- While a high protein intake does increase the workload of the kidneys, healthy kidneys are more than equipped to handle this
- In people who already have kidney disease, high dietary protein can lead to a more rapid loss of kidney function, and intake should be limited 
- High protein intake increases blood urea nitrogen (BUN) – high BUN is also a marker of impaired kidney function .
If you’re physically active or trying to lose weight, you’ve probably heard that you should eat a lot of protein – and with good reason. High protein diets have been shown to improve gains in muscle mass in people undergoing resistance training, and to mitigate loss of lean mass while dieting  .
But can a high protein diet “overwork” the kidneys, and cause long-term damage? To start, let’s look at the kidneys’ role in protein digestion and metabolism.
The stomach and small intestine contain enzymes which break down proteins into individual amino acids, which are either used by the body or degraded. Amino acid degradation results in the formation of ammonia, which is toxic. To get rid of ammonia, the urea cycle functions in the liver to convert ammonia into urea. The kidneys are charged with getting rid of the urea to be excreted .
When we eat a lot of protein, and subsequently produce more urea, the volume of fluid filtered by the kidneys, also known as the glomerular filtration rate (GFR), increases . An increased GFR allows the kidneys to get rid of more waste products from protein. Such increases in GFR can be observed in other instances in which kidney demand is increased, such as pregnancy and following nephrectomy, without complications  . Acute changes in GFR in response to dietary protein increases may explain why protein restriction tends to preserve renal function among patients who already have kidney disease  
High Protein Diets – Cause For Concern?
Some have suggested that a high protein intake in healthy people contributes to kidney disease    . The idea was propagated by Brenner and colleagues in 1982, who suggested that increased kidney demand from a high protein diet caused renal hypertension and subsequent kidney damage. However, they drew conclusions from animal studies and patients receiving amino acids intravenously, rather than people with healthy kidney function .
A recent review paper, from the American Journal of Kidney Diseases, raised concern based on acute changes in GFR in response to protein increases, reports of increased urinary protein excretion on high protein diets, and associations between high meat intake and kidney stones. However, the risks outlined were purely speculative. The author stated that “there are no clear renal-related contraindications to high-protein diets in individuals with healthy kidney function” .
It should be noted that in people with underlying kidney disease, protein often needs to be limited. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) patients with kidney disease who are not on hemodialysis should consume 0.6-0.8g/kg protein . People with kidney disease should speak with a nephrologist regarding their protein intake.
High Protein Diets in Healthy People
Despite common misperceptions, research demonstrates that a high protein diets are safe in people with normal kidney function. A 2018 systematic review and meta-analysis of randomized-controlled trials, looking at 1,358 people, found that high-protein diets did not lead to significant long-term change in GFR as compared to normal protein diets . Several epidemiological studies have also supported the safety of high protein intakes in people with healthy kidney function   . One of these studies looked at protein intake and kidney function in over 4,000 people at two points in time, about 14 years apart. While protein intake was directly associated with odds of kidney function decline in people who already had impaired kidney function, there was no association in people with healthy kidneys at baseline .
Even very high protein intakes, common among strength athletes, do not appear to impair kidney function. One study looked at clinical markers of 20 young male bodybuilders, who consumed an average of 169g of protein per day (1.94g/kg), and found key clinical renal parameters to be within normal reference ranges .
Intakes as high as 3.4g/kg, or 1.5 g/lb, have also been demonstrated to be safe for the kidneys   . In a one-year study, 14 resistance-trained men consumed their habitual protein intake (~214g/day or 2.5g/kg) and a really high-protein diet (284g/day or 3.3g/day), for 6 months each. There were no significant differences before and after each diet on any clinical markers for kidney function . A recent follow-up from that study looked at 5 of those participants, who continued to consume ~300g protein for an additional year (2 years total). Clinical markers of kidney function remained within normal limits .
Blood Urea Nitrogen
One final point to consider is protein’s impact on blood urea nitrogen (BUN). BUN is a blood marker that is often elevated in cases of impaired kidney function. When the kidneys are unable to excrete urea efficiently, nitrogen builds up in the blood. High protein intake can also modestly increase BUN levels, as it increases urea production as previously described. Importantly, the increased BUN observed as a result of high protein intake is NOT independently associated with kidney disease .
Concerns regarding the impact of a high protein diet on kidney health are unfounded, except for cases of underlying kidney disease. While eating a lot of protein will engender acute changes in the kidneys, these changes are adaptive and do not pose a health risk. People with underlying kidney disease should talk to a nephrologist to determine an appropriate protein intake given their medical condition.
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