Do you need to restrict protein intake to protect your kidneys?

Healthy kidneys do not need protein restriction. A review found no association between protein intake and loss of kidney function in people with normal kidney function. In fact, eating less than 0.6 grams of protein per kilogram of body weight per day is associated with a higher risk of mortality, even in people with healthy kidneys, according to a large cohort study (NHANES). Protein restriction for prevention in healthy individuals is therefore not useful and can even be counterproductive.

Chronic kidney disease (CKD) is a different matter. A high protein intake raises the pressure in the small filtering structures of the kidney. When the kidneys are already damaged, this can accelerate their decline. Multiple reviews show that a protein-restricted diet of 0.6 to 0.8 grams per kilogram per day slows the reduction in kidney function and delays the start of dialysis. This is not entirely conclusive, however: the largest randomised trial in this area (the MDRD study) did not produce a clear-cut result.

In people with an eGFR below 60 (a measure of the kidney's filtering capacity), an intake of 1.4 grams of protein per kilogram or more was associated with a 37% higher risk of mortality in the NHANES cohort study. This is an association, not causal evidence, but it is consistent with the biological explanation that overburdened kidneys deteriorate more quickly.

For those who already have severe kidney disease (eGFR below 30), an even stricter plant-based diet with specialised amino acid substitutes (known as keto analogues) appears to be more effective than a standard protein-restricted diet in delaying dialysis. The randomised study that demonstrated this found a strong benefit in patients with the most severely impaired kidney function. However, only 14% of the screened patients were able to sustain it, so this is not a feasible approach for everyone.

An important risk of protein restriction in kidney patients is malnutrition: insufficient protein and energy in the body, resulting in muscle breakdown. Multiple reviews emphasise that guidance from a dietitian is indispensable. The type of protein also matters: animal protein produces more acid, which places an additional burden on the kidneys. Plant-based protein is considered less burdensome, although the precise magnitude of this effect has not yet been well established.