Can a High-Protein Diet Be Harmful?

Proteins play a fundamental role in the diet of any person, whether they are athletes or not and regardless of their age (being in fact even more important at advanced ages). This macronutrient is essential, among many other functions, to maintain and increase muscle mass and strength, as well as to regenerate damaged tissues after an exercise session.

The Benefits of a High-Protein Diet

Although the classic guidelines recommend somewhat limited protein intakes (0,8 g/kg/day), there is evidence that higher intakes produce greater benefits. For example, a few years ago a review was published that, after analyzing 49 studies, concluded that higher daily protein intakes were associated with greater strength and muscle mass gains in people who strength trained to reach at least 1,6 g/kg/day of protein, from which no great additional benefits were observed.¹ Similarly, more recently, another review that analyzed data from 74 studies also concluded that a protein intake of 1,6 g/kg/day or higher provided the greatest benefits in muscle mass and strength in people trained in this discipline.² That is why it is common for many athletes, especially those who train in strength disciplines, to consume high amounts of protein. But it is not only strength athletes who consume a lot of protein, as very high intakes of this macronutrient (reaching ~2,8-3,8 g/kg/day) have also been reported in endurance athletes, such as professional cyclists during periods of high load such as stage races ^3–6.

Controversy surrounding high protein intake

Despite the benefits of high-protein diets, certain sectors have suggested that high consumption of this macronutrient could also have certain adverse effects. It has mainly been suggested that high protein intake would lead to an increase in the load of solutes (e.g. urea, a waste product of protein metabolism) at the kidney level, which could 'overload' the kidney and cause conditions such as compensatory hyperfiltration with the consequent risk of kidney damage.  In fact, traditionally in people who already have kidney damage, low-protein diets have been prescribed to avoid this problem. ⁷

However, there is no consistent evidence regarding the negative effects of high-protein diets in healthy subjects. ⁸ For example, a review of 28 studies showed that high protein intakes (>1.5 g/kg/day, >20% of total daily energy, or >100 g per day) did not result in significant changes in kidney function indicators such as glomerular filtration rate.⁹ Studies in athletes who consumed high doses of protein have also not observed adverse effects. For example, one study randomized a group of bodybuilders to consume either their usual diet, which was already high in protein (2.5 g/kg), or even a higher dose of protein (3.3 g/kg). After following them for a year, the researchers found no indicators of adverse effects either at the lipid level or in kidney function,¹⁰ and when they were measured again the following year, when they had already been on a very high protein diet for two years, no adverse effects were found either.¹¹

Conclusions

A high-protein diet (with intakes significantly higher than traditional recommendations) is generally beneficial and even necessary to maintain or increase muscle mass, as well as to improve performance and recovery. It is important to exercise caution in people who already have kidney dysfunction because high protein consumption could aggravate their condition. However, in healthy people with normal kidney function, there is no evidence that high protein intake can have adverse effects. However, given that the benefits of consuming very high amounts of protein (e.g. >2 g/kg/day) seem to be minimal, we can be conservative and avoid consumption much higher than these doses as they may not have additional benefits.

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References

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2. Nunes EA, Colenso-Semple L, McKellar SR, et al. Systematic review and meta-analysis of protein intake to support muscle mass and function in healthy adults. J Cachexia Sarcopenia Muscle. 2022;13(2):795-810. doi:10.1002/jcsm.12922
3. Rehrer NJ, Hellemans IJ, Rolleston AK, Rush E, Miller BF. Energy intake and expenditure during a 6-day cycling stage race. Scand J Med Sci Sport. 2010;20(4):609-618. doi:10.1111/j.1600-0838.2009.00974.x
4. Sánchez-Muñoz C, Zabala M, Muros JJ. Nutritional intake and anthropometric changes of professional road cyclists during a 4-day competition. Scand J Med Sci Sports. 2016;26(7):802-808. doi:10.1111/sms.12513
5. Muros JJ, Sánchez-Muñoz C, Hoyos J, Zabala M. Nutritional intake and body composition changes in a UCI World Tour cycling team during the Tour of Spain. Eur J Sport Sci. 2019;19(1):86-94. doi:10.1080/17461391.2018.1497088
6. Saris WHM, Van Erp-Baart MA, Brouns F, Westerterp KR, Ten Hoor F. Study on food intake and energy expenditure during extreme sustained exercise: The Tour de France. Int J Sports Med. 1989;10(SUPPL. 1):26-31. doi:10.1055/s-2007-1024951
7. Hahn D, Hodson EM, Fouque D. Low protein diets for non-diabetic adults with chronic kidney disease. Cochrane Database Syst Rev. 2020;2020(11). doi:10.1002/14651858.CD001892.pub5
8. Antonio J. High-protein diets in trained individuals. Res Sport Med. 2019;27(2):195-203. doi:10.1080/15438627.2018.1523167
9. Devries MC, Sithamparapillai A, Brimble KS, Banfield L, Morton RW, Phillips SM. Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets: A Systematic Review and Meta-Analysis. J Nutr. 2018;148(11):1760-1775. doi:10.1093/jn/nxy197
10. Antonio J, Ellerbroek A, Silver T, et al. A High Protein Diet Has No Harmful Effects: A One-Year Crossover Study in Resistance-Trained Males. J Nutr Metab. 2016;2016:0-4. doi:10.1155/2016/9104792
11. Antonio J, Ellerbroek A. Case reports on well-trained bodybuilders: Two years on a high protein diet. J Exerc Physiol Online. 2018;21(1):14-24.