In a 609-person, year-long study, dieters lost an average of about 12 pounds—regardless of whether they were trying to stick to a low-fat or a low-carb diet and regardless of whether they carried genetic variations linked to success on one of those diets.

The lackluster finding, published by Stanford researchers this week in JAMA, knocks back hopes that we’re at the point of harnessing genetic information to tighten our waistlines. Previous studies had whetted dieter’s appetites for the idea, picking out specific blips in metabolic genes that appeared to help explain why some people easily shed poundage on a given diet, while others struggled. Biotech companies have even begun serving up DNA tests that claim to help hungry dieters pair their menus with their biological blueprints.

But according to the new study, that order isn’t up yet.

The authors, led by nutrition researcher Christopher Gardner, enrolled 609 participants, who were aged 18 to 50 and had body mass indexes from 28 to 40 (spanning overweight to extreme obesity), with a mean of 33 (obese) and an average weight of around 212 pounds. Of those, 305 participants were randomly assigned to eat a “healthy low-fat diet” for a year, while the remaining 304 were assigned a “healthy low-carbohydrate diet.”

The dieters weren’t strictly monitored or required to stick to a rigid plan. Instead, they were offered 22 hour-long classes led by registered dietitians on how to follow their assigned diet without feeling deprived, as well as general advice on healthy eating.

Lean data

For instance, the low-fat group was advised to avoid oils, fatty meats, full-fat dairy, and nuts, while the low-carb group was cautioned to avoid cereals, grains, starch vegetables, and legumes. But both diet groups were told to “(1) maximize vegetable intake; (2) minimize intake of added sugars, refined flours, and trans fats; and (3) focus on whole foods that were minimally processed, nutrient dense, and prepared at home whenever possible.” Dietitians also went over emotional awareness—to avoid stress binges, for instance—and common behavioral modifications—such as setting goals—that can help with dieting.

Otherwise, the dieters were given general targets: members of the low-carb group tried to get their daily carb intake down to 20 grams in the first eight weeks. The low-fat dieters tried to get their daily fat intake down to 20 grams in the first eight weeks. Then, both groups were instructed to find the minimal level that they thought they could maintain indefinitely.

Both groups cut back but on average didn’t hold to the targets, based on interviews. The low-carb dieters reported eating, on average, about 246.5 grams of carbohydrates per day at the beginning of the trial. They got that down to an average of 97 at the three-month mark but crept back up to 132 by the end. The low-fat dieters were, on average, eating 87 grams of fat per day at the start. They were down to 42 after three months and inched up to 57 by the end.

Though the researchers didn’t tell dieters to count or cut calories, both groups were eating around 500 to 600 fewer calories each day throughout the study.

At the end, the low-carb group lost an average of about 13.2 pounds per person, while the low-fat dieters lost an average of 11.7 pounds. The difference between the two groups was not statistically significant. But, there was wild variation within both groups. Some dieters lost upwards of 60 pounds, while others gained more than 20.

To see if genetics could help explain that weighty wobbling, the researchers turned to small genetic differences in three genes involved in fat and carbohydrate metabolism. Earlier work had suggested that these could predict the success of certain diets. Among the 304 low-carb dieters, 97 (32 percent) had a matching low-carb genotype, 114 (37.5 percent) had a mismatched low-fat genotype, and the rest had genotypes that matched neither. Among the 305 low-fat dieters, 130 (42 percent) had a matching low-fat genotype while 83 (27 percent) had a mismatched low-carb genotype.

Statistics on the side

The researchers ran the data to see if those with the matching genotypes did better on their diets than others. They didn’t. There were no statistically significant differences in success or failure among the genotypes in either dieting group.

The researchers also collected insulin data from the participants to see if that could predict dieting outcomes. The idea here is that people whose bodies don’t release enough insulin—which is involved in metabolizing carbohydrates—may have a better chance of losing weight on a low-carb diet, which would demand less insulin. But, that too, didn’t pan out. Insulin secretion didn’t link to better or worse weight-loss outcomes.

Overall, researchers concluded that neither the genetic variations nor insulin measurements were “helpful in identifying which diet was better for whom.”

Of course, the study had limitations. Despite being large and randomized, it relied on self-reported diet information from participants who didn’t adhere to strict menus. This replicates what happens in real-life conditions for most dieters who try but do not always succeed at consistently following a strict—and potentially punishing—food plan. That said, stricter diets could have led to different outcomes during the study. Also, while the genetic variations the authors homed in on didn’t seem to predict weight-loss outcomes, other genetic factors or combinations may one day prove useful in this front.

The researchers are currently picking through leftover genetic data from the participants to see if any other bits of code may help explain the weight-loss variation.

JAMA, 2018. DOI: 10.1001/jama.2018.0245 (About DOIs).