The purpose of this systematic review is to assess the evidence behind the dietary requirement of
protein and to assess the health effects of varying
protein intake in healthy adults. The literature search covered the years 2000-2011. Prospective cohort, case-control, and intervention studies were included. Out of a total of 5,718 abstracts, 412 full papers were identified as potentially relevant, and after careful scrutiny, 64 papers were quality graded as A (highest), B, or C. The grade of evidence was classified as convincing, probable, suggestive or inconclusive. The evidence is assessed as: probable for an estimated average requirement of 0.66 g good-quality
protein/kg
body weight (BW)/day based on
nitrogen balance studies, suggestive for a relationship between increased all-cause mortality risk and long-term low-
carbohydrate-high-
protein (LCHP) diets; but inconclusive for a relationship between all-cause mortality risk and
protein intake per se; suggestive for an inverse relationship between cardiovascular mortality and
vegetable protein intake; inconclusive for relationships between
cancer mortality and
cancer diseases, respectively, and
protein intake; inconclusive for a relationship between
cardiovascular diseases and total
protein intake; suggestive for an inverse relationship between blood pressure (BP) and
vegetable protein; probable to convincing for an inverse relationship between soya
protein intake and
LDL cholesterol; inconclusive for a relationship between
protein intake and bone health, energy intake, BW control, body composition, renal function, and risk of
kidney stones, respectively; suggestive for a relationship between increased risk of
type 2 diabetes (T2D) and long-term LCHP-high-fat diets; inconclusive for impact of physical training on
protein requirement; and suggestive for effect of physical training on whole-body
protein retention. In conclusion, the evidence is assessed as probable regarding the estimated requirement based on
nitrogen balance studies, and suggestive to inconclusive for
protein intake and mortality and morbidity.
Vegetable protein intake was associated with decreased risk in many studies. Potentially adverse effects of a
protein intake exceeding 20-23 E% remain to be investigated.