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David Katz
David L. Katz, MD, discusses the implications of the JAMA study that compared low-fat and low-carbohydrate diets for weight loss and health improvement and also profiled the genes of the participants to determine if the genetic patterns thought to predict success on a given diet actually did so. The following edited extract is reproduced with his permission.

“The study, run by Professor Christopher Gardner at Stanford University, randomized over 600 adults to either a fat-restricted or carbohydrate-restricted diet making both diets as ‘good’ as possible. The result was that significant weight loss, and health improvement measured in all the usual ways, occurred in both treatment assignments. Going from the generally poor baseline diet that prevails in America to either a healthy, low-fat diet that emphasized vegetables, and whole, minimally processed foods; or to a healthy low-carbohydrate diet that also emphasized vegetables, and whole minimally processed foods produced significant, and very comparable benefits.

Study participants were not told to track or reduce calories, but by shifting to a much higher-quality diet, they did so as a by-product. Both groups cut their daily calorie intake by about 500 to 600 kcal daily (2100–2510 kilojoules), and lost the amount of weight that would portend. They achieved this by eating wholesome foods in sensible combinations, and by applying some rules and discipline to diets that had neither at the start.

Genetic markers of expected success on one type of diet versus the other proved to be of no value. Weight loss in each diet arm was indistinguishable between those with a genetic profile saying they should do especially well, and those with profiles suggesting they should struggle. Nutrigenomically customized weight loss, despite the buzz it generates, is clearly not ready for prime time.

The study also found no relationship between baseline insulin status and success on a given diet assignment. The high-profile claims that weight loss is all about reducing carbs to reduce insulin prove to be apocryphal. A shift to wholesome foods in sensible combinations is effective at lowering weight and improving health regardless of fat or carbohydrate levels, even among those with insulin resistance at the start.

Fundamentally, then, this study suggests that the best way to lose weight and improve health with diet is not by fixating on macronutrients or calories, but by eating wholesome foods in some sensible combination, and emphasizing whole, minimally processed plant foods. The study also indicates that the fundamentals of generally healthful eating pertain to us all, regardless of our genes or insulin levels.

Boy with horse

These findings of science may surprise some, but they don’t much surprise me, and the reason is – sense. Horse sense, in particular. With horses, as with every species other than our own, we tend to think about the kind of diet that is generally right for the kind of animal, rather than the need to customize diet to each individual. Of course, the one does not preclude the other; horses can all be fed like horses, but some horses will need extra grain to maintain their weight, some will do better on certain varieties of hay. But from the start, the focus is on the common theme of a healthful diet for an entire species, and only after that, variations on the theme.

The DIETFITS study findings collectively indicate that the fundamentals of a health-promoting dietary pattern for Homo sapiens matter more than customizing on the basis of inter-individual variations. As with horses, the one does not preclude the other – but the science we own at present better empowers us to customize diet based on preference, rather than genes. It is good to know that when it comes to dietary patterns that are best for health, we do have choices among the variants on a common theme.

Dr. Katz proudly notes that DIETFITS Principal Investigator, Prof. Gardner, is a science advisor to his company, DQPN, LLC, devoted to reinventing dietary intake assessment for the digital age.

About David L. Katz 
• Director, Yale University Prevention Research Center; Griffin Hospital
• Immediate Past-President, American College of Lifestyle Medicine
• Senior Medical Advisor,
• Founder, The True Health Initiative

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