How Metabolic Rate Works: BMR, TEF, NEAT, and What Actually Changes It

Metabolism is not a single thing. It’s a term that gets attached to everything from cellular chemistry to weight management, which is part of why it causes so much confusion.

The actual question most people are asking when they say “I have a slow metabolism” is: why do I burn fewer calories than I expect? The answer is almost always more interesting than a malfunctioning engine.

The Four Components of Total Energy Expenditure

Your total daily energy expenditure (TDEE) has four distinct parts.

Basal metabolic rate (BMR) is the energy needed to keep you alive while at rest: heartbeat, breathing, temperature regulation, cell maintenance, organ function. It accounts for roughly 60-70% of total energy use. BMR is largely determined by body size, lean mass, age, sex, and thyroid hormone levels.

The thermic effect of food (TEF) is the energy cost of digesting and processing what you eat. It’s roughly 10% of total calories, with some variation by macronutrient. Protein has the highest thermic effect, around 20-30% of its calories go to digestion. Carbohydrates run about 5-10%. Fat is lowest at 0-3%. This is real, but the absolute numbers are modest compared to the other components.

Non-exercise activity thermogenesis (NEAT) is every calorie burned in movement that isn’t formal exercise: walking to your car, typing, fidgeting, doing household tasks, standing. NEAT is the most variable component between individuals (Levine, 2004, American Journal of Physiology), varying by up to 2,000 calories per day between people of similar size.

Exercise activity thermogenesis (EAT) is formal exercise: running, lifting, cycling. For most people, this is a smaller contributor than NEAT unless they train very frequently.

What Actually Determines BMR

Lean mass is the strongest predictor of BMR. Muscle tissue burns more calories at rest than fat tissue. The difference is meaningful but not dramatic: roughly 13 calories per kilogram of muscle per day at rest vs. about 4.5 calories per kilogram of fat. Building 5kg of muscle raises resting metabolism by maybe 65 calories per day. Not transformative, but real and cumulative.

Body size matters simply because bigger bodies require more energy to maintain. A 100kg person burns more calories at rest than a 60kg person, all else equal.

Thyroid hormone directly regulates cellular metabolic rate. Hypothyroidism (low thyroid hormone) genuinely slows metabolism and causes weight gain. This is real and diagnosable. But clinical hypothyroidism explains a fraction of metabolic complaints. Most people with stubborn weight aren’t clinically hypothyroid.

Age is a legitimate factor. BMR declines with age, partly due to reduced muscle mass (sarcopenia) and partly due to reduced cellular metabolic activity. Research by Poehlman (1989, Medicine and Science in Sports and Exercise) showed exercise training substantially mitigates age-related metabolic decline by preserving muscle mass.

Why “Slow Metabolism” Rarely Explains It

Studies using doubly labeled water (the gold standard for measuring total energy expenditure) consistently find that obese individuals have higher absolute metabolic rates than lean individuals, not lower ones, because they have more total body mass to maintