Body Composition and Metabolic Changes With Aging

Body composition analysis provides a means to partition body weight into various compartments, organs, and tissues, providing valuable information regarding an individual’s health and functional status. Body weight reflects the combined weight of all the body’s tissues, while body composition refers to the relative proportions of fat and lean mass. Fat-free mass includes cellular water, protein, and minerals within adipocytes. Fat mass (FM) refers to energy stores, with tri-glycerol accounting for ≈ 60%-90% of adipose tissue. Lean body mass (LBM) refers to the sum of all lean compartments, organs, and tissues except bone and including nonadipose lipids. Body composition analysis serves multiple clinical uses, including assessment of changes associated with aging and certain disease states. Both simple and advanced methods for body composition analysis exist and are based on the techniques by which the body is partitioned. These include various anthropometric methods (e.g., BMI, waist circumference, and skinfold thickness), which are widely used in population studies, and certain research techniques (e.g., DXA, CT, MRI, NMRS, and ultrasound, impedance, isotopic body water measures, and hydrodensitometry). Aging is associated with decreases in LBM, increases in total body and regional FM, and decreases in bone mass. Decreases in LBM contribute to sarcopenia, increased functional impairment, falls, frailty, disability, and mortality. The corresponding increases in central adiposity contribute to insulin resistance (IR), type 2 diabetes, dyslipidemia, increased inflammation and thrombosis, and endothelial and cardiovascular dysfunction. Body composition and hormonal changes with age, especially decreases in GH/IGF-I, testosterone, and DHEA/DHEAS, contribute to accelerated atherogenesis. Aging is associated with multiple changes in metabolic functions leading to (1) decreased anabolism and increased catabolism and (2) IR, dysglycemia, and type 2 diabetes. Factors contributing to IR include changes in diet, exercise and body composition, comorbid conditions and medications, and diminished brown fat and hormone production. Age-related increases in IR are associated with enhanced risks of obesity, frailty, Type 2 diabetes mellitus, atherosclerotic cardiovascular disease, dementia, and cancer, and serve as a biomarker of ill health and decreased longevity. Further research is warranted to identify effective, safe, and practical nonpharmacological and targeted pharmacological interventions to increases in insulin sensitivity, and to determine whether enhancing insulin sensitivity will improve healthspan.