Medical researchers are homing in on a new wonder drug that can significantly reduce the risk of heart disease, cancer, diabetes and many other diseases - sunshine.

A study released Tuesday found that men who are deficient in the so-called sunshine vitamin, vitamin D, have more than double the normal risk of suffering a heart attack.

Another study last week found that low levels of vitamin D increase the risk of diabetes, and a study last month linked deficiencies to an increased risk of dying from breast cancer.

The new findings join a growing body of evidence indicating an adequate level of the vitamin, which most people can get from 20 daily minutes in the sun, is crucial to maintaining good health.

Not every scientist agrees, and there is controversy about what should be considered an adequate level of vitamin D in the blood. But sentiment gradually is shifting toward a higher intake.

“We don’t have a cause-and-effect relationship here yet,” said biochemist Hector DeLuca of the University of Wisconsin, to prove higher doses of vitamin D prevent these diseases. DeLuca was the first to demonstrate how the vitamin interacts with the endocrine system, which manages the body’s hormonal balance.

But the links are so suggestive “that we have to pay attention to keeping blood levels up where they will protect,” he said. Until the protective effect is proved, he added, “What’s wrong with keeping an adequate level of vitamin D in the blood in case it is?”

Until recently, vitamin D was viewed primarily as a protective agent against diseases of the bone, such as rickets, osteomalacia and osteoporosis. Recommendations for the vitamin are based on preventing these disorders and call for a relatively small intake - a minimum of 400 international units (IUs) per day and perhaps twice that for elderly people, who don’t get outdoors much.

The vitamin is produced from natural precursors in the body by exposing skin to ultraviolet B in sunlight. Caucasian sunbathers can get 20,000 IUs in 20 minutes at noon in summer. But any additional exposure simply damages skin.

Darker skinned people need three to five times the exposure to produce the same amount. Sunblock interferes with production by screening out ultraviolet light.

The primary sources of vitamin D in the diet are milk, which is fortified to yield about 100 IUs per glass, and oily fishes, which have a high content. To have an adequate intake, most people must take supplements or spend more time in the sun - a recommendation that dermatologists generally oppose because of the risk of skin cancer.

Current guidelines call for blood levels of about 30 nanograms per milliliter. By that definition, perhaps 10 percent to 15 percent of the white U.S. population and 50 percent of the black population is deficient in summer, with levels rising in winter when there is less sunlight.

Many researchers now say that we should be striving for average blood levels of 50 to 60 nanograms per milliliter, at which level the bulk of the U.S. population would be considered deficient.

Most researchers in the field take supplements of at least 1,500 IUs per day. Most recommend taking no more than 4,000 IUs because of potential toxicity.

Experts attribute the vitamin D deficiency, in part, to modern lifestyles, which have taken people off the farm and into offices and factories. Video games and computers have brought children indoors, minimizing their exposure to sunlight. Fear of cancer and increasing use of sunblock also might have contributed.

In the new analysis, Dr. Edward Giovannucci of the Harvard School of Public Health and his colleagues studied 18,225 men enrolled in the Health Professionals Follow-up study, a subgroup of a much larger ongoing study.

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The men submitted blood samples when they enrolled in the study, mostly in 1993 to 1995, and the samples were stored.

In 10 years of follow-up, the team identified 454 men who had a nonfatal or fatal heart attack. They matched these men with about 900 other study members who did not have an attack, then measured vitamin D levels at study entry.

They reported in the current issue of the Archives of Internal Medicine that men with blood levels below 15 nanograms per milliliter had 2 1/2 times the risk of having an attack or dying. When they controlled for all other possible factors, such as hypertension, obesity and high lipid levels, the risk was still twice as high as it was for the controls.

Men with levels between 15 and 29 nanograms per milliliter also had an increased risk. Unfortunately, Giovannucci said, there were not enough men in the group with levels above 35 nanograms per milliliter to determine whether higher levels are more protective.

The findings are “not out of left field,” he said. Many epidemiological studies have found a lower rate of heart attacks at higher latitudes, at lower altitudes and in winter - all of which correlates to decreased exposure to sunshine.

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About 869,000 Americans die of heart disease each year, according to the American Heart Association.

“They certainly have made the link between diabetes and cardiovascular disease,” said Dr. Mason Weiss, a cardiologist at Centinela Hospital Medical Center in Inglewood, Calif., who was not involved in the study. “Now the research must be on what the mechanism is.”

Giovannucci speculated that several mechanisms could be responsible for the observation. Previous studies have suggested, for example, that low vitamin D levels lead to a buildup of calcium in atherosclerotic plaques on the walls of arteries, increasing the risk of heart attacks.

It also could affect blood pressure or even have a direct effect on functioning of heart muscles, making them more susceptible to arrhythmia.

“We obviously need to understand the mechanism better,” Giovannucci said. “But that requires randomized trials, which is a big undertaking.”

Weiss joined the chorus of researchers calling for changes in federal guidelines to reflect the new data.

“The next time they review the daily requirements, they should look at all these articles,” he said. Increasing the recommended intake of vitamin D “could have a significant health benefit,” he said, and would be a very cost-effective change.