The harms of high protein intake: conjectured, postulated, claimed, and presumed, but shown?

High-protein (HP) diets and protein-supplemented foods and beverages have become increasingly popular in adults due to potential benefits relating to appetite, energy intake, body weight, and body composition, and questions have been posed regarding whether current dietary recommendations for protein are too low. At the same time, health concerns relating to high-protein diets have been widespread in the literature for >60 y. However, the conjectured harms of HP diets, which remain prevalent in the lay and sometimes academic literature, are often without strong scientific evidence or may actually be contradicted to a reasonable degree of certainty by scientific evidence. In this paper, we discuss several of the postulated harms cited in academic and lay publications and investigate the strength of evidence to support or refute these asserted harms. We highlight areas of caution relating to experimental design and interpretation of results and propose areas of research that would be helpful to better determine the potential risks associated with high dietary protein intake.