Supplements are offered for sale at a Wal-Mart store in Chicago, Illinois. An investigation by the New York State attorney generals office into store-brand supplements being sold at Wal-Mart, Walgreens, GNC and Target found that many did not contain the herbs shown on their labels and some included potential allergens not identified in the ingredients list. (Photo by Scott Olson/Getty Images)

Annual sales in the dietary supplement industry in this country have more than doubled to $13 billion over the past decade, even though the science behind these products remain questionable. And new federal data on Tuesday shows just how much Americans are flocking to these products.

In 2012, 17.7 percent of adults used nonvitamin, nonmineral dietary supplements, making it the most popular "natural" health method in the country, according to a new Centers for Disease Control and Prevention report. That makes dietary supplements more popular than a wide range of alternative methods meant to boost health, including deep-breathing exercises — which was the second most popular, at 11.6 percent — yoga, meditation, chiropractic care, homeopathy and more.

The CDC asserts that the health benefits of dietary supplements are "unclear." Despite this, dietary supplements have maintained their top ranking among alternative health-care approaches since 2002 and 2007, the last two times the CDC studied this topic.

As you can see below, Americans particularly love taking fish oil. Usage increased from 4.8 percent of adults in 2007 to 7.8 percent in 2012.

Do fish oil supplements even work?

The CDC points out that some research has suggested that fish oil supplements, itself now more than a $1 billion industry, can help protect heart and mental health, but "its benefits are not well understood." Indeed, new studies last year questioned whether fish oil — a source of omega-3 fatty acids recommended by nutritionists — can prevent prevent heart disease as claimed. The CDC also found more American adults are taking supplements of melatonin, supposed to be a sleep aid, and probiotics and prebiotics for digestive health.

Meanwhile, 4.9 percent of kids ages 4-17 are taking these dietary supplements, according to a second CDC survey. That's not nearly at the same rate that adults are taking these products, but dietary supplements are still the most popular of any alternative health care method among this age group.

The CDC adds caution here, too: "Little is known about the possible benefits or adverse effects of nonvitamin, nonmineral dietary supplements in children." The rate of children using a supplement to treat a health condition decreased from nearly half of all users in 2007 to 35.5 percent in 2012, according to the CDC.

The American Academy of Pediatrics says supplements can be important for some children, but you should consult a doctor before using them. However, a 2013 NIH-sponsored survey of children using dietary supplements suggests this rarely happens — only 15 percent of supplements used by kids had been recommended by a physician or health-care provider.

And if you're still wondering which dietary supplements actually seem to be legit, my colleague Christopher Ingraham has got you covered here.