The scientific community knows that high protein diets induce early marked metabolic changes in human and animal models, especially when the diet contains at least 50 percent of energy as protein, but the physiological and functional consequences of a long-term high protein (HP) diet have not been fully explored. Now, a long-term study involving male rats has found that a protein intake of three times the requirements did not produce any adverse effects in key systems.
A New Study
Researchers are aware that no long-term interventional human studies on the issue exist nor are there any complete toxicological studies on high protein diet effects. This has led to a new investigation of the wide range of biochemical, anatomical and histological parameters to determine whether long-term ingestion of a high protein diet could have adverse and/or beneficial effects in an obesity prone strain of rats. The authors of "A long-term high-protein diet markedly reduces adipose tissue without major side-effects in Wistar male rats," are Magali Lacroix, Claire Gaudichon, Celine Morens, Veronique Mathe, Daniel Tome, and Jean-Francois Huneau, all from the Physiologie de la Nutrition et du Comportement alimentaire, Institut National Agronomique Paris-Grignon, Paris; and Antoine Martin, at the Service d'Anatomie pathologique, Hôpital Avicenne, Bobigny, both in France. Their findings appear in the Articles in Press section of the American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. The journal is one of 14 scientific publications issued each month by the American Physiological Society (APS) (www.the-aps.org).
Methodology
This study used male Wistar rats that were administered either a no protein diet or a high protein diet for six months. Detailed body composition, biomarkers of oxidative stress [reduced and oxidized glutathione, thiobarbituric acid reactive substances, expression of glutamy-cysteine-ligase, and detoxification function (Glutathione-S-Transferase), plasma hormones (insulin, cortisol and leptine)] and liver and kidney histopathology were investigated. Calcium balance was also assessed over four months after this protocol.
Results
This study sought to estimate the consequences of a long-term intake of high levels of protein in an obesity-prone rat strain. The results showed that the long-term effect of eating protein at will led to a markedly reduced food intake and lowered white adipose tissue. At the same time, basal blood insulin, leptin and triglyceride levels, and glucose tolerance were improved. Calcium balance was not affected by a high intake of milk proteins. Moreover, in contrast to what has generally been admitted, no adverse effects of the high protein diet were reported, particularly regarding kidney and liver health. After six months of the experiment, the body weight of rats fed the high protein diet was 18 percent lower than that of rats fed the non-protein diet. Body composition measurements revealed remarkable differences between the two groups, especially concerning the subcutaneous fat pad.
This study also revealed that the weight reduction in rats fed the high protein diet was strongly associated with lower basal blood sugar and insulin levels, as previously described, and improved glucose tolerance. As the isocaloric exchange between high and low protein diets was performed on carbohydrate, the flattening effect of the high protein diet on insulin and glucose basal levels could be attributed to its reduced carbohydrate content. This contrasts with the results of many studies that have acknowledged the fact that high protein intakes induce an increase in glucose and insulin concentrations.
Conclusions
This unique long-term study found that in male rats, a protein intake of three times the requirements did not produce any adverse effects on the renal and hepatic functions, on oxidative stress or on the calcium balance. On the contrary, exchanging carbohydrates for proteins was beneficial regarding body composition, basal triglycerides, glucose, leptin, and insulin plasma concentrations. The results of the present study agree with the idea that long-term dietary management is of major importance to preventing obesity.
This study will not settle the long-standing debate regarding the merits of a high protein diet. However, these findings will provide support to those who advocate such a regimen for weight loss and better health.
Source: Online edition of the American Journal of Physiology--Regulatory, Integrative and Comparative Physiology. The journal is one of 14 published each month by the American Physiological Society (www.the-aps.org).
The American Physiological Society (APS) was founded in 1887 to foster basic and applied science, much of it relating to human health. The Bethesda, MD-based Society has more than 10,000 members and publishes 3,800 articles in its 14 peer-reviewed journals every year.