John Blundell and colleagues have come out with an interesting new paper, “Role of resting metabolic rate and energy expenditure in hunger and appetite control: A New Formulation.” The paper provides a fascinating insight into human body weight regulation, appetite and the recurring drive to eat. The resting metabolic rate (RMR) refers to the energy needed to keep our bodies functioning. Of all of the components of energy expenditure, it is the largest, accounting for about 50%-70% of total energy expenditure. The liver contributes about 20% to the RMR, the brain another 20%, the heart 11%, the GI tract 9% and skeletal muscle another 20%. The drive to eat is predicated on getting enough energy to these systems to keep them running.
Blundell and colleagues’ paper explores a long-running research question as to the influence energy expenditure has on energy intake. As many people know, one of the problems with the ‘eat less, exercise more’ prescription is that exercising often leads to greater hunger, then to more eating, negating or lessening the weight-related benefits of exercise in the first place. They noted previous research that found that exercise caused a significant increase in levels of hunger but this was highly variable between individuals. However, they also observed an increase in post-feeding satiety signalling. What the researchers found was that fat-free body mass but not fat mass or BM is strongly correlated with meal size and daily energy intake. The write, “The strong implication of this relationship is that some privileged molecules arising from FFM (fat free mass) or some physiological consequences that reflect the activity of FFM, act as a signal to drive food intake.” They note, further, that RMR is positively associated with meal size and daily energy expenditure. It is also a strong predictor of fasting levels of hunger and influences the daily profile of hunger. Ergo, RMR is the driver of food intake. They suggest three components of appetite regulation: a drive for food arising from the physiological demand for energy; an inhibition of eating arising from signals of energy storage, primarily adipose tissue; and additional inhibitory signals arising from the mouth and GI tract.
Regarding weight management, the authors note that their observations bring additional dissatisfaction with the Body Mass Index (BMI) as a measure of excess weight. They note that persons with obese, as well as people with large muscle mass, would have a higher tendency to consume larger meals just to maintain their RMR and greater difficulty in tolerating dietary restrictions. RMR is not the cause of obesity; it is the mechanism for preserving body weight. The amount of food actually consumed is determined by energy density. A high RMR could influence weight gain by maintaining a high level of hunger but a positive energy balance (i.e. gaining additional weight) would “depend on the energy density and palatability of the diet.” Increasing fat-free mass and RMR would increase the drive to eat “meaning that weight gain becomes part of a positive, rather then a negative, feedback system. Increasing body weight therefore could facilitate further weight gain and increase the difficulty of weight loss or maintenance.” See PubMed: Role of Resting Metabolic rate and Energy Expenditure
The universal call to “maintain a healthy weight”, “avoid weight gain” and “eat less to lose weight” begs the question, “How many calories should I be consuming?” The answer is your resting metabolic rate. Going well below that rate induces strong hunger signals and a return to earlier calorie consumption levels. Almost all diet advice tries to get dieters to consume just about their RMR to encourage weight loss and avoid triggering hunger cues. RMR can be measure in the laboratory using very sophisticated tools. But most consumers will look to web-based calculators. And this is a sorry story indeed.