Meals Containing Equivalent Total Protein from Foods Providing Complete, Complementary, or Incomplete Essential Amino Acid Profiles do not Differentially Affect 24-h Skeletal Muscle Protein Synthesis in Healthy, Middle-Aged Women

Background: Dietary protein quality can be assessed by skeletal muscle protein synthesis (MPS) stimulation. Limited knowledge exists on how consuming isonitrogenous meals with varied protein qualities affects postprandial and 24-h MPS. Objectives: We assessed the effects of protein quality and complementary proteins on MPS. We hypothesized that meals containing a moderate amount of high-quality, complete protein would stimulate postprandial and 24-h MPS. Meals containing two complementary, plant-based incomplete proteins would stimulate MPS less, and meals containing plant-based incomplete proteins at each meal, but complementary over 24 h would not stimulate MPS. Methods: This quasi-experimental study included a randomized, crossover design to assess protein quality and a nonrandomized low-protein control. We measured postprandial and 24-h MPS responses of healthy middle-aged women (n = 9, age 56 ± 4 y), to 3 dietary conditions: isonitrogenous meals containing 23 g protein/meal from 1) complete protein (lean beef), 2) 2 incomplete, but complementary protein sources (navy/black beans and whole wheat bread), and 3) single incomplete protein sources (black beans or whole wheat bread at 1 meal), but providing a complete amino acid profile over 24 h. In the low-protein group women (n = 8, 54 ± 5 y) consumed a single breakfast meal containing 5 g of protein. Venous blood and vastus lateralis samples were obtained during primed, constant infusions of L-[ring-¹³C₆]phenylalanine to measure mixed muscle fractional synthetic rates (FSR). Results: Meals containing complete, complementary, or incomplete proteins did not differentially influence FSR responses after breakfast (P = 0.90) or 24 h (P = 0.38). At breakfast, the complete (P = 0.030) and complementary (P = 0.031) protein meals, but not the incomplete protein meal (P = 0.38), had greater FSR responses compared with the low-protein control meal. Conclusions: Isonitrogenous meals containing a moderate serving of total protein from foods providing complete, complementary, or incomplete essential amino acid profiles do not differentially stimulate muscle protein synthesis after a meal and daily. Trial registration number: This clinical trial was registered at clinicaltrials.gov as NCT03816579. URL: https://www.clinicaltrials.gov/ct2/show/NCT03816579?term=NCT03816579&draw=2&rank=1.