Directions to Weight Loss? Don’t ask your genes.

David L. Katz, MD

A large, robust study called DIETFITS, just published in the Journal of the American Medical Association, is a precautionary tale for those thinking of banking on, or investing in, genetically customized approaches to weight loss.  The randomized trial in over 600 adults indicates that we are not there yet, and perhaps that there simply is no there, there.

At the risk of spoiling the suspense, an emphasis on wholesome foods resulted in reduced calorie intake, whether the dietary pattern assigned was low in fat, or low in carbohydrate.  The pattern of single-nucleotide polymorphisms, or SNPs, suggesting preferential effects of one or the other dietary pattern made no difference in the outcomes.  Neither did baseline measures of insulin sensitivity.  Weight loss, and overall improvement in biomarkers, was quite comparable whether fat or carbohydrate was restricted.

The study itself was fairly complex, examining an array of outcomes and inputs, and their interactions.  All the more reason for us to keep the story line here as clear and uncluttered as possible.

The study affirms that restricting dietary fat or dietary carbohydrate can lead to weight loss provided energy intake goes down.  That does not mean there is a need to count calories, something avoided in the DIETFITS study.  Rather, by focusing on wholesome foods, and imposing some discipline and rules on dietary choices, calories go down as a by-product.  That happened in both diet assignments of the study, and to a roughly comparable degree.  There is, in other words, no value, at least for weight loss, in claims directed at one macronutrient versus another.  Cutting either fat or carbohydrate can cut calories, which in turn causes weight to fall.

Why might that finding be different this time than in prior studies that suggested there was macronutrient-specific magic to be had?  Because this group of investigators was uniquely fair to both diet assignments, focusing on developing both a fat-reduced and a carbohydrate-reduced dietary pattern made up of wholesome foods.  In most prior studies of this topic, one diet was optimized, and the other set up to fail.

Matching diets to genes made no difference in the outcomes.  The investigators randomized over 300 adults to each diet, and then analyzed outcomes in relation to their gene patterns.  Genetic indications of favorable responses to low-fat or low-carb diets did not translate into meaningful differences in outcomes.  Weight loss and health improvement with either diet assignment were comparable between those with genes saying they should do especially well, and those with genes saying they should struggle.  Pretty much everyone who adhered to a diet that emphasized vegetables, whole foods, and the avoidance of highly processed foods lost weight and improved their health, whether their intake of total fat or total carbohydrate was high or low.