency since they are at higher risk for cardiovascular events."
These studies come as the Institute of Medicine (IOM), an independent organization that advises the U.S. government on health and medical matters, is considering new recommendations for vitamin D intake. Currently, the dietary reference intake for vitamin D is 200 international units per day (IU/day) for adults ages 14 to 50, 400 IU/day for adults 50 to 71, and 600 IU/day for those older than 71.
Holick says the IOM should raise the limit on the amount of vitamin D currently deemed safe, or tolerable, which now stands at 2,000 IU/day for adults.
"If we raise it, more food can be fortified with vitamin D and fortified at a higher level," he says. "We think [2,000 IU/day] is the baseline of what we need to take, not the upper safe limit."
Holick advocates raising the daily limit to 10,000 IU. Many researchers agree and have suggested that the toxic level of vitamin D is closer to 10,000 IU/day.
Unlike May, however, Holick does not think that screening for vitamin D deficiency is necessary.
"We should all be taking supplements of vitamin D," he says. "There's no downside to increasing vitamin D intake, and if it does turn out that there is a benefit, you're ahead of the game."
Copyright Health Magazine 2011