Research suggests that high protein diets are effective at causing weight loss. For example, in a recent study published in the American Journal of Clinical Nutrition1, researchers used a meta-analysis to compare the effects of standard protein calorie restricted diets to high protein calorie restricted diets, on weight loss in overweight subjects. The studies included in the analysis were isocaloric for fat, but had variation in the carbohydrate and protein content. The results showed that high protein diets cause significantly more favourable losses in body weight (-0.79 kg) and body fat (-0.87 kg) over 12 weeks compared to lower protein diets. The higher protein diets also caused significant increases in fat free mass (+0.43 kg) and resting metabolic rate (+142 kcal per day) compared to standard protein diets. These results support a growing body of evidence showing that high protein diets are beneficial at causing weight loss. The mechanisms by which such diets might be beneficial therefore deserves some consideration.
One of the most credible theories as to why high protein diets are beneficial at causing weight loss is because of the satiety that they cause. In fact in the above study, the authors reported that satiety was greater for higher protein diets in 3 of the 5 studies included in the analysis. This satiety effect may be caused by the presence of undigested protein stimulating the release of cholecystokinin (CCK) from I cells located in the mucosa of the duodenum. The CCK then enters the plasma and stimulates receptors on the wall of the stomach reducing stomach motility, and this may inhibit further food intake due to an increase in the length of time that the chyme is present in the upper intestine. Plasma CCK also signals the hypothalamus to reduce appetite. The consistent reports of reduced appetite with high protein diets supports this theory and suggest that protein plays an important role in satiety. However, high protein diets can also be low carbohydrate diets, and while high protein may increase satiety, so too may low carbohydrate diets.
The reason that low carbohydrate diets can induce satiety is because high intakes of refined starch, as found in Western diets, can cause detrimental blood sugar changes. Refined carbohydrates cause an accelerated glucose absorption that stimulates a rapid rise in blood sugar, and this stimulates an excessive insulin release. Elevated insulin levels cause rapid glucose disposal, a precipitous fall in blood sugar, and thus create a hypoglycaemic state. Hypoglycaemia is a particularly strong driver of appetite and so following consumption of refined carbohydrates, a strong desire to eat may be experienced postprandially, particularly in insulin insensitive individuals. Because insulin is also a potent inhibitor of orectic peptides such as neuropeptide Y, and can increase hypothalamic sensitivity to satiety signals such as CKK, resistance in the insulin receptors can prevent proper hypothalamic feedback inhibition of appetite and cause hyperphagia. Low carbohydrate diets reverse these physiological changes and return correct appetite regulation.
The possibility therefore arises that the benefits of high protein diets might result from the fact that by definition, they can also be low carbohydrate diets. Research is increasingly seeing refined carbohydrates as the cause of the current obesity epidemic due mainly to the dysfunctional metabolic changes fructose can cause regarding blood sugar and insulin homeostasis. That such diets lead to metabolic changes that include distortions to the normal energy regulatory systems, including detrimental appetite changes, suggests that these factors should be considered in any experiment that alters the macronutrient ratios in order to cause weight loss. Increasing protein within a diet should not therefore be considered in isolation without addressing the confounding variable that is carbohydrate intake. It was such lack of due process that mistakenly identified high saturated fat diets as a risk factor for cardiovascular disease, when infact high saturated fat diets and low fibre diets are synonymous and cannot be considered mutually exclusive.
Because a mechanism exist by which protein can cause satiety, it is likely that it may have beneficial effects. In particular the improved nitrogen retention of such diets explains their ability to reduce loss of skeletal muscle during periods of weight loss. This is an important consideration, because it is skeletal muscle mass that dictates the resting metabolic rate of the individual, and this can have a dramatic effect on 24 hour energy expenditure. Athletes are already aware of the importance of a positive nitrogen balance during periods of heavy training in order to maintain or increase skeletal muscle mass. Therefore logic suggests that high protein diets are useful tools in those wishing to lose excess body fat. However, that high protein diets may also low carbohydrate diets should raise particular questions as to where the main benefit lies. This highlights the difficulty faced by researchers in macronutrient studies, because it is not possible to alter one macronutrient intake without affecting that of another.
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