Every week, the Nuance will go beyond the basics, offering a deep and researched look at the latest science and expert insights on a buzzed-about health topic.


Which diet is best for weight loss: low carb or low fat?

It seems like a straightforward question — one that a single well-designed study should be able to answer. But after nearly 20 years of such studies, the debate rages on. Taken together and applied to big groups, existing research suggests the two diets are about equally effective. But at an individual level, the effectiveness of these plans varies dramatically; some people lose 50 pounds or more, while others on the same diet end up gaining weight.

If only dietitians and doctors could predict how a patient would respond to a specific diet — in terms of weight loss, but also longevity, disease risk, and other health outcomes — it would revolutionize the field of human dietetics.

Enter “nutritional genomics,” sometimes shorted to “nutrigenomics.” This is the idea — now well-founded in science — that a person’s DNA and diet choices are interwoven in ways that mitigate or increase that individual’s risk for obesity, heart disease, and other health complications. By understanding the genetic markers that predispose a person to reacting well or poorly to different dietary components, nutrigenomics should be able to answer the “low-carb or low-fat” question, along with countless others.

“I have been working on this for almost 30 years, and during my career I have had ups and down in terms of my confidence,” says José Ordovás, PhD, a professor of nutrition and genetics at Tufts, when asked about nutrigenomics and personalized nutrition. “But the downs were mostly about the lack of technology needed for looking at the genome of every individual.”

Ordovás says those technological barriers have mostly been cleared, and he and many other researchers are now working to identify the precise genetic traits associated with diet success or failure. “We now have examples where, depending on your genes, you can have a diet relatively high in saturated fat and not be negatively affected for increased risk of obesity or heart disease,” he says. “But we’re still at the beginning of the race in terms of personalized nutrition.”

Ordovás is on the scientific advisory board of the DNA-testing company Habit, which aims to provide users with nutrition guidance based on their genetic and blood biomarkers. He says he has no financial interest in Habit—just one of a handful of new companies jockeying for market share in the realm of personalized, DNA-based nutrition.

And while Ordovás says Habit and other testing services are based on real science and may provide some users with effective guidance, “the information they can obtain today is probably not enough.” He points out that genetics are not the only determinant of diet success; the human microbiome also plays a part, and it’s not shaped solely by a person’s DNA.

Other nutrition researchers reiterate Ordovás’ optimism in nutrigenomics, as well as his assessment of the current state of the science.

“It would be lovely if DNA could predict the perfect diet,” says Marion Nestle, PhD, professor emerita of nutrition and food studies at New York University. “Maybe someday, but unfortunately not yet.”

“There are a few specific genes that are associated with food intolerances, but for the most part those are long known and well established,” adds Michael Jensen, MD, a professor of medicine at Mayo Clinic in Rochester, Minnesota. When it comes to DNA testing companies that offer diet-specific advice, he’s less enthusiastic. “The new stuff is basically a way to get people to pay for a whole genome test with some can’t-be-proven claim that it will point them to the magic diet for them,” Jensen says. “In truth, I suspect they just recommend any one of several healthful diets and hope for a big placebo effect.”

Christopher Gardner, PhD, is a professor of medicine at the Stanford University School of Medicine. He and his colleagues just wrapped a five-year, $8 million study that attempted to link certain genetic variables to weight-loss success on either healthy low-carb or low-fat diets. The team tested one genotype pattern — out of “hundreds we could have looked at,” Gardner says — and it didn’t work.

This doesn’t mean DNA can’t predict diet success or failure, he says. (Gardner jokes that if he could clone himself and raid the U.S. Treasury for funding, he’d love to keep running these sorts of trials.) “The world of personalized nutrition is filled with possibilities, and is certainly plausible, but at this time the field is in its infancy, and most people would be better off with common sense than with some of the companies advertising that they know how to help people personalize their diets,” he says.

What does “common sense” look like in diet form? Gardner lists four things he calls the “foundation” of a healthy diet: Eat less (or eliminate) added sugar, eat less (or eliminate) refined grains, eat more vegetables, and choose whole foods over packaged or processed ones. “Those four factors appear to be agreed on by everyone in the field, including the zealous proponents of fringe or fad diets,” he says. “If we made progress in just those four areas, I suspect many of our concerns about diet and health would be addressed.”

Ordovás says he agrees that most people — around two-thirds of us — would do well following these commonsense guidelines. It’s the other third — whose genes put them at above-average risk for disease — who would benefit most from DNA-based diet personalization. “But we’re still in the early days,” he adds.