A common criticism of high protein intakes/diets is the concern that they are damaging to the kidneys. This belief seems to stem from the fact that, in individuals with preexisting kidney damage, protein intake often has to be reduced to prevent further development of the disease. Incorrectly, this has been turned around to suggest that high-protein intakes are damaging to the kidneys (1).
There is at best a weak case to be made for a risk of high protein intakes on kidney function; quite in fact, some research suggesting a beneficial effect of higher protein intakes on kidney function (2). Simply put, the adaptations to kidney function that are often cited as indicating ‘strain’ or damage are more likely to simply be normal adaptive effects of varying protein intake (1).
Unfortunately, very little research has directly examined the impact of high protein intakes on kidney function in athletes. One study examined the impact of 2.8 g/kg protein on the kidney function of bodybuilders, no negative effect was seen (3). To my knowledge, higher intakes have not been studied.
Empirically, it’s worth considering that athletes have been habitually consuming large amounts of protein for at least several decades without any reported increase in the incidence of kidney problems. If such a problem were going to occur, it seems likely that it would have shown up by now. While this certainly doesn’t prove that high protein intakes aren’t potentially detrimental to kidney function, the data in support of that idea would seem to be lacking both from a scientific and real-world point of view.
Interestingly, while it’s always been stated that high dietary protein intakes increases fluid requirements, this idea appears to have originated from a military study examining nitrogen balance under conditions of water and energy restriction (1). There is no indication that individuals who are sufficiently hydrated need to go out of their way to increase fluid intake when they are consuming large amounts of protein.
Perhaps one of the most pervasive criticisms of high protein intakes has to do with the impact of protein on bone health and calcium status. This goes back to early nutritional studies which gave purified protein diets and saw a loss of calcium from the body.
Later studies, using whole food proteins (which included other nutrients such as phosphorous) found very different effects. Frankly, the early studies on this topic are irrelevant to normal human nutrition since the consumption of protein in the total absence of other nutrients would be extremely rare; all whole food proteins and protein powders contain micronutrients.
The impact of protein on overall calcium status is more complex than having a simple positive or negative effect as dietary protein can impact on both calcium excretion as well as calcium absorption and utilization. It is the combined effect of these processes which determines the end result in terms of bone health.
In epidemiological studies, a high intake of animal protein increases the risk of bone fractures; as well, a high ratio of animal to vegetable protein intake has also been associated with an increased risk of bone loss (4). In contrast, high intakes of protein improve bone healing, following a fracture for example. This is mediated both by increased calcium absorption as well increased levels of insulin-like growth factor 1(IGF-1), a hormone involved in tissue growth (5). How can this contradiction be reconciled?
Fundamentally, it’s too simplistic to look at protein intake in isolation in terms of its effects on bone health as the protein content of food interacts with other nutrients in that food or in the total diet (6). For example, recent studies suggest an interaction between protein and calcium intake.
When calcium intake is low, high protein intakes appear to have negative effects on bone health. In contrast, when calcium and vitamin D intake are sufficient, protein intake has a beneficial effect on bone health (7). This suggests that ensuring adequate calcium intake (through a sufficient intake of dairy foods, or calcium supplements) is crucial for bone health when a high protein intake is being consumed.
This most likely serves to explain the above contradiction. In the studies where dietary protein intake was found to have a negative impact on bone health, there were other dietary factors playing a role. Calcium or Vitamin D intake may have been insufficient causing an overall negative effect. However, when sufficient calcium and Vitamin D are provided (as they typically are following bone injury), dietary protein has a beneficial impact.
Related to the issue of dietary protein and bone health is a concept referred to as net renal acid load (NRAL). When foods are consumed, they have the potential to produce either a net acidic or net alkaline (basic) effect, which the body, primarily the kidneys has to deal with. NRAL refers to the total amount of acid produced that the kidneys have to process.
Simplistically, protein foods tend to increase the net renal acid load, as does a high intake of sodium relative to potassium. In contrast, fruits and vegetables, along with foods high in potassium, tend to buffer this net acid load and have an overall alkalizing effect on the body. With an excess of acid forming foods in the diet relative to the number of base producing foods, a metabolic acidosis can occur.
The modern diet, with its high reliance on animal proteins and high intake of sodium, along with a low intake of fruits, vegetables and potassium is thought to generate a sub-clinical metabolic acidosis (8). Even a slight increase in the overall acid status of the body can have a number of negative health effects, not the least of which is an impact on hormones important to athletes (9). Ensuring sufficient intake of basic foods (fruits and vegetables) to balance out the acid produced from a high protein intake is one key to avoiding this problem.
From both a bone health and performance standpoint, any athlete consuming a high protein diet must ensure sufficient intake of other foods including plenty of fruits and vegetables to buffer any potential negative effects (10). Using a potassium salt or mixed sodium/potassium salt to ensure adequate potassium intake to offset the high levels of sodium in the modern diet is not a bad idea either.
As a final comment related to this issue, it has been suggested that the impact of diet on the body’s acid balance can impact on exercise performance. It’s well established that low-carbohydrate diets tend to decrease the body’s ability to buffer acid produced during high intensity exercise, for example. This hurts performance during those types of events. Reducing protein intake and increasing carbohydrate intake for 3-5 days prior to an important event has been theorized to increase exercise performance in events lasting 3-7 minutes (11).
Colon Cancer/Heart Disease/Overall Health
A large meat intake, especially red meat, is often claimed to be involved in the development of a number of diseases, especially heart disease and colon cancer. A great deal of this research is based on observational work where individuals consuming a meat-based diet are more likely to get such diseases. As well, there is ample evidence to suggest health benefits with vegetarian diets (12).
