Effects of Within-Day Dietary Protein Distribution on Changes in Plasma Amino Acid Concentrations and Body Composition
The increasing prevalence of adults with overweight and obesity in the United States emphasizes the need for research on novel behavioral and lifestyle interventions such as dietary manipulation to modulate the epidemic. While consuming an energy-restricted diet is a proven method of reducing adiposity, adhering to and adopting a diet that is not self-chosen is difficult. Not unimportantly, reducing energy intake also typically results in a reduction in skeletal muscle mass. Skeletal muscle is important for facilitating functional movements, metabolizing glucose, and contributing to resting energy expenditure. Multitudinous research has focused on the macronutrient composition of an energy-restricted diet both to improve adherence and reduce skeletal muscle loss. Previous research showed that consuming a higher protein diet (1.2-1.5 g/kg/d) may attenuate the loss of skeletal muscle mass vs a lower-protein diet with similar reductions in body weight. Adults in the United States and in most developed countries typically consume very little protein at breakfast, slightly more at lunch, and the majority at dinner. This skewed distribution pattern limits the meals available for increasing protein quantity. While the exact within-day protein distributions from the studies assessing the effect of consuming a higher-protein diet vs a lower-protein diet on skeletal muscle changes are unknown, it is feasible that the protein contents of breakfast and lunch were increased, in part, due to the practical limitations of adding more protein to dinner. In effect, consuming a higher protein diet may results in a more even protein distribution pattern. Evenly distributing protein throughout the day could feasibly upregulate muscle protein synthesis more frequently, attenuating skeletal muscle loss. Study 1 (Chapter 2) of this document was designed to assess the effects of within-day protein intake distribution on changes in body composition during dietary energy restriction and resistance training. We hypothesized that consuming an even protein distribution would result in greater lean mass retention. Contrary to our hypothesis, the effectiveness of dietary energy restriction combined with resistance training to improve body composition is not influenced by the within-day distribution of protein when adequate total protein is consumed. Consuming an even protein distribution inevitably results in higher protein-containing breakfasts and lunches. Breakfast is traditionally characterized as being carbohydrate-rich and altering habitual breakfast habits can be particularly challenging, in part, due to practical limitations such as convenience, appetite, and cultural influence. One method to increase the protein quantity of breakfast may be to leverage the rapid digestion and absorption kinetics of whey protein. Amino acids from meals peak in the plasma at ~180 minutes postprandial. Conversely, amino acids from rapidly digestible whey protein appear in the plasma within 15 minutes and peak at 60 minutes postprandial. In study 2 (Chapter 3), we hypothesized that consuming a 20-g whey protein snack 2 hours after a standard mixed-macronutrient, lower protein breakfast (10 g) would result in peak and composite postprandial plasma essential amino acid (EAA) responses that were not different from consuming a 30-g protein breakfast alone. We found that consuming a rapidly digested whey protein snack 2 hours after a slowly digested, lower protein breakfast resulted in a greater peak plasma EAA concentration but comparable plasma EAA availability than consuming a single higher protein breakfast. Consuming a protein supplement 120 minutes after breakfast may also impact body weight by manipulating ingestive behaviors. Limited observational and clinical trial evidence suggests that consuming energy-containing beverages between meals effectively increases daily meal frequency and contributes to higher body weights. If true, it’s possible that the timing of protein supplementation ingestion relative to meal times would impact changes in body weight in adults consuming a "self-chosen" diet. In study 3 (Chapter 4) we conducted a systematic review of literature to investigate whether the existing research studies support consuming protein supplements between meals versus with meals to differentially change body composition in adults who initiate resistance training regimens. Regardless of protein supplement timing in relation to meals, lean mass is likely to increase in response to resistance training. However, consuming protein supplements with meals rather than between meals may be a more effective dietary strategy to improve resistance training-induced changes in body composition by reducing fat mass, which may be relevant for adults undergoing purposeful weight loss to improve their health status. Conversely, consuming protein supplements between meals may be more effective at increasing overall body mass.
Campbell, Purdue University.
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