Replacing the carbohydrate in the diet with protein produces a low carbohydrate high protein diet, sometimes simply called a high protein diet. Both the Atkins and Zone diets have relatively high amounts of protein, which replaces the carbohydrate content of the diet. Low carbohydrate high protein diets appear to show beneficial weight and fat loss effects when compared to low fat diets. Low carbohydrate high-protein diets may have these beneficial fat loss effects for a number of reasons. Firstly, lowering the carbohydrate component of the diet improves insulin sensitivity. This occurs because the carbohydrates most often consumed in the typical Western diet are refined grains or refined crystalline sugars, the latter including the compound fructose. Protein also shows beneficial glycaemic effects because protein is the rate limiting step in gastric emptying, and this reduces starch digestion and sugar absorption rates. Lastly, protein has been shown to have satiety inducing effects, which increases adherence to the diet.
A number of studies have compared low carbohydrate diets with traditional low fat energy restricted diets. Meta analyses of such studies have been performed to elucidate the effectiveness of the two diet types in comparison. In one such meta analysis1, the authors compared the weight loss effects of low carbohydrate high protein diets with low fat energy restricted alternatives from studies performed between 2007 and 2009. The results of the analysis showed that the attrition rates in low fat energy restricted groups was significantly higher than in the low-carbohydrate high-protein groups. This supports the contention that high-protein diets induce satiety and this allows easier adherence to the diet. Also, the low-carbohydrate high-protein diets caused significantly more weight loss (-4.02 kg at 6 months) compared to the low fat energy restricted diets. The extra weight loss in the low carbohydrate high protein diet was accompanied by a reduction in blood pressure, as would be expected.
As had been shown previously, total cholesterol increased in the low carbohydrate high protein group, when compared to the low fat energy restricted group. This effects is due to increases in both plasma low density lipoprotein (LDL) and high density lipoprotein (HDL) concentrations. However, low carbohydrate high protein diets tend to favourably affect the LDL to HDL ratio, which is a stronger determinant of cardiovascular risk than total cholesterol. In addition, low carbohydrate high protein diets tend to increase the LDL and HDL particle size which further decreases cardiovascular risk. It is unclear as to whether the reduced cardiovascular risk associated with low carbohydrate high protein diets is caused by the metabolic effects of the diet or is secondary to the weight loss. However, this is a moot point as the fact that cardiovascular risk decreases is the important point, not how it is achieved. Similarly the lowering of blood pressure on such diets may result from the weight loss they cause.