Dietary protein intake and renal function.
It has been speculated that high dietary intakes of protein, because they could give rise to large amounts of nitrogenous breakdown products, could result in excessive demands on the kidneys with resultant increased risk of renal damage or failure. This speculation has not been substantiated, and as Millward (1999) points out "there is now only a weak case for risk for renal function". Furthermore, the assumption that high protein diets could result in increased urinary calcium loss has also now been shown to be largely unfounded, and rather than the risk of osteoporosis which has been predicted, benefits may occur (Millward, opus cit. "For bone health the established views of risk of high protein intakes are not supported by newly-emerging data, with benefit indicated in the elderly. There is also circumstantial evidence for benefit on blood pressure and stroke mortality.").
Recent studies are entirely supportive of the proposition that relatively high dietary protein intakes have no negative consequences for renal function in either short or long term in subjects with healthy kidneys. Skov et al. (1999), for example, concluded that the kidney adapts well to moderate changes in protein intake without adverse effects, while Poortmans an Dellalieux (2000) concluded that protein intakes under 2.8 g/kg/day (about 200 g/day for the "standardized" male of 70 kg) did not impair renal function as indicated by the usual testing methods. Brandle et al. (1996) concluded that protein intake is a controlling factor for glomerular filtration rate in those with normal renal function, in that glomerular filtration rate increases as protein intake rises. Kitazato et al. (2002), however, showed that animal protein had a greater influence on glomerular filtration rate and renal plasma flow than did vegetable protein.
It may be concluded that the healthy kidney will not be affected adversely by protein intakes that are up to 2 or 3 times the normally accepted RDA for protein, thus up to about 2.4 g protein/kg/day. Practically, this confers a wide safety margin, since the benefits of high protein diets, whether used for sports purposes, for weight loss or for paediatric nutritional purposes, tend to level off in the range of 1.3 - 1.6 g protein/kg/day, and administering higher amounts does not result in proportionally greater benefits.
While the protein itself does not create risks of renal damage, the form of the protein may have connotations. Knight et al. (2003), in a prospective study lasting 11 years, showed that animal protein in the form of meat could cause a worsening of kidney function in women who had initial evidence of mild impairment of renal function, but that both dairy and vegetable proteins were without negative effect. This can be readily explained by the fact that meat contains nitrogenous substances other than protein, such as nucleic acids and creatine, which also have to be eliminated via the kidneys after some degree of metabolic breakdown, and in the presence of significant meat intake this excretion may impose a burden on the kidneys. Isolated plant and dairy proteins, such as soy protein isolate, whey protein isolate and caseinates, do not contain these materials and are thus unlikely, as confirmed by this study, to impose an extra burden on the kidneys. In the other studies cited in this discussion, the main emphasis was on the nitrogenous breakdown products derived from the amino-acids. In the case of isolated and purified plant (soy) and milk proteins, the only nitrogenous breakdown products are, in fact, from the amino-acids. However, when protein is given as natural and unprocessed proteins, particularly of animal origin, the metabolism of the nucleic acids must also be taken into account, since these will result in increased amounts of uric acid being formed. This is especially the case with highly cellular "organ meats" such as liver, that are proportionately rich in nucleic acids, and in fact, such "rich food" has been known to be associated with disorders caused by excessive amounts of uric acid for many years, particularly gout. A corollary of this is that high levels of uric acid in plasma may also predispose to uric acid kidney stones.
High protein intakes are not recommended for those with reduced renal function. While there is some controversy at present about the protein intake that is optimal for those in chronic renal failure, there is unanimity that such patients require relatively low protein intakes. This is logical, since other studies summarized here have shown the ability of the healthy kidney to adapt glomerular filtration rates over a wide range in response to dietary and metabolic factors. This ability is lost in renal failure, and dietary intake has to be tailored to the residual renal function.