However, as with the protein and bone health issue, you can’t simply isolate protein/meat intake from other aspects of the diet. This is important when looking at the research as most of it tends to be epidemiological in nature, that is it looks at large populations of individuals and tries to draws correlations between different measured variables. This can lead researchers to draw incorrect conclusions.
For example, modern meat based diets are also typically very high in fat with typical cuts of red meat being high in saturated fat, a known risk factor for various diseases. In contrast, lean red meats, trimmed of visible fat, have a drastically different impact on the risk of cardiac disease (13). As well, unprocessed lean red meat doesn’t increase markers of inflammation or oxidation (14). In addition to potential cancer promoting factors, meat also contains a number of cancer preventing factors (15). Replacement of carbohydrate with lean red meat has also been shown to lower of blood pressure (16). The key here, of course, is that lean red meat, as opposed to the fattier cuts commonly consumed were studied.
Diets high in meat are often low in fruits and vegetables (meaning a low intake of important micronutrients as well as fiber) and research suggests that it is the lack of those foods (fruits, vegetables) more so than the presence of red meat that is responsible for any increased cancer risk (17). High fat intakes have also been associated with low food variety and low intakes of fruits and vegetables (18); this would further contribute to the apparent link between consuming fatty meat and health risk.
Put differently, there is going to be a fairly large difference in the overall impact of a diet that is high in animal protein, high in fat, low in fruits and vegetables (and thus low in fiber and other important nutrients) which may be accompanied with other health risks such as inactivity, being obese, etc. This would be held in complete contrast to an athletic diet containing large amounts of lean meats along with a large fruit and vegetable intake, high levels of activity, maintenance of a low level of body fat, etc.
As I mentioned above with regards to bone health any diet high in animal protein must be accompanied by a high intake of fruits and vegetables. As well, leaner cuts of meat (especially red meat) should be chosen whenever possible.
A number of health risks have been attributed to the consumption of high protein intakes, this includes potential problems with the kidneys, bone health, metabolic acidosis and certain types of cancers. For the most part, these risks tend to be extremely overstated.
While high protein intakes may cause problems when there is pre-existing kidney disease, no research suggests that high protein intakes cause kidney damage. While there is potential for high protein intakes to cause body calcium loss, this appears to only occur when calcium intake is insufficient in the first place; high protein intakes with high calcium intakes improves bone health. Ensuring sufficient vegetable intake along with a high protein intake is a key aspect not only to bone health but to preventing a small metabolic acidosis which may occur when large amounts of protein are consumed by themselves.
Concerns over heart disease and cancer are more related to the high fat content of many cuts of meat, along with other nutritional factors such as insufficient fruit and vegetable intake that contributes. Other lifestyle factors that typically accompany the consumption of higher fat cuts of meat are also a likely contributor to the overall health risk. The consumption of lean cuts of meat has actually been shown to improve overall health; both athletic and diets for general health should ideally contain plenty of fruits and vegetables for this reason.
- Martin WF et. al. Dietary protein intake and renal function. Nutr Metab (2005) 2: 25.
- Millward DJ. Optimal intakes of protein in the human diet. Proc Nutr Soc. (1999) 58(2): 403-13.
- Poortmans JR and Dellalieux O. Do regular high protein diets have potential health risks on kidney function in athletes? Int J Sport Nutr Exerc Metab. (2000) 10(1):28-38.
- Dawson-Hughes B. Calcium and protein in bone health. Proc Nutr Soc. (2003) 62(2): 505-9.
- Bonjour JP. Dietary protein: an essential nutrient for bone health. J Am Coll Nutr. (2005) 24(6 Suppl): 526S-36S.
- Massey LK. Dietary animal and plant protein and human bone health: a whole foods approach. J Nutr. (2003) 133(3):862S-865S.
- Dawson-Hughes B. Interaction of dietary calcium and protein in bone health in humans. J Nutr. (2003) 133(3):852S-854S.
- Frassetto L et. al. Diet, evolution and aging–the pathophysiologic effects of the post-agricultural inversion of the potassium-to-sodium and base-to-chloride ratios in the human diet. Eur J Nutr. (2001) 40(5):200-13.
- Wiederkehr M, Krapf R. Metabolic and endocrine effects of metabolic acidosis in humans. Swiss Med Wkly. (2001) 131(9-10):127-32.
- Barzel US and LK Massey Excess dietary protein can adversely affect bone. J Nutr. (1998) 128(6):1051-3.
- Fogelholm M. Dairy products, meat and sports performance. Sports Med. (2003) 33(8):615-31.
- Sabate J. The contribution of vegetarian diets to human health. Forum Nutr. (2003) 56:218-20. 13. Li D et. al. Lean meat and heart health. Asia Pac J Clin Nutr. (2005) 14(2):113-9.
- Hodgson JM et. al. Increased lean red meat intake does not elevate markers of oxidative stress and inflammation in humans. J Nutr. (2007) 137(2):363-7. Links
- Biesalski HK.Meat and cancer: meat as a component of a healthy diet.
- Eur J Clin Nutr. (2002) 56 Suppl 1:S2-11.
- Hodgson JM et. al. Partial substitution of carbohydrate intake with protein intake from lean red meat lowers blood pressure in hypertensive persons.Am J Clin Nutr. (2006) 83(4):780-7.
- Hill M. Meat, cancer and dietary advice to the public. Eur J Clin Nutr. (2002) 56 Suppl 1:S36-41
- Elmadfa I, Freisling H. Fat intake, diet variety and health promotion. Forum Nutr. (2005) (57):1-10.