Factors Affecting Energy Metabolism: SarcopeniaTuesday, June 19, 2018
Continuing with citations from my article in Obesity Reviews on an aeteological framework for assessing obesity, we continue discussing factors that can affect metabolic rate:
The importance of fat-free mass as the key determinant of resting metabolic rate, even in a very obese individual [sic], cannot be over emphasized. Obese individuals [sic] can present with wide variations in lean body mass, almost entirely accounted for by differences in skeletal muscle mass. Thus, any change in muscle mass can markedly affect basal energy requirements. In this context it is important to remember that in ambulatory individuals, the mass of weight‐bearing muscles is directly proportional to BMI, as heavier individuals require a greater skeletal muscle mass to support and move their excess weight. This alone accounts for much of the higher basal and activity‐related energy requirements of larger individuals.
Although inactivity may be the most common cause of decreased skeletal muscle mass and reduced basal metabolic needs in obese individuals [sic], it is important to consider other causes of muscular atrophy that can likewise markedly reduce energy demands. A wide range of nutritional, neuromuscular, endocrine, renal, cardiac, pulmonary, inflammatory, infectious or neoplastic conditions can result in muscular wasting and sarcopenia. Reduced skeletal muscle mass and weight gain is also noted after many cancer treatments, although the mechanisms remain unclear. Any reduction in skeletal muscle mass not accounted for by a decrease in physical activity and ambulation should prompt investigations for other causes of muscular wasting.
Commentary: Sarcopenic obesity is perhaps even more prevalent than most people may think – especially in people who have slight overweight or even moderate obesity. It is particularly common in certain ethnic groups such as South Asians, even at “normal” BMIs. Clinically, this is where body composition studies can be helpful. Although a reduction in muscle mass does reduce resting metabolic rate (RMR), it is important to remember that overall skeletal muscle only accounts for about 15% of RMR. This is why, the notion that building up muscle mass will help with weight loss by burning more calories is not really an effective weight loss strategy.