Is too much protein really an issue?

Since there is evidence that protein intakes above the RDA may be beneficial to athletes, a risk–benefit analysis may be useful. An important consideration is the potential harm that may arise from elevated protein intakes. There is little research into the maximum tolerable protein intake in healthy individuals. It has been suggested that excessive protein intakes may increase calcium loss, thus affecting bone health. However, since a major portion of bone is protein, excessive protein does not appear to influence bone health. High protein intakes have been suggested to pose a risk for the kidneys but, in healthy individuals with no underlying kidney disease (presumably most elite athletes), there is no evidence for harm to kidneys with higher intakes. Certainly, it would be detrimental for an athlete to consume excess protein at the expense of other nutrients required to support the necessary level of training and competition. There is a suggestion that intakes greater than 40% of total energy intake might be the upper limit. Protein intakes greater than 40% may limit intake of fat and/or carbohydrates, thus compromising the benefits of these nutrients. However, given the high energy intakes of most elite athletes, protein intakes higher than 40% are unlikely in most. Even a small female restricting energy intake and consuming only 1500 kcal would need to consume 150 g of protein to reach 40%.

Indeed, the recent study Dawson-Hughes et al. did not confirm the perception that increased dietary protein results in urinary calcium loss.[36] According to Dawson-Hughes et al., "Theconstellation of findings that meat supplements containing 55 g/d protein, when exchanged for carbohydrate did not significantlyincrease urinary calcium excretion and were associated withhigher levels of serum IGF-I and lower levels of the bone resorption marker, N-telopeptide, together with a lack of significant correlationof urinary N-telopeptide with urinary calcium excretion in thehigh protein group (in contrast to the low protein) point tothe possibility that higher meat intake may potentially improvebone mass in many older men and women."

Finally, the cross-cultural and population studies that showed a positive association between animal-protein intake and hip fracture risk did not consider other lifestyle or dietary factors that may protect or increase the risk of fracture.[35] It is of some interest that the author of the most cited paper favoring the earlier hypothesis that high-protein intake promotes osteoporosis no longer believes that protein is harmful to bone.[34] In fact, he concluded that the balance of the evidence seems to indicate the opposite.

Despite its role in nitrogen excretion, there are presently no data in the scientific literature demonstrating the healthy kidney will be damaged by the increased demands of protein consumed in quantities above the Recommended Dietary Allowance (RDA). Furthermore, real world examples support this contention since kidney problems are nonexistent in the bodybuilding community in which high-protein intake has been the norm for over half a century.[3] Recently, Walser published comprehensive review on protein intake and renal function, which states: "it is clear that protein restriction does not prevent decline in renal function with age, and, in fact, is the major cause of that decline. A better way to prevent the decline would be to increase protein intake. there is no reason to restrict protein intake in healthy individuals in order to protect the kidney."[4]

Conclusion

Although excessive protein intake remains a health concern in individuals with pre-existing renal disease, the literature lacks significant research demonstrating a link between protein intake and the initiation or progression of renal disease in healthy individuals. More importantly, evidence suggests that protein-induced changes in renal function are likely a normal adaptative mechanism well within the functional limits of a healthy kidney. Without question, long-term studies are needed to clarify the scant evidence currently available regarding this relationship. At present, there is not sufficient proof to warrant public health directives aimed at restricting dietary protein intake in healthy adults for the purpose of preserving renal function.[