How do low-carb and low-fat diets affect appetite? Original paper

According to the carbohydrate-insulin model of obesity, since carbs spike insulin, eating carbs should promote fat gain more than eating fat.^[1] However, fat has more calories per gram and a lower TEF, and some studies have shown it to be less satiating.^[2] The present study looked at the effects of dietary carbs and fat on caloric intake.

For this crossover study, 20 adults spent 4 weeks in a laboratory. They followed two diets for 2 weeks each, in random order and with no washout period.

The low-carb diet (10% of calories from carbs, 75% from fat) was animal based. The low-fat diet (10% of calories from fat, 75% from carbs) was plant based. Both diets were low in highly processed foods and had equal amounts of non-starchy vegetables. For both diets, the participants could eat as much as they wanted.

Caloric intake was measured at every meal. Body weight was measured daily. DXA scans were taken weekly. Urine was collected for five consecutive days in the second week of each diet. Fasted blood was collected at baseline and at the end of each diet. A mixed-meal test and an oral glucose tolerance test (OGTT) were performed at the end of each diet. The participants assessed meal palatability and their own satiety over three separate days in the second week of each diet.

Urine levels of β-hydroxybutyrate were elevated in the low-carb group, suggesting that the diet was, in fact, ketogenic.

The two diets led to similar weight loss (1–2 kg / 2.2–4.4 lb, over 2 weeks). However, the low-fat diet led to a greater reduction in fat mass, whereas the low-carb diet led to a greater reduction in fat-free mass. The authors point out that the fat-free mass lost could be mostly water, not muscle.

The participants ate almost 700 fewer kilocalories over the 2-week low-fat diet than over the 2-week low-carb diet. Most of the difference (550 kcal) was between the second week of one diet and the second week of the other, when the participants may have been slightly more adapted to their current diet.

Despite the researchers trying to match protein intake, the participants on the low-fat diet ate, on average, 135 fewer kilocalories per day from protein. They had lower nitrogen excretion, however, which suggests lower protein breakdown.

On the low-fat diet, caloric expenditure was lower by about 50 kcal/day. Respiratory quotient was higher, however, which suggests that more carbohydrate was used for energy than on the low-carb diet.

The low-fat diet resulted in lower systolic and diastolic blood pressure and heart rate, higher postprandial blood glucose, higher daily insulin, and higher peak triglycerides yet lower fasting triglycerides. In the mixed-meal test, it produced higher blood glucose, insulin, c-peptide, and lactate, but lower fatty acids. Accordingly, mean lactate was higher and free fatty acids were lower at the end of the low-fat diet.

The low-carb diet produced lower mean and 2-hour glucose in the OGTT. When on the low-carb diet, 9 of the 20 participants met the criteria for “impaired glucose tolerance”, compared to only 3 on the low-fat diet.

Finally, the two diets were similar with regard to insulin sensitivity, insulin levels, c-peptide levels, palatability, and satiety.

The fact that protein nitrogen excretion was greater in the low-carb group may not be a sign of greater muscle breakdown. Instead, as the same group of authors posited in a previous article, it may be a sign of greater utilization of dietary protein for energy,^[3] which might just be an ordinary aspect of switching to a keto diet. If you’d like to know more, check out this NERD article.

Most studies have reported that low-fat diets and low-carb diets have the same effect on weight loss.^[4][5][6]