Impact of dietary alteration, digestive and metabolic health on regulating circulatory amino acids

Abstract

Amino acids (AAs), the building blocks of protein, are active metabolites central to many important physiological processes. Among the AAs, the branched-chain amino acids (BCAAs) have drawn significant attention considering their potential benefits, particularly on securing digestive and metabolic health. In addition to acting as potent nutrient signals that have diverse physiological and metabolic roles, elevated circulatory levels of BCAAs are evident in individuals with poor metabolic health. It has been suggested that circulatory AA concentrations are reflective of dietary protein intake, while digestion and absorption kinetics play a significant role in determining the subsequent appearance of AAs into circulation. There remains a gap in understanding whether the scenarios that affect dietary protein quality and related food matrix also induce an altered digestive response and postprandial AA dynamics. Further, whether suboptimal digestive and metabolic health status of the person consuming the food may affect the digestive and metabolic fate of ingested protein and reflective circulatory AAs remains poorly understood. While factors, internal and external to the food, including protein quality and cooking conditions together with digestive function of the person consuming the diet may determine the nature of postprandial digestive responses, manipulation of dietary intake and metabolic health of the individuals may determine the short-term protein metabolism. Therefore, the research detailed in this thesis aimed to investigate the acute and short-term impact of meals, diet and subsequently lifestyle (weight loss) on circulatory BCAA (and other AA) dynamics to gain more insight relevant to improving our understanding of protein digestion and AA homeostasis. Among the animal-based dietary proteins, meat and milk are the two primary sources of high-quality protein with higher digestibility. With the aim of examining the acute impact of variations in meat cooking on postprandial circulatory BCAAs and other AAs, healthy participants consumed either pan-fried (PF) or sous-vide (SV) cooked meat. Differences in the cooking conditions are likely to impact the protein ultrastructure. These effects further contribute to differential changes in the texture and tenderness of red meat, which may influence meat digestibility. Interestingly, we have shown that a significant variation in meat cooking methods (in terms of time-temperature conditions) demonstrated minimal effect on circulating AAs in healthy individuals. This investigation was then further extended to gain more insight into our understanding of whether digestion of dairy protein is impacted in individuals with altered digestive function. Postprandial AA kinetics were measured after providing three different milk-types (conventional milk (CON): containing lactose and A1 βcasein; a2 Milk™ (A2M): containing lactose but no A1 β-casein; and lactose-free conventional milk (LF-CON): containing A1 and A2 β-casein but no lactose) to individuals with divergent digestive function (lactose intolerant (LI), non-lactose dairy intolerant (NLDI) or dairy tolerant (DT)). Our work revealed for the first time that acute milk protein digestion, as determined by circulatory AAs, is largely unaffected by the presence of measured milk intolerances. Yet, the circulating AA response was dependent on the nature of digestive intolerances with the absorption of BCAAs increased in response to A1 β-casein-free milk compared to other milk types in those individuals with digestive symptoms (both the LI and NLDI). Therefore, we suggest that similar to the cooking-oriented effects in protein structure; digestive dysfunction is not a major driver of subsequent digestive and circulatory AA responses. Instead, elemental meal composition and the associated AA profiles may have a more significant impact on circulating AA concentrations. Short-term metabolic regulation of circulatory BCAAs in relation to altered dietary intake and metabolic health status in humans remains poorly defined. In healthy individuals, we showed that BCAA concentrations remain within a stable range even with modification of BCAAs from dairy intake through habitual dietary changes for four weeks. We also demonstrated that this manipulation of dietary BCAA intake had no impact on insulin sensitivity. Further investigation on continuous feeding with a low energy diet was carried out to determine whether it could improve the dysregulated protein metabolism in individuals with compromised insulin sensitivity. Continuous dietary energy restriction for eight weeks resulted in no change in fasting circulatory BCAA concentrations in pre-diabetic individuals. Dietary energy restriction contributed towards normalisation of insulin sensitivity and improved overall metabolic health. This outcome suggests that neither shortterm dietary energy deprivation (in terms of dietary protein or from total diet) nor short-term improvement in insulin sensitivity is necessarily reflected in a reduction in fasting circulatory BCAAs. This highlights that there may be other factors, including catabolic changes in metabolism during weight loss, which should be investigated as causes underpinning the maintenance of BCAA homeostasis. The series of studies conducted in this thesis highlight that protein modification and food structure was not a major driver of associated changes in circulatory AAs, including the BCAAs in healthy individuals as well as individuals with digestive intolerances. However, for individuals with digestive intolerances, this research prioritises to choose food sources selectively that provide better digestive benefits. Additionally, the current data could not identify the circulatory BCAAs responsive to short-term restriction of dietary-energy or manipulation of a single dietary protein source. Importantly, consistent with the available literature, we also showed that the circulatory BCAA levels could be effectively utilised as a marker of metabolic health, however, not as a sensitive tool for measuring interventioninduced metabolic improvement. Together, these findings offer an avenue for future research to address the long-term features of digestive and metabolic responsiveness, potentially critical for BCAA regulation.