High protein diets are often recommended for the purpose of building muscle and aiding weight loss.
That is because dietary protein increases muscle protein synthesis, decreases muscle protein breakdown, improves satiety and glycaemic control, and increases metabolic rate. As a result, high protein diets have been shown to have a metabolic advantage [1,2].
That means you can effectively consume the same number of calories or even more on a high protein diet and still achieve more favourable body composition changes.
However, there has also been a lot of debate as to the effect high dietary protein intake has on kidney function. The kidneys filter the blood from harmful substances, such as toxic medications and other chemical substances and excrete them in the urine.
High protein diets are believed to worsen kidney function because having too much protein can cause waste to build up in the blood and the kidneys may not be able to remove all the extra waste .
An early 2004 study indicated that there are no clear kidney related contraindications to high protein diets in individuals with healthy kidney function, but that high protein diets have the potential to cause significant harm in individuals with chronic kidney disease . This study is often cited by fitness enthusiast and those in the evidence-based community in defence of high protein diets.
However, more recent studies suggest that worsening of kidney function may occur even in individuals without impaired kidney function.
A recent 2020 research review reported that “High dietary protein intake can cause intraglomerular hypertension, which may result in kidney hyperfiltration, glomerular injury, and proteinuria” and it is possible that long-term high protein intake may lead to chronic kidney disease .
As a result, very high dietary protein intakes, greater than 3g per kg per day, would probably be reserved for weight loss phases to improve satiety, spare lean tissue, and increase the thermic effect of feeding.
During a muscle building or maintenance phase, a moderate protein intake of around 1.6-2g per kg per day or 1g per lb would be more appropriate for individuals with healthy kidney function. When calories are at maintenance or above studies show that there is no benefit in going above 1.6g of protein per kg per day .
However, this is still double the Recommend Daily Allowance (RDA) for protein which is a 0.8g per kg per day. Individuals with chronic kidney disease are advised to consume no more than 0.8g of protein per kg per day .
The quality of dietary protein also appears to have an impact on kidney function. Compared with plant based dietary protein sources, animal proteins, especially processed red meat, have been associated with a greater risk of kidney disease in several observational studies . Potential causes of kidney damage arising from animal protein include increased dietary acid load, high phosphate content, and dysbiosis, which results in inflammation .
In addition to the kidneys, high dietary protein may also increase liver stress. Although protein is important for liver detoxification because amino acids are essential cofactors for phase II liver detoxification, the liver is also responsible for deaminating amino acids.
One study showed that the maximal rate at which the liver can deaminate amino acids is between 3.5-4.5g of protein per kg per day . As a result, we do not recommend exceeding 3.5g of protein per kg per day, even in individuals with healthy kidney function.