Americans have cut back on all manner of spending during the recession, from dining out to flat screen televisions, but one nonessential line of products has thrived: vitamin supplements.

The industry, which began taking off in the mid-1990s, brought in $9.6 billion in the U.S. last year, up from $7.2 billion in 2005. Its popularity is generally considered a testament to the almost religious belief some people have that vitamins can prevent disease and extend life.

"Vitamins have surged recently as a result of the economic downturn," said Cara Welch, vice president for scientific and regulatory affairs for the Natural Products Association. "People have become proactive about disease prevention as an alternative to the high cost of doctors and prescription medicine."

The only problem is, there's little evidence to support the hope. Although some clinical trials have shown a link between certain supplements and positive health outcomes, vitamin pills have fared poorly in major study after major study in recent years. The gloomy headlines may dry up funding for further study.

The most recent discouraging studies were published last week. A decade-long clinical trial of roughly 35,000 healthy middle-aged men found vitamin E not only did not prevent prostate cancer, it increased trial subjects' risk of developing the disease by 17 percent compared to a placebo. Another study found a higher risk of dying among nearly 39,000 older women who took multivitamins, vitamin B6, folic acid, magnesium, zinc, copper or, especially, iron over a 19-year period, compared with women who didn't.

Those studies were just the latest bad news for vitamins. In the past decade, vitamin pills have either failed to show a benefit or increased the risk of disease in clinical trials involving the heart, stroke, dementia and cancer, where hope was once the greatest.

Industry groups have criticized the studies as flawed or biased. The Council for Responsible Nutrition called last week's study of post-menopausal women and vitamins "a hunt for harm."

It's been confusing for people, many told by their doctor to take a multivitamin as part of a good prevention plan. It doesn't help that scientists aren't sure why vitamins in food promote health but those in pill form might contribute to disease.