A popular notion among many dieters is that some people are blessed with a fast metabolism, meaning they can eat whatever they want without gaining significant weight. (Or, conversely, a slow metabolism that makes it hard to stay slim and easy to put on fat.)
We’ve all heard such comments and maybe even said it about ourselves, but does this idea really hold true?
Resting metabolic rate (RMR) simply refers to the amount of energy required by your bodily organs to stay alive and accounts for approximately 70% of total energy expenditure in sedentary individuals. With that in mind, is it possible that lean individuals have a higher or ‘faster’ metabolic rate when compared to those who struggle with maintaining or losing weight?
Although there can indeed be differences in RMR between individuals, the difference is a lot less than often assumed. In fact, most people, at a given body weight, will remain within a range of just 100-300 kcal/day (the equivalent of a chocolate bar) to sustain the body’s most basic functions.
And contrary to popular belief, individuals who have excess body fat have a higher metabolic rate due to carrying a heavier body mass.
The greater discrepancy among sedentary individuals and the most variable component of total daily energy expenditure is 'non-exercise activity thermogenesis' or NEAT.
NEAT involves spontaneous activities such as fidgeting, maintaining one’s posture, walking etc. and has been shown to be significantly more responsible for an individual’s likelihood to gain weight.
One classic overfeeding study found that NEAT can account for a ten-fold difference in body fat accumulation (0.36kg to 4.23kg) over an eight-week period, despite participants being over-fed the exact same number of calories.
Age can also affect metabolic rate, which decreases slightly in later years. Part of this decline is simply an effect of the age-related loss of lean body mass, coupled with a reduction in overall activity and food intake. Getting older, however, does not have to be synonymous with a decrease in metabolic rate. Interestingly, the decline in RMR with age does not seem to occur in those who maintain exercise volume and/or energy intake to match their younger counterparts.
To sum up, it is true that some people find it harder than others to lose body fat but the idea that it’s down to your metabolism is almost certainly false, at least for the vast majority. That's why 'eat less and move more' is still the best advice if you want to lose body fat.