Deep Dive: Keto, carbs, and calories

According to U.S. News, the most searched diet in 2020 was the ketogenic diet. The ketogenic diet is a high-fat (HF), low-carbohydrate diet that was first developed in the 1920s to treat children with epilepsy^[1]. Since then, the evidence has been growing for other therapeutic uses^[2] of the keto diet for pathologies, including obesity, diabetes, and cardiovascular disease, while some studies even suggest a potential for neurological diseases, cancer, and more. However, as with most diets, identifying the mechanism behind any associated benefits is rather difficult because food is complex and has intricate interactions with behavior and physiology that cannot completely be accounted for with general macronutrient restrictions.

One of the debated benefits of a ketogenic diet is that it may improve appetite control. This is supported by the carbohydrate-insulin model^[3] of obesity, which suggests that elevations in postprandial insulin from intake of high-glycemic carbohydrates and decreases in circulating levels of metabolic fuel promote increased body fat mass, and subsequently increase hunger and energy intake. A systematic review and meta-analysis reported decreased hunger^[4] and desire to eat in participants following a ketogenic diet. However, only three studies were included. Of these three studies, only one reported energy intake and verified a state of ketosis, which is thought to suppress appetite^[5]. Moreover, there is evidence to suggest that high-fat foods actually promote overconsumption^[6] and excess energy intake, which is attributed to their weak effect on satiety and reduction in food hedonics^[7] (food wanting and liking) after consumption, as well as their higher energy density. However, these types of findings have generally not been reported in the context of carbohydrate restriction, and advocates of very low-carbohydrate diets might suggest that the levels of carbohydrate may not be low enough to invoke the positive benefits.

It’s interesting that whether participants follow an HF ketogenic diet or a low-fat high-carbohydrate (HC) diet, studies either fail^[8] to show long term weight loss beyond a few pounds, or find stronger efficacy but little difference between HF and HC diets. Lack of efficacy could be explained by a decreased diet adherence due to increased appetite^[9] that is proportional to bodyweight loss, along with inadequate behavioral and lifestyle adaptation to the diet. Thus, an understanding of which aspects of dietary modification influence adherence and appetite^[10] could be key to developing better strategies for weight loss management. Even when provided with the food, outpatient study participants often do not adhere^[11] to prescribed diets, while inpatient studies (that adequately control and monitor diet adherence) have not investigated HF versus HC diets that are sufficiently low in the macronutrient of interest to reveal the claimed health benefits.

Since no studies have compared the effects of high-fat versus high-carbohydrate diets on energy intake (as an objective indicator of appetite) in a highly controlled manner (to ensure adequate adherence), researchers from the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland conducted an inpatient crossover randomized controlled trial comparing ad libitum energy intake on a ketogenic diet versus a low-fat HC diet.

Despite being an increasingly popular diet, understanding of the high-fat ketogenic diet and its supposed health benefits is limited. One of the most debated benefits of high-fat (HF) versus high-carbohydrate (HC) diets is appetite control, with evidence supporting either side limited by inadequate adherence, diet prescription, and control. To determine whether a HF or HC diet induces greater energy intake, the researchers of the study under review conducted an inpatient crossover randomized controlled trial comparing ad libitum energy intake on an HF ketogenic diet versus an HC diet.