The best efforts of the scientific community to prove the health benefits of vitamins keep falling short.

This week, researchers reported the disappointing results from a large clinical trial of almost 15,000 male doctors taking vitamins E and C for a decade. The study showed no meaningful effect on cancer rates.

Another recent study found no benefit of vitamins E and C for heart disease.

In October, a major trial studying whether vitamin E and selenium could lower a man’s risk for prostate cancer ended amidst worries that the treatments may do more harm than good.

And recently, doctors at Memorial Sloan-Kettering Cancer Center in New York warned that vitamin C seems to protect not just healthy cells but cancer cells, too.

Everyone needs vitamins, which are critical for the body. But for most people, the micronutrients we get from foods usually are adequate to prevent vitamin deficiency, which is rare in the United States. That said, some extra vitamins have proven benefits, such as vitamin B12 supplements for the elderly and folic acid for women of child-bearing age. And calcium and vitamin D in women over 65 appear to protect bone health.

But many people gobble down large doses of vitamins believing that they boost the body’s ability to mop up damaging free radicals that lead to cancer and heart disease. In addition to the more recent research, several reports in recent years have challenged the notion that megadoses of vitamins are good for you.

A Johns Hopkins School of Medicine review of 19 vitamin E clinical trials of more than 135,000 people showed high doses of vitamin E (greater than 400 IUs) increased a person’s risk for dying during the study period by 4 percent. Taking vitamin E with other vitamins and minerals resulted in a 6 percent higher risk of dying. Another study of daily vitamin E showed vitamin E takers had a 13 percent higher risk for heart failure.

The Journal of Clinical Oncology published a study of 540 patients with head and neck cancer who were being treated with radiation therapy. Vitamin E reduced side effects, but cancer recurrence rates among the vitamin users were higher, although the increase didn’t reach statistical significance.

A 1994 Finland study of smokers taking 20 milligrams a day of beta carotene showed an 18 percent higher incidence of lung cancer among beta carotene users. In 1996, a study called Caret looked at beta carotene and vitamin A use among smokers and workers exposed to asbestos, but the study was stopped when the vitamin users showed a 28 percent higher risk for lung cancer and a 26 percent higher risk of dying from heart disease.

A 2002 Harvard study of more than 72,000 nurses showed that those who consumed high levels of vitamin A from foods, multivitamins and supplements had a 48 percent higher risk for hip fractures than nurses who had the lowest intake of vitamin A.

The Cochrane Database of Systematic Reviews looked at vitamin C studies for treating colds. Among more than two dozen studies, there was no overall benefit for preventing colds, although the vitamin was linked with a 50 percent reduction in colds among people who engaged in extreme activities, such as marathon runners, skiers and soldiers, who were exposed to significant cold or physical stress. The data also suggested vitamin C use was linked with less severe and slightly shorter colds.

In October 2004, Copenhagen researchers reviewed seven randomized trials of beta carotene, selenium and vitamins A, C and E (alone or in combination) in colon, esophageal, gastric, pancreatic and liver cancer. The antioxidant users had a 6 percent higher death rate than placebo users.

Two studies presented to the American College of Cardiology in 2006 showed that vitamin B doesn’t prevent heart attacks, leading The New England Journal of Medicine to say that the consistency of the results “leads to the unequivocal conclusion” that the vitamins don’t help patients with established vascular disease.

The British Medical Journal looked at multivitamin use among elderly people for a year but found no difference in infection rates or visits to doctors.

Despite a lack of evidence that vitamins actually work, consumers appear largely unwilling to give them up. Many readers of the Well blog say the problem is not the vitamin but poorly designed studies that use the wrong type of vitamin, setting the vitamin up to fail. Industry groups such as the Council for Responsible Nutrition also say the research isn’t well designed to detect benefits in healthy vitamin users.