High Protein Diets Are Bad for Kidneys, Liver, and Bones: Myth or Truth

The claim that protein is bad for you goes back to the last century, when it was assumed that nitrogen and ammonia produced during protein digestion and breakdown (1) were bad for your health. The negative light on protein is also cast by cohort studies that show a correlation between protein intake, CVD (2) and kidney disease (3, 13) in the general population.

However, with the advent of sports nutrition research in the last 20-30 years, this thesis is not a modern position in nutrition and has been refuted numerous times in randomized controlled trials (RCTs) and meta-analyses (4, 5, 6, 7, 8, 16, 17).

Effects of Protein on Sports

A high protein diet, defined by the International Society of Sports Nutrition (ISSN) as 1.4-2.0 g protein/kg body weight in sports, has been well established as a safe and effective diet for achieving athletic performance (9). Dietary protein, in addition to its role in muscle hypertrophy (9, 10), also plays a role in satiety (11), fat loss, and muscle mass maintenance on a hypocaloric diet (12). Therefore, protein intake recommendations for active individuals are higher than the 0.8 g/kg recommended for the average person.

Bones and Protein

Contrary to another belief that protein has a negative effect on bone and calcium loss, increasing protein to as little as 0.75 g/kg has been shown to improve bone markers in older adults (14). Overall, there is no scientific evidence linking higher protein intakes with adverse outcomes in healthy, athletic individuals (9). On the other hand, excessively high (2.4 g/kg) protein intake over a long period of time (several months) may result in insufficient intake of other nutrients, including calcium, which in turn will lead to bone loss. Thus, the ISSN position is that active older adults need protein intakes in the range of 1.4 to 2.0 g/kg, and that this intake level is safe (9).

Why is there still an anti-protein agenda in medicine?

Indeed, there is a link between protein intake and health problems in cohort studies (3, 13, 15). However, it is true for people with already existing kidney (3) and heart (13) diseases. The fundamental problem with cohort studies on the general population, is the logical fallacy of ‘association does not mean causation’ and should not be extrapolated on individual level as they are presented, but rather used to find trends and associations. It is likely that the average person with protein also consumes more fat, salt and generally has an unbalanced diet (15). Furthermore, in the same studies (3), low protein intake (<60 g per day) had a pronounced negative effect on health and overall mortality. 

Looking at randomised controlled trials (RCTs) that look more at the individual level, the picture is more clear. Two meta-analyses, which together reviewed more than 200 RCTs, show that increasing protein in controlled conditions leads only to positive effects on the body, such as increasing muscle mass, lowering blood triglycerides and lowering blood pressure (16, 17).

Recommendations

For average people, recommendations of 0.8 g per kg of body weight remain preferable, because low awareness of nutrition and nutritional science in the population can lead to insufficient intake of other nutrients with a poorly planned high-protein diet that can drag the increase in fat and salt intake. For athletes of any kind of sports, especially strength sports, the recommendations are 1.4-2.0 g/kg of dietary protein. It should be emphasized that the total protein is taken into account, including from plant sources, and not only from animals.