Dietary protein seems to be more satiating – able to satisfy hunger, that is – than carbohydrate and fat.
The typical Western (especially American) diet derives about 55-60% of total calories from carbohydrates. When carbohydrate intake is very low, under 20-30 grams per day for example, fat stores are utilized as a source of energy to replace carb calories, resulting in fat breakdown waste products called ketone bodies. These are ketogenic diets. In them, carbs are replaced usually by both extra fat and extra protein.
Each of 17 obese men, 20 to 65 years old, were placed on two separate diets for four weeks each time. Average weight was 111 kg. Average body mass index was 35. This was a residential program, but the subjects were allowed to leave and go to work.
- Diet 1: high-protein, low-carbohydrate, ketogenic. 30%, 4%, and 66% of energy (calories) as protein, carbohydrate, and fat, respectively.
- Diet 2: high-protein, medium-carbohydrate, nonketogenic. 30%, 35%, and 35% of calories as protein, carb, and fat, respectively.
Actually 20 men signed up, but three dropped out for personal reasons after starting.
They could eat as much as they wanted.
Subjects had no overall preference for either diet. No differences in the diets for desire to eat, preoccupation with food, or fullness. Weight loss was greater for the low-carb diet tahn with the medium-carb diet: 6.34 kg vs 4.35 (P < 0.001). Subjects lost more weight on their first diet than on their second. Fasting glucose and HOMA-IR (a test of insulin resistance) was lower than baseline for the low-carb diet but not the other. Total and LDL cholesterol were tended to fall in response to both diets, but to a statistically significantly great degree only on the medium-carb diet. When eating the low-carb diet, subjects ate 300 calories per day less than on the medium-carb diet. [ketones were measures?]
We have to assume that study subjects were of Scottish descent. Applicability of these results to other ethnic groups is not assured. Similarly, results don’t necessarily apply to women.
I’m surprised the medium-carb dieters, eating all they wanted, lost weight at all. Must be a result of the high protein content or lower-than usual carbohydrate content of the study diet. Study authors cite others who found that doubling protein intake from 15 to 30% of calories reduces food intake, which should lead to weight loss.
Since protein content was the same on both diets, the greater weight loss seen on the low-carb ketogenic diet was the result of lower caloric intake, in turn due to less hunger. The reduced energy intake could be due to lower carb or higher fat intake, or both. The researchers cite one study finding no satiating effect of fat. Some say that ketone bodies reduce appetite.
Although the medium-carb diet showed greater improvements in total and LDL cholesterol, the low-carb diet changes trended in the “right” direction (down).
On the low-carb ketogenic diet, lower glucose levels and insulin resistance would tend to help people with (or prone to) type 2 diabetes, prediabetes, and some cases of metabolic syndrome.
Steve Parker, M.D.
Johnstone, Alexandra, et al. Effects of a high-protein ketogenic diet on hunger, appetite, and weight loss in obese men feeding ad libitum. American Journal of Clinical Nutrition, 87 (2008): 44-55.
Weigle, D.S., et al. A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations. American Journal of Clinical Nutrition, 82 (2005): 41-48.