High protein diets are beneficial at causing weight loss. The exact reason for this is likely multifactorial in nature. The presence of undigested proteins in the stomach inhibit gastric emptying and this maintains any carbohydrate in the stomach for longer. This has a beneficial glycaemic effect and allows the glucose from starch to enter the blood more slowly, and this is thought to have beneficial effects on insulin sensitivity. In addition, protein is used structurally in the body and this means that it contributes less towards energy needs than carbohydrate and fat and is therefore less likely to be stored in adipose tissue. High protein diets are also low carbohydrate diets and as refined carbohydrates and crystalline fructose are probable drivers of weight gain and obesity, high protein diets are beneficial at preventing this outcome. The anecdotal success of higher protein diets such as the atkins and zone diet support the large body of clinical and epidemiological data showing a role for high protein diets in weight loss.
However, there are claims that high protein diets are detrimental to the health in the long term. One problem of high protein diets involves the effects of protein metabolism on bone health. It was Wachman and Bernstein who hypothesised in 1968 that high protein diets could increase metabolic acidosis and that this in term would increase the resorption of minerals from bone. These minerals enter the blood where they act as buffers to return the pH to normal physiological levels. Over time this would lead to bone loss and eventually osteoporosis. A recent study published in the American Journal of Clinical Nutrition1 investigated the effects of two high protein weight loss diets on bone health. Obese women were randomly assigned to receive either more than 90 grams (high) or less than 80 grams (normal) of protein per day for 24 months, which equated to 32 and 22 % of calories as protein respectively. The subjects had a mean energy intake of 1314 kcals per day and this included 1300 mg of calcium from dietary sources.
As has been shown for high protein diets previously, the subjects in both diets lost weight. The weight loss in the high protein group was 7.9 kg and the weight loss in the normal protein group was 8.9 kg. Some of the weight loss experienced by both groups was lean mass, and this amounted to 1.7 kg in the high protein group and 2 kg in the normal protein group. The higher protein diet might therefore have spared skeletal muscle mass compared to the normal protein group, although this difference was small. When the authors analysed the bone mineral density from the lumbar spine vertebra, the forearm, the femoral neck and the hip they found that the mean loss was 1 to 2 % but that there was no difference in bone loss between the two diets, and no difference in the loss of calcium in the urine between the two diets. This was despite some biomarkers for bone loss being affected by diet type over time. The 1 to 2 % loss of bone mineral density was well within the range of that expected in postmenopausal women.
Dr Robert Barrington’s Nutritional Recommendation: High protein diets are beneficial for weight loss. Despite a large body of evidence supporting a role for a high protein intake in health, accusations continue to be made about their detrimental effects. Of course in such circumstances it is always interesting to consider confounding variables. The theory that high protein diets can cause bone loss stems from their ability to cause metabolic acidosis which in turn increases bone resorption due to the necessity to buffer the low pH with minerals from bone. Because plant food are blood alkalising agent due to their high content of potassium, high protein diets that also contain high amounts of plant foods are not able to cause metabolic acidosis. Western diets contain high amounts of animal protein and are a likely contributory factor in the development of osteoporosis. However, it is not possible to blame the high protein content of the Western diet for increasing the risk of osteoporosis as high protein diets that contain high amounts of plant foods do not have this effect. Of course, athletes are renowned for eating high protein diets and those that perform strength training are known to have particularly dense levels of bone minerals in their skeletons. As long as a high protein content to a diet is balanced by a high intake of plant foods, there is no reason to believe that detrimental health outcomes would result.
RdB
