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What happens to diets when you control food quality?

Dieters often control their intake of either carbs or fat. But when dieting, the overall quality of food you eat can also change. Do low-fat and low-carb diet effects differ, even if you control for food quality?

Study under review: Visceral adiposity and metabolic syndrome after very high–fat and low-fat isocaloric diets: a randomized controlled trial

Introduction

Many controlled trials have compared low-carbohydrate to low-fat diets in their ability to promote weight loss. Among trials lasting longer than six months in duration, both diets appear similarly effective[1]. However, another meta-analysis[2] focusing exclusively on trials where the low-carbohydrate group consumed less than 120 grams of carbohydrates per day found that low-carbohydrate diets led to greater weight loss.

As the popularity of low-carbohydrate diets for the treatment of obesity continues to rise, there is still concern over their effects on metabolic health, particularly because of the potential for these diets to be high in saturated fat. For instance, even when participants with obesity are eating in a caloric deficit, consuming a diet high in saturated fat produces changes[3] in skeletal muscle insulin signaling that may lead to insulin resistance over the long-term.

However, as we have learned in previous NERDs, not all food sources of macronutrients are the same. NERD #9, Volume 1, Got milk (fat globule membrane), discussed a study in which a diet where the saturated fat was provided from cream led to more favorable blood lipid profile than an otherwise identical diet where the saturated fat came from butter. Similarly, in NERD #8, Blast from the past: a paleo solution for type 2 diabetes, it was shown that a paleo diet was more effective than a diet based on the recommendations of the American Diabetes Association for improving insulin sensitivity, glycemic control, and blood lipids despite both diets having equal macronutrient composition (i.e., only food sources of macros differed).

A limitation of previous low-carb vs. low-fat studies is that food quality was largely uncontrolled for. Many people on a low-fat diet may eat more processed carbohydrates and added sugar while people on a low-carb diet may exclude these foods, only to drink buttered coffee every morning. Moreover, protein and polyunsaturated fat consumption, which are known to have profound effects on body composition and metabolic health, respectively, are not always controlled between low-fat and low-carb diets.

The current study sought to compare the effects of a low-carbohydrate diet (10% kcal) to a low-fat diet (less than 30% kcal) on body composition and metabolic health when both diets were minimally processed, based on the same foods, and had similar amounts of calories, protein, and polyunsaturated fat.

Both low-carbohydrate and low-fat diets are effective for promoting weight loss. However, effects on metabolic health continue to be an area of concern. The current study compared a low-fat to a low-carb diet, with the crucial difference from other research being that this study used diets that were both minimally processed, based on the same foods, and had similar amounts of calories, protein, and polyunsaturated fat.

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