Study compares a low-fat vegetable-based diet to a low-carb animal-based diet

People on a low-fat, plant-based diet ate less daily calories, but had higher levels of insulin and blood glucose, compared to when they ate a low-carb animal-based diet, according to a small but highly controlled at the National Institutes of Health. Led by researchers at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), the study compared the effects of the two diets on calorie intake, hormone levels, body weight and more. The results, published in Nature Medicine, broaden the understanding of how restricting carbohydrates or fats in the diet can affect health.

“Foods high in fat are believed to result in excessive calorie intake because they have too many calories per bite. Alternatively, foods rich in carbohydrates can cause large fluctuations in glucose and insulin in the blood that can increase hunger and lead to overeating. “said NIDDK Senior Investigator Kevin Hall, Ph.D., lead author of the study. “Our study was designed to determine whether diets high in carbohydrates or fats result in higher calorie intake.”

The researchers housed 20 adults without diabetes for four continuous weeks at the NIH Clinical Center’s Metabolic Clinical Research Unit. Participants, 11 men and nine women, received a low-fat, vegetable-based diet or a low-carbohydrate diet based on animals for two weeks, immediately followed by two weeks on the alternative diet. The low-fat diet was rich in carbohydrates. The low-carb diet was high in fat. Both diets were minimally processed and contained equivalent amounts of starchless vegetables. Participants were given three meals a day, plus snacks, and could eat as much as they liked.

The main results showed that people who were on a low-fat diet ate 550 to 700 fewer calories per day than when they were on a low-carbohydrate diet. Despite the large differences in calorie intake, participants did not report differences in hunger, pleasure in meals or satiety between the two diets. Participants lost weight on both diets, but only the low-fat diet led to a significant loss of body fat.

“Despite eating foods with an abundance of high glycemic index carbohydrates that resulted in pronounced fluctuations in blood glucose and insulin, people who ate the low-fat, plant-based diet showed a significant reduction in calorie intake and loss of body fat, which defies the idea that high-carbohydrate diets alone lead people to overeat. On the other hand, the low-carb, animal-based diet did not result in weight gain, despite being rich in fat, “said Hall.

These findings suggest that the factors that result in overeating and gaining weight are more complex than the amount of carbohydrates or fat in the diet. For example, Hall’s lab showed last year that a diet rich in ultra-processed foods leads to overeating and weight gain compared to a minimally processed combined diet for carbohydrates and fat.

The low-fat, vegetable-based diet contained 10.3% fat and 75.2% carbohydrates, while the low-carbohydrate diet based on animals contained 10% carbohydrates and 75.8% fat. Both diets contained about 14% protein and were matched for the total calories presented to individuals, although the low-carb diet has twice as many calories per gram of food as the low-fat diet. On the low-fat menu, dinner can consist of roasted sweet potatoes, chickpeas, broccoli and oranges, while a low-carb dinner can be braised meat with cauliflower rice. Participants could eat what and how much they wanted from the meals they received.

“Interestingly, our findings suggest benefits for both diets, at least in the short term. While the low-fat, plant-based diet helps curb appetite, the low-carb, animal-based diet resulted in insulin lower and more stable and glucose levels, “said Hall. “We still don’t know whether these differences would be sustained in the long run.”

The researchers noted that the study was not designed to make dietary recommendations for weight loss, and the results may have been different if the participants were actively trying to lose weight. In addition, all meals were prepared and provided to participants in an inpatient setting, which can make it difficult to repeat results outside the laboratory, where factors such as food costs, food availability and meal preparation restrictions can make the adherence to diets is a challenge. The tightly controlled clinical environment, however, ensured the objective measurement of food intake and the accuracy of the data.

“To help us achieve good nutrition, rigorous science is critical and of particular importance now, in light of the COVID-19 pandemic, as we aim to identify strategies to help us stay healthy,” said NIDDK director Griffin P. Rodgers , MD “This study brings us closer to answering long-sought questions about how what we eat affects our health.”

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