CHICAGO — They may be popular in health food stores, but beta carotene pills won’t help ward off heart disease or cancer or promote a longer life, two new studies indicate.

The findings, published Wednesday in The Journal of the American Medical Association, underline previous studies that cast doubt on the supposed benefits of supplements containing beta carotene, a nutrient abundant in green and yellow vegetables.

In the first study, researchers divided the 1,805 subjects into four equal groups according to their beta carotene blood levels prior to the study.

The researchers found that those who began the study with the highest levels before taking beta carotene supplements were 40 percent less likely than those with the lowest levels to die of heart disease, cancer and other causes during the study’s eight years of follow-up.

But taking beta carotene supplements of 50 milligrams a day — about 10 times the minimum amount recommended in a healthy diet — had no effect on participants’ health risks, said researchers led by Dr. E. Robert Greenberg of Dartmouth Medical School in Lebanon, N.H.

Beta carotene provides the body with a building block for vitamin A. Like vitamin E, beta carotene is an antioxidant, meaning it can protect against cell-damaging biological oxidation.

Scientists have been disappointed that clinical trials of beta carotene supplements have not been consistent with population studies indicating that people who eat lots of fruits and vegetables rich in beta carotene suffer cancer and heart disease at much lower rates.

Greenberg said one possible explanation is that a diet high in beta carotene may contain other nutrients that are actually providing the benefits. Or, a lifetime of beta carotene supplements might be necessary for the benefit, he said.

People buying beta carotene supplements in health food stores are “hoping for a fairly prompt reduction in risk, and that’s just not going to happen based on our study and others that have addressed the topic,” he said.

There is no official recommended dietary allowance for beta carotene, but experts say a healthy amount in a normal diet is 5 to 6 milligrams daily.

In the second study, involving more than 29,000 male smokers ages 50 to 69, Finnish researchers found that 20-milligram beta carotene supplements daily did not decrease the risk of developing chest pain over 4 1/2 years of follow-up. But vitamin E supplements were linked to a 9 percent decrease in heart-pain risk over that period.