Metabolic Advantage of a High Protein, Low-Carbohydrate Diet

The metabolic advantage in low-carbohydrate diets – greater weight loss than isocaloric diets of different composition.

It is broadly held that a calorie is a calorie and by this it is usually mean that two isocaloric diets lead to the same weight loss. A calorie is a measure of heat energy and when food is being referenced, it represents the total amount of energy stored in food. Used in this way, all calories are equal, whether from fat, protein, or carbohy-draates. However, the idea that a diet calorie is a calorie as far as how it effects useable and storable energy, body weight, and composition under all conditions is simplistic at best.

Many factors influence how much energy is actually derived from dietary macro¬nutrient intake. Calories can be "wasted" in many ways. Decreased absorption from the GI tract and increased excretion are two obvious ways. An increase in thermo¬genesis and energy expenditure will also waste calories. In the case of thermogenesis (thermic effect of feeding), or the heat generated in processing food, the thermic effect of nutrients is approximately 2-3% for lipids, 6-8% for carbohydrates, and 25-30% for proteins. This in itself is almost enough to explain the metabolic advantage of low-carb, high-protein diets. But there's more involved. For example it has been shown that increasing dietary protein increases fat oxidation. As well, the calorie cost in the use of the various macronutrients for energy also differs, with protein being the least efficient.

Through the interaction of both cytoplasmic and mitochondrial pathways it is possible to store both carbohydrates and protein as body fat. In the case of protein, it involves both the glucogenic and ketogenic amino acids. The ketogenic influence on body fat is obvious, since ketones are readily metabolized to two carbon units and can be directly used for lipogenesis. The glucogenic amino acids can enter the TCA cycle as intermediates and either through a short or long pathway end up as 2 carbon units that can be exported to the cytoplasm for lipogenesis.

Low-carbohydrate and higher-protein diets do more than increase weight loss. It's also been shown that low-carbohydrate, high-protein diets favorably affect body mass and composition and that these changes are independent of energy intake.

But this is nothing new. Prior research also found that low-carbohydrate diet results in a significant fat loss and an increased retention of muscle mass, either alone or in comparison with a high-carbohydrate diet.

For example, in 1971 a group of researchers looked at the effects of three diets that had the same calorie and protein levels, but varying fat and carbohydrate content.They found that, as the carbohydrates in the diets went down, there was an increased weight and fat loss. In other words, the men who were on the lower-carbohydrate diets lost the most weight and body fat.

In 1998, another study, this time involving obese teenagers, came up with similar results. After 8 weeks on a low-carbohydrate diet, the teens not only lost significant amounts of weight and body fat, but even managed to increase their lean body mass.

In the study, a 6-week carbohydrate-restricted diet resulated in a favorable response in body composition (decreased fat mass and increased lean body mass) in normal-weight men. The results of this study indicate that a low-carbohydrate diet mobilizes and burns up body fat more than a high-carbohydrate diet, while at the same time preserving muscle mass.

Insulin, by varying the amount of fat and carbohydrates storage, can also make the body more efficient in the use of dietary calories. For example, reduced insulin levels, increased insulin sensitivity and even lack of an insulin receptor in fat tissue leads to increase in energy expenditure and helps to protect against obesity even in obesiogenic environments.

Calories from different macronutrient mixes can affect appetite, satiety, compliance, short- and long-term compensatory responses, and changes in the oxidation of other substrates and thus make a difference as far as weight loss and body composition are concerned. For example, one study found that fat mass status and the macronutrient composition of an acute diet intake influence substrate oxida¬tion rates. This study found that the intake of a high-protein, lower-carbohydrate single meal improved postprandial lipid oxidation in obese women and produced an increased thermic response. These responses were due to elevated insulin levels that occur with higher-carbohydrate meals as well as the increased energy needs associated with the higher-protein meal.

The enhanced weight loss on protein-enriched diets as compared with balanced diets has been often assigned to a greater food-derived thermogenesis, an effect generally attributed to the metabolic costs of peptide-bond synthesis and breakdown, urogenesis, and gluconeogenesis.