Every time Rebecca Castle sat down to eat her favorite foods, she says she suffered excruciating abdominal pain.
"I cut out carbs. I wasn't eating dairy," she said. Nevertheless, she experienced sharp, shooting pain, bloating and distention for more than two years.
Castle saw multiple doctors. Then she spent several hundred dollars to take a DNA metabolic test.
What she learned? "I was allergic to starch," she said. "That's mostly root vegetables, corn, peas, sweet potatoes."
Nutritionist Nicci Schock says a typical client for such a DNA test is an athlete looking to improve performance. "And then folks like Rebecca, they know their body and they know something is off."
Ahmed El-Sohemy, the chief science officer at Nutrigenomix, said, "Even though it's a genetic test, it's effectively dietary advice and counseling an individual on how to eat better.
"Individual genetic differences can help us understand why some people respond differently from others."
Nutrigenomix markets a DNA test that looks at 45 genetic markers. The company makes recommendations based on the patient's genetic profile, pointing out attributes like "an elevated risk for low iron."
"We're talking about metabolic tests," El-Sohemy told correspondent Michelle Miller. "They're genetic tests, but they affect the way that you metabolize various substances that you consume."
But Dr. David Agus, a CBS News contributor, says DNA tests for diet and exercise recommendations are not validated by medical research.
"There are very few of them that actually have data behind them," he said. "And to me, that's a problem. You know, you have companies like 23andMe that were because they had to show data with regard to human disease. Well, I think this same thing needs to happen with nutrition and exercise."
The FDA says it supports tests "that may provide consumers with direct genetic information that can inform health related decisions."
But the agency points out it "does not actively regulate these products."
"2,400 years ago, Hippocrates did something amazing: he would eat something, and he would write down how he felt after he ate it," Dr. Agus said. "To me, that's the best way in the world to know what's right for you."
Rebecca Castle says the test produced more than 30 pages of results about her body, which means she can now avoid the foods that set off her stomach.
"I think it's worth it," she said. "You don't need your blood taken; you're literally just spitting into a test tube. You could do it in your sleep!"