Effect of Increased Dietary Protein on Tolerance to Intensified Training

The purpose of the present study was to examine the effect of increased protein intake on short-term decrements in endurance performance during a block of high-intensity training. Trained male cyclists (VO(2max) = 64.2 ± 6.5 mL·kg(-1)·min(-1)) completed two 3-wk trials both divided equally into normal (NOR), intensified (INT), and recovery (REC) training. In a counterbalanced crossover experimental design, cyclists received either a high-protein (PRO; 3 g protein·kg(-1) body mass (BM)·d(-1)) or a normal diet (CON; 1.5 g protein·kg(-1) BM·d(-1)) during INT and REC. Dietary carbohydrate content remained constant at 6 g·kg(-1) BM·d(-1). Energy balance was maintained during each training week. Endurance performance was assessed with a VO(2max) test and a preloaded time trial. Alterations in blood metabolite responses to exercise were measured at rest, during, and after exercise. Cyclists completed the Daily Analysis of Life Demands for Athletes (DALDA) questionnaire each day. Increased dietary protein intake led to a possible attenuation (4.3%; 90% confidence limits ×/÷5.4%) in the decrement in time trial performance after a block of high-intensity training compared with NOR (PRO = 2639 ± 350 s; CON = 2555 ± 313 s). Restoration of endurance performance during recovery training possibly benefited (2.0%; ×/÷4.9%) from additional protein intake. Frequency of symptoms of stress described as "worse than normal" reported after a block of high-intensity training was very likely (97%) attenuated (17; ±11 AUC of "a" scores part B, DALDA for INT + REC) by increasing the protein content of the diet. No discernable changes in blood metabolite concentrations were observed in PRO. Additional protein intake reduced symptoms of psychological stress and may result in a worthwhile amelioration of the performance decline experienced during a block of high-intensity training.