ADRENERGIC RECEPTOR (ADR) GENOTYPE INFLUENCES THE EFFECTS OF STRENGTH TRAINING ON MID-THIGH INTERMUSCULAR FAT
Sarcopenia is the age-associated loss of muscle mass and strength. These losses are also associated with a deterioration in health status and with adverse effects on functional abilities in the elderly (1). Aging adversely affects the quality, as well as the quantity, of skeletal muscle (SM). For example, in African American women, fat infiltration of muscle increases with age (2), leading to the accumulation of intermuscular fat (IMF) and the development of low density muscle (LDM). Elevated levels of thigh IMF have been linked to insulin resistance in muscle and to the development of type 2 diabetes (3, 4). In addition, higher LDM is associated with lower muscle strength (5), with poorer leg function (6) and with greater incidence of mobility limitations in the elderly (7).
Despite this relationship of limb IMF and LDM to health and functional status and that ST is now commonly prescribed for the prevention and treatment of sarcopenia (8), little or no information is available on the effects of ST on limb IMF or LDM. In this regard, Sipila et al. (9) reported a reduced percent of thigh IMF in response to ST, but no information on absolute IMF change was provided. The reduced percentage of fat may have just been due to the increase in thigh muscle mass, which lowers the percent of fat tissue, even if the total mass of fat doesn’t change. One study showed that the combination of ST with aerobic exercise training decreased LDM (10) and another study showed that ST together with a low-calorie diet reduced thigh IMF
(11). Unfortunately, neither of these two studies can tell us the independent effects of ST on IMF or LDM. Thus, the first purpose of this study was to examine the effects of ST on IMF and LDM.
ST has been showed to increase sympathetic nerve activity (12). Norepinephrine (NE) derived from sympathetic nerves regulates lipolysis by binding to stimulatory adrenergic receptors (ADR) (β1, 2, or 3), mainly ADRβ2 in skeletal muscle (13), and inhibitory ADRα2b receptors. The balance between ADRβ2 and ADRα2b can thus determine the relative efficacy of NE as a lipolytic hormone. ADRβ2 Gln27Glu polymorphisms have been associated with fat adaptation to exercise training in some
(14, 15), but not all studies (16). The combined effects of ADRβ2 Gln27Glu and ADRα2b Glu12/Glu9 polymorphisms on the total body fat reduction induced by aerobic exercise training (15). However, studies investigating genotype influences of ST effects on fat phenotypes are unavailable. Thus, the second purpose of this study was to investigate ADR genotype influences on IMF and LDM responses to ST. We hypothesized that ST will significantly reduce IMF and LDM, and ADRβ2 and ADRα2b gene variants will significantly influence these effects.