A novel triple-tracer approach to assess postprandial protein turnover

Serum Metabolites Associated with Muscle Hypertrophy after 8 Weeks of High- and Low-Load Resistance Training

The mechanisms responsible for the similar muscle growth attained with high- and low-load resistance training (RT) have not yet been fully elucidated. One mechanism is related to the mechanical stimulus and the level of motor unit recruitment; another mechanism is related to the metabolic response. We investigated the electromyographic signal amplitude (sEMG) and the general metabolic response to high-load RT (HL) and low-load resistance training (LL). We measured muscle thickness by ultrasound, sEMG amplitude by electromyography, and analysis of metabolites expressed through metabolomics. No differences were observed between the HL and LL groups for metabolic response and muscle thickness. A greater amplitude of sEMG was observed in the HL group. In addition, a correlation was observed between changes in muscle thickness of the vastus lateralis muscle in the HL group and levels of the metabolites carnitine, creatine, 3-hydroxyisovalerate, phenylalanine, asparagine, creatine phosphate, and methionine. In the LL group, a correlation was observed between changes in muscle thickness of the vastus lateralis muscle and levels of the metabolites acetoacetate, creatine phosphate, and oxypurinol. These correlations seem to be related to the characteristics of activated muscle fibers, the metabolic demand of the training protocols used, and the process of protein synthesis.

Circulatory amino acid responses to milk consumption in dairy and lactose intolerant individuals

Background/objectives

Self-reported digestive intolerance to dairy foods is common. As dairy can be an important source of dietary protein, this study aimed to identify whether milk protein digestion is compromised in individuals with digestive intolerance.

Subjects/methods

Adult women (n = 40) were enroled in this double-blinded, randomised cross-over trial, with digestive symptoms characterised using a lactose challenge and self-reported digestive symptom questionnaire. Participants were classified as either lactose intolerant (LI, n = 10), non-lactose dairy intolerant (NLDI, n = 20) or dairy tolerant (DT, n = 10). In a randomised sequence, participants consumed three different kinds of milk (750 ml); conventional milk (CON), a2 Milk™ (A2M), and lactose-free conventional milk (LF-CON). Circulatory plasma amino acid (AA) concentrations were measured at baseline and every 30 min until 3 h post-ingestion.

Results

In all participants across all milk types, plasma AA concentrations (AUC_0-180) increased after milk ingestion with no significant differences in responses observed between milk types or participants (P > 0.05), with the exception of the suppressed lysine response in the DT group following A2M ingestion, relative to the other two groups and milk types (P < 0.05).

Conclusion

Milk protein digestion, as determined by circulatory AAs, is largely unaffected by dairy- and lactose- intolerances.

New advances in stable tracer methods to assess whole-body protein and amino acid metabolism

PURPOSE OF REVIEW

Stable isotope methods have been used for many years to assess whole-body protein and amino acid kinetics in healthy conditions and in response to aging, exercise and (clinically stable) disease states.

RECENT FINDINGS

In recent years, tracer research expanded to the anabolic response to feeding in critical illness and its use during acute metabolic stressors. Furthermore, new isotope approaches and tracer insights have been obtained. In the postabsorptive state, the novel tracer pulse approach has several advantages above the established continuous tracer approach because of the metabolic information that can be obtained, easy applicability, and low tracer costs. The use of bolus versus sip-feeding approaches to assess the anabolic response to a meal is dependent on the research question and its feasibility. Promising new tracer approaches have been developed to measure the anabolic capacity, and protein digestibility and absorption. Advances have been made in the field of mass spectrometry in low enrichment analysis.

SUMMARY

Novel tracer approaches are available that can more readily be used in critical illness and during acute metabolic stressors. Besides the use of tracer application in various clinical conditions, more research is needed on how to incorporate isotopes on an individual level.

Recent Advances in the Characterization of Skeletal Muscle and Whole-Body Protein Responses to Dietary Protein and Exercise during Negative Energy Balance

In a review published in 2012, we concluded that higher-protein diets preserve muscle mass during energy deficit via stimulated mammalian target of rapamycin complex 1 signaling, coincident increased muscle protein synthesis (PS), inhibited ubiquitin-mediated proteolysis, and suppressed muscle protein breakdown (PB). Since then, there have been significant advances in understanding the fundamental effects of higher-protein diets, with or without exercise training, on muscle and whole-body protein homeostasis during negative energy balance. Therefore, an update on the evolution of this field of research is warranted to better inform recommendations on best practices for healthy weight loss and muscle preservation. We will review the most recent studies examining the effects of higher-protein diets and negative energy balance on body composition, muscle PS, muscle PB, associated intracellular regulatory pathway activities, and whole-body protein homeostasis. In addition to critically analyzing contemporary findings, knowledge gaps and opportunities for continued research will be identified. Overall, the newest research confirms that consuming higher-protein diets, particularly when coupled with resistance exercise, preserves muscle mass and maintains whole-body protein homeostasis during moderate energy deficits (i.e., normal weight loss). However, these newer findings also indicate that as the magnitude of energy deficit increases, the efficacy of higher-protein diets for mitigating losses of fat-free mass is diminished. Further, recent results suggest that alterations in muscle PS, more so than muscle PB, may be primarily responsible for changes in muscle mass that occur in response to negative energy balance.