Evidence suggests that high protein low carbohydrate diets are effective at causing weight loss when compared to higher carbohydrate lower protein equivalents. High protein diets may be beneficial during weight loss because they improve nitrogen retention and therefore are more efficient at maintaining lean body mass. In addition, protein slows digestion by limiting food expulsion from the stomach and therefore reduces the glycaemic effects of carbohydrate. Further, some evidence suggests that protein is able to decrease appetite following a meal due to the regulation of satiety hormones. Study design and methodology can have a large influence on the outcome of research, and as a result some studies have shown no benefit to high protein diets with regard weight loss. Because of this, pooling data in a meta-analysis can be a useful tool to help clarify the effects of high protein diets to researchers.
For example a meta-analysis published in the American Journal of Clinical Nutrition in 20061 used meta-regression to determine the effects of variations in protein and carbohydrate intake on weight loss during energy restriction. To be included, the studies had to have been published in the English language, feature energy restriction of more than 1000 kcal per day and have a duration of over 4 weeks. After analysing the 87 studies, the authors reported that in studies of less than 12 weeks, protein intakes greater than 1.05 g/kg were associated with 0.60 kg additional fat free-mass retention compared to diets with protein intakes below 1.05 g/kg. This improved retention of lean mass increased to 1.21 kg in studies that were over 12 weeks in duration. These results therefore support the contention that protein is effective at maintaining lean mass during energy restriction, in the short-term.
When the authors analysed carbohydrate intake, they found that diets comprising of 35 to 41.4 % energy from carbohydrate were associated with a 1.74 kg greater weight loss, a 2.05 kg greater loss of fat mass, a 1.29 % greater loss in percentage body fat, but a 0.69 kg greater loss of fat-free mass, when compared to diets of higher carbohydrate content. In studies over 12 weeks in duration, the same low carbohydrate intakes were associated with a 6.56 kg greater weight loss, a 5.57 kg greater loss of fat mass, a 3.55 % greater loss in percentage body fat, but a 1.74 kg greater loss of fat-free mass. No weight loss effects were seen from increasing protein intake if not combined with lowered carbohydrate. Therefore while higher protein diets appear to prevent loss of lean mass during energy restriction, lower carbohydrate diets appear to increase weight loss.
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