Abstract
Globally, people 65 years of age and older are the fastest growing segment of the population. Physiological manifestations of the aging process include undesirable changes in body composition, declines in cardiorespiratory fitness, and reductions in skeletal muscle size and function (i.e., sarcopenia) that are independently associated with mortality. Decrements in muscle protein synthetic responses to anabolic stimuli (i.e., anabolic resistance), such as protein feeding or physical activity, are highly characteristic of the aging skeletal muscle phenotype and play a fundamental role in the development of sarcopenia. A more definitive understanding of the mechanisms underlying this age-associated reduction in anabolic responsiveness will help to guide promyogenic and function promoting therapies. Recent studies have provided evidence in support of a bidirectional gut-muscle axis with implications for aging muscle health. This review will examine how age-related changes in gut microbiota composition may impact anabolic response to protein feeding through adverse changes in protein digestion and amino acid absorption, circulating amino acid availability, anabolic hormone production and responsiveness, and intramuscular anabolic signaling. We conclude by reviewing literature describing lifestyle habits suspected to contribute to age-related changes in the microbiome with the goal of identifying evidence-informed strategies to preserve microbial homeostasis, anabolic sensitivity, and skeletal muscle with advancing age.
Highlights
By the year 2030, all Baby Boomers will be at least 65 years of age, and this portion of the population will comprise more than 20% of individuals residing in the United States [1].The aging process is associated with pervasive physiological declines that are exemplified by reductions in size and function of skeletal muscle [2]
The regulation of skeletal muscle mass is dictated by temporal fluctuations in muscle protein synthesis (MPS) and muscle protein breakdown (MPB) [4]
Evidence is lacking to indicate whether sleep restoration may help to improve gut microbiome health, it is worthy to note that sleep habits are demonstrated to influence myofibrillar protein synthesis [192], and that poor sleep duration and/or quality has been linked to an increased likelihood of sarcopenia [193]
Summary
By the year 2030, all Baby Boomers will be at least 65 years of age, and this portion of the population will comprise more than 20% of individuals residing in the United States [1]. The gut microbiota of long-lived individuals (e.g., centenarians) will be considered as this population provides researchers with an insightful model of healthy aging With these mechanisms and considerations in mind, we will evaluate lifestyle habits suspected to contribute to age-related changes in the microbiome with the goal of identifying evidenceinformed strategies to preserve microbial homeostasis and potentially improve anabolic sensitivity in older adults. Several reports examining the microbiome of healthy, long-lived animal and human models (≥90 years) have observed microbial signatures containing elevated levels of SCFA producers [87,88,89] Interpreted in unison, these findings highlight that age-related changes to protein digestion and absorption in the small intestine, via microbial interactions with proteolytic enzymes, may contribute to anabolic responsiveness. The microbiome may have its greatest impact on protein metabolism within the large intestine through the production of various protein metabolites with implications for down-stream protein synthetic processes, which will be discussed in greater detail in future sections
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