High protein diets are popular for their weight loss effect. In this regard, high levels of dietary protein is able to cause weight loss possibly through a combination of factors. These include appetite suppression, lean mass accumulation and inefficient metabolic storage. Increasing the protein content of the diet is usually performed in combination with reductions in carbohydrate content, in order to maintain an isoenergetic intake. Carbohydrate reductions can also lead to weight loss effects if the carbohydrate removed from the diet is of the low quality refined variety. This is particularly true for sugars, which have an obesogenic effect. High protein low carbohydrate diets are therefore popular, and they can be useful tools to improve body composition. High protein low carbohydrate diets can also be healthy, as long as they are based on plant foods. High protein diets however, can be problematic if care is not taken to maintain adequate intakes of plant foods. This relates to the acid forming nature of high protein diets.
Consuming high protein diets can therefore be a double edged sword. While they can contribute to a weight loss effect, they may also lower the pH of the blood and thus alter the delicate acid-base balance of the blood. Researchers have assessed the changes to the acid-base balance caused by high protein low carbohydrate diets. For example, in one study1 researchers fed subjects a balanced diet that was based on the subjects habitual diet. Then the subjects were fed a severely carbohydrate limited diet for 2 weeks. This diet was based on the induction phase of the Atkins diet and contained reasonable high levels of protein and fat to compensate for the low carbohydrate intake. Subjects then consumed a diet similar to the maintenance phase of the Atkins diet, which had a more balanced level of carbohydrate, protein and fat. The authors then monitored a number of physiological parameters in the subjects including the urinary pH, calcium excretion and urinary uric acid levels.
The results of the study showed that the pH of the urine decreased from 6.09 to 5.56 and then back to 5.67 as the protein content of the diet increased and then decreased with the changing protein levels. Acid excretion increased by 56 mEq per day with the Atkins induction diet and by 51 mEq per day with the Atkins maintenance diet, from the baseline habitual diet. Urinary excretion of urate (dissociated uric acid) also increased significantly with increasing protein intake. In addition, the urinary calcium excretion increased from 160 mg per day with the habitual diet to 258 mg per day with the Atkins induction diet and to 248 mg per day with the Atkins maintenance diet. However, the increased calcium excretion was not compensated for by an increase in intestinal absorption of calcium. Calcium balance therefore decreased by 130 mg per day and 90 mg per day with the induction and maintenance Atkins diets, respectively. High protein diets may therefore acidify the blood and increase urinary calcium excretion.
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