Understanding Metabolism: What You Can and Cannot Change

“I have a slow metabolism” is one of the most common explanations for difficult weight loss. Sometimes there is truth in it; often the picture is more complicated. Here is how metabolism works and what you can do about it.

The four parts of daily energy use

What determines resting metabolism

Larger bodies burn more energy at rest. Lean tissue, especially organs and muscle, uses more energy than fat. Men usually have higher RMR than women because they are larger and carry more muscle. When adjusted for body size and composition, differences between people shrink, though some variation of roughly 5–10% remains.

Age and metabolism

A landmark 2021 study in Science by Pontzer and colleagues measured energy expenditure in over 6,000 people aged 8 days to 95 years. After adjusting for body size and composition, metabolism was stable from about age 20 to 60, then declined slowly. Weight gain in middle age is therefore driven more by reduced activity, muscle loss and lifestyle than by a sudden metabolic slowdown.

Genetics

Genes influence body weight through appetite, food preferences, fat distribution and how much people move, more than through resting metabolism itself. They load the dice but do not decide the outcome. See hormonal causes of weight gain.

Medical causes of a slower metabolism

• Hypothyroidism: common, especially in women; easily diagnosed with a TSH blood test. See thyroid conditions and weight.
• Cushing’s syndrome (excess cortisol), rare
• Some medications: certain antidepressants, antipsychotics, insulin, corticosteroids and beta blockers can promote weight gain.
• Low testosterone in men

What really changes your metabolism

1. NEAT: the biggest lever. Two people of the same size can differ by several hundred calories a day in non-exercise movement. See NEAT and daily movement.
2. Muscle: each kilogram of muscle burns roughly 13 calories a day at rest, modest but meaningful over years, and muscle improves insulin sensitivity.
3. Protein: about 20–30% of protein calories are used in digesting it, compared with 5–10% for carbohydrates.
4. Exercise: adds directly to daily energy use.
5. Sleep: poor sleep increases appetite and reduces daily activity.

Metabolism during dieting

When you lose weight, energy use falls because your body is smaller, and sometimes by an extra amount called adaptive thermogenesis. That is why weight loss slows over time. See metabolic adaptation.

How to estimate your own energy needs

Online calculators estimate resting metabolism using formulas such as Mifflin-St Jeor, then multiply by an activity factor. They are reasonable starting points but can be off by 10–20% for an individual. The most reliable method is practical: track food intake and body weight for two to three weeks. If weight is stable, your average intake is close to your maintenance calories. Adjust from there.

Myths

• “Eating six small meals boosts metabolism.” Meal frequency does not meaningfully change daily energy use.
• “Spicy food or green tea burns lots of fat.” Effects are tiny.
• “Starvation mode stops weight loss.” Metabolism adapts, but a true deficit still leads to weight loss.

Canadian lifestyle factors

Long commutes by car, desk work and cold winters lower daily movement for many Canadians. Statistics Canada data suggest most adults do not meet the guideline of 150 minutes of moderate-to-vigorous activity a week. Building movement into the day, such as walking meetings, stairs, indoor walking in winter and active commuting where possible, often matters more than any metabolism “hack”. See fat loss with a desk job.

Frequently asked questions

Photo: Burst, StockSnap (public domain).

General information, not medical advice.