What factors influence your metabolism?
Muscle mass, abdominal fat, age, sex and the way you lose weight together determine how fast your metabolism runs. Building and maintaining muscle through strength training is the most direct lever you have in your own hands.
Muscle mass is the most powerful driver of your resting metabolism. Muscles and organs (together known as lean mass) burn more energy than fat tissue, even when you are sitting still. People with more muscle mass have a higher resting metabolic rate. This applies to athletes, but also after surgery: people who lose muscle mass following gastric bypass surgery see their resting metabolism decline and are, in the long run, more likely to regain weight.
Abdominal fat around the organs, also known as visceral fat, is a second major player. Too much visceral fat disrupts metabolism through several pathways at once: higher blood lipids, more inflammatory substances in the blood, and reduced sensitivity to insulin. How much visceral fat a person accumulates depends in part on age, sex, genetics and ethnic background. The good news is that weight loss through lifestyle changes preferentially targets this abdominal fat.
Drastically crash-dieting on calories has a hidden side effect. After extreme weight loss, the body permanently shifts into a more economical mode: resting metabolism drops substantially and stays that way, even years later. Participants in an intensive weight-loss programme still had a resting metabolic rate that was, on average, around 700 kilocalories per day lower than before the diet, six years after the programme ended. Even people who later regain weight do not automatically have a 'recovered' metabolism. This makes crash diets counterproductive in the long term.
Ageing gradually slows metabolism, partly because the production of growth hormone and sex hormones declines. Hormone replacement as a solution works only to a limited extent: for most people, the side effects do not outweigh the benefits, except in specific situations such as after menopause or in cases of a diagnosed hormone deficiency. Regular physical activity, both endurance training and strength training, combined with a balanced diet, is proven to be more effective as a counterweight to age-related metabolic decline.
Sex also shapes how your metabolism responds. In women, muscle mass is associated with protein and fat intake; in men, that relationship is less clear. Metabolism is therefore not just a matter of weight or lifestyle, but also of who you are biologically.
This answer is based on multiple human studies, including research involving athletes, bariatric patients, people with post-polio syndrome and participants in an intensive weight-loss programme who were followed for six years. The evidence for muscle mass as a predictor of metabolism is strong and consistent. The findings on metabolic adaptation after crash dieting come from a single long-term study in a specific group, which limits extrapolation to the general population. The hormonal claims are based on reviews without quantified effect sizes.