Based on those findings, a U.S. government-back panel issued draft recommendations that echo its previous conclusion: it cannot recommend for or against taking vitamins and minerals to prevent those conditions.

"At this point in time the science is not sufficient for us to estimate how much benefit or harm there is from taking vitamin or multivitamin supplements to prevent cancer or heart disease," Dr. Michael LeFevre said.

LeFevre is co-vice chair of the U.S. Preventive Services Task Force (USPSTF), which issues recommendations to help guide doctors and health systems. The USPSTF sponsored the new analysis.

The panel's draft statement also says neither beta-carotene nor vitamin E should be taken to prevent heart disease or cancer.

Previously, beta-carotene was found to further increase the risk of lung cancer among people who are already at an increased risk.

Approximately 600,000 people die of heart disease in the U.S. every year, according to the Centers for Disease Control and Prevention. Another 580,000 die of cancer, the American Cancer Society says.

Cancer and heart disease share a number of risk factors including inflammation, researchers wrote in the Annals of Internal Medicine. Animal and lab studies have suggested supplements may guard against some of those risk factors.

Americans spend an estimated $12 billion each year on supplements.

For the new analysis, Dr. Stephen Fortmann and his colleagues from the Kaiser Permanente Center for Health Research in Portland, Oregon, analyzed studies that examined vitamin and mineral use to prevent cancer and heart disease.

After searching online medical research databases for studies published between January 2005 and January 2013, the researchers included data from 26 studies.

Those studies examined the effects of multivitamins or specific vitamins and minerals taken together or alone.

The studies included anywhere from 128 to over 72,000 participants with average ages between 22-77. The average age for most studies was greater than 50, however.

Participants were not taking supplements for any known nutritional deficiencies, such as low vitamin D levels.

The researchers found there was no difference in deaths between people taking multivitamins and people taking placebo pills or nothing. Also, there was no effect of multivitamins on fatal or non-fatal heart disease.

Two trials did show a small reduction in new cancers over a 10-year period, but only in men.

Fortmann and his colleagues concluded there is no consistent evidence that supplements -- multivitamins or otherwise -- affect the risk of heart disease, cancer or death among adults.

That, they write, is consistent with earlier findings. However, they caution that there are few quality studies on supplements other than vitamin E and beta-carotene.

After reviewing six trials on each, the researchers found vitamin E did not have any benefits. Beta-carotene increased the risk of lung cancer among smokers.

"The main message is that there is not much evidence of a long term health benefit to taking most of the vitamins that people are taking," Fortmann told Reuters Health.

"But one has to qualify that comment, because we only looked at evidence through heart disease, cancer and deaths and it's hard to show an effect on those things."

Steve Mister, president and CEO of the Council for Responsible Nutrition (CRN) said most people don't take vitamins or minerals to protect against heart disease or cancer.

"We have both government data and our own consumer study that we do every year," Mister said. "Primarily, they take these vitamins or multivitamins because they know they're not eating the diet that they should … They help to fill in those gaps."

CRN is a Washington, D.C.-based trade group that represents dietary supplement manufacturers and ingredient suppliers.

"We tell consumers to talk with a healthcare provider and recognize that a multivitamin is part of a healthy lifestyle," Mister, who was not involved with the new analysis, told Reuters Health. "It's not a silver bullet that's going to save you from all the things that you do."

Fortmann emphasized that message and said people shouldn't take supplements as an excuse for having a poor diet.

"If you're taking a vitamin, you shouldn't be too confident that it's preventing any heart disease or cancer," he said.

LeFevre, who is also a professor at the University of Missouri in Columbia, said the findings don't mean nutrition doesn't matter.

"Good nutrition is important and probably important for heart disease and cancer," he told Reuters Health.