Metabolism Explained: Genetics, Lifestyle and What You Can Change
Metabolism is often blamed for weight gain. It does vary between people, but less than most think, and the parts you control add up.
SteroidsCanada.org editorial teamUpdated · 2 min read
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“I have a slow metabolism” is one of the most common explanations for stubborn weight. Metabolism is real and it does differ between people. But the science shows the differences are usually smaller than we feel they are, and much of what drives daily calorie burn is within your control.
What makes up your daily burn
| Component | Share of daily calories | What affects it |
|---|---|---|
| Resting metabolic rate | About 60 to 70% | Body size, muscle mass, age, sex, genetics, thyroid |
| Thermic effect of food | About 10% | How much you eat; protein costs the most to digest |
| Exercise | About 5 to 10% | Training volume and intensity |
| Non-exercise activity (NEAT) | About 15 to 30% | Steps, standing, fidgeting, job type |
NEAT is the most variable piece: two people of the same size can differ by several hundred calories a day simply through how much they move outside the gym.
Genetics
Genes influence resting metabolism, appetite, where you store fat and how you respond to overfeeding. Some people really do gain weight more easily. But for people of similar size and composition, measured resting metabolism usually differs by only a modest amount. Appetite and activity differences often matter more than the engine itself.
Age
A large study of more than 6,000 people found that, after adjusting for body size and composition, daily energy expenditure stays remarkably stable between about 20 and 60, then declines slowly. Much of the “middle-age slowdown” comes from losing muscle and moving less, both of which can be addressed.
Dieting and metabolic adaptation
When you lose weight, you burn fewer calories, partly because a smaller body needs less energy and partly through adaptation: lower NEAT, more efficient muscles and hormonal changes. The adaptation beyond what weight loss predicts is usually modest, but it is real and it explains why fat loss slows. Diet breaks, keeping steps up, strength training and high protein all limit it.
Practical ways to raise daily burn
- Walk more: 8,000 to 10,000 steps a day can add 200 to 400 calories.
- Lift weights: maintain or build muscle, which supports metabolism and health.
- Eat enough protein: it has the highest thermic effect and protects muscle.
- Sleep well: short sleep lowers daily activity and raises appetite.
- Break up sitting: stand or walk for a few minutes every hour.
When metabolism might be the problem
An underactive thyroid, some medications and certain hormone conditions can lower calorie burn or raise appetite. If you have symptoms such as fatigue, feeling cold and unexplained weight gain, see our guides to thyroid conditions and hormonal imbalances, and ask your doctor about testing.
Myths about metabolism
| Myth | Fact |
|---|---|
| Eating six small meals boosts metabolism | Total intake, not meal frequency, sets the calories burned digesting food |
| Starvation mode stops weight loss | Adaptation slows loss but does not stop it in a real deficit |
| Some foods have negative calories | Even celery provides more energy than it costs to digest |
Estimating your own numbers
Equations like Mifflin-St Jeor estimate resting metabolism within about 10 percent for most people. The macro calculator uses it, then you refine from your real weight trend over a few weeks. That trend is your metabolism, measured.
Questions
Does metabolism really slow down with age?
A large 2021 study found metabolism stays fairly stable from about age 20 to 60 when adjusted for body size and composition, then declines gradually.
Can certain foods boost metabolism?
Spicy food, green tea and caffeine have tiny effects. Protein has the largest effect of the macronutrients because it costs more energy to digest.
Does building muscle raise metabolism?
Yes, a little. Each kilogram of muscle burns roughly 13 calories a day at rest, more than fat but less than often claimed.
What is NEAT?
Non-exercise activity thermogenesis: calories burned through daily movement like walking, standing and fidgeting. It varies by hundreds of calories between people.
Sources
General information, not medical advice. Written for adults in Canada. Anabolic steroids are Schedule IV under the Controlled Drugs and Substances Act. Talk to a licensed healthcare provider and get bloodwork before using any compound.
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