To explore whether changes in dietary protein sources can lower plasma branched-chain amino acids (BCAAs), aromatic amino acids and sulfur amino acids (SAAs) that are often elevated in the obese, insulin-resistant state and in type 2 diabetes.

Thirty-six subjects (mean age 31���2�years) underwent a voluntary abstinence from meat, poultry, eggs, and dairy products for 6�weeks, while enriching the diet with fish, in fulfillment of a religious fast. Subjects were assessed 1�week before the fast (V1), 1�week after initiation of the fast (V2) and in the last week of the fast (V3). Thirty-four subjects completed all three visits.

Fasting�plasma BCAAs decreased at V2 and remained low at V3 (P�<�0.001 for all). Valine showed the greatest decline, by 20 and 19�% at V2 and V3, respectively. Phenylalanine and tryptophan, but not tyrosine, also�decreased at V2 and V3. The two proteinogenic SAAs, methionine and cysteine, remained stable, but the cysteine product, taurine, decreased from 92���7�μmol/L to 66���6 (V2; P�=�0.003) and 65���6�μmol/L (V3; P�=�0.003). A progressive decline in plasma glutamic acid, coupled with an increase in glutamine, was observed. Plasma total and LDL cholesterol decreased at V2 and V3 (P�<�0.001 for all).

Changing dietary protein sources to plant- and fish-based sources in an ad libitum setting lowers the plasma BCAAs that have been linked to diabetes risk. These findings point to habitual diet as a potentially modifiable determinant of fasting plasma BCAA concentrations.