Protein and kidneys: a small trial in trained men does not settle the safety of high intakes

Two protein intakes, two training approaches

The researchers randomly assigned 48 resistance-trained men, with an average age of 26, to four groups. They combined an intake of 1.6 or 3.2 grams of protein per kilogram of body weight per day with resistance training alone or combined training.

The intervention lasted 16 weeks, with four sessions per week. The analysis covers the 44 participants with complete data before and after the intervention. These intakes are the trial’s experimental conditions, not dietary recommendations.

Blood markers, not a direct measure of kidney damage

Fasting blood samples were used to measure markers including blood urea nitrogen and cystatin C. The researchers also estimated glomerular filtration rate, an indicator of kidney function, using equations based on creatinine, cystatin C or both.

The abstract reports no direct measurement of kidney filtration. It also provides no numerical magnitude for the observed changes, limiting assessment of their clinical importance.

Why changes within each group are not enough

In both groups consuming 3.2 g/kg/day, blood urea nitrogen and cystatin C increased, while estimates of kidney filtration decreased. These changes were statistically significant within the groups. No significant change was observed in the groups consuming 1.6 g/kg/day.

However, analyses comparing changes by protein intake detected no statistically significant difference, regardless of the training approach. A significant change in one group and a nonsignificant change in another does not, on its own, demonstrate that the groups changed differently.

The authors therefore consider the observed within-group changes descriptive. Presenting them as proof of kidney damage caused by protein would go beyond the findings.

What this trial tells us

The opposite interpretation would also go too far: failing to detect a difference does not prove that the two intakes are equivalent or that a high intake is risk-free. The authors stress that the limited sample size prevents them from ruling out clinically important differences.

These findings concern men who were already trained and a period of 16 weeks. They do not support conclusions about long-term effects, effects in women or effects in people with kidney disease.

What we can take away

This trial demonstrates neither kidney damage attributable to high protein intake nor its long-term safety. The absence of a detected difference remains an answer limited by sample size and duration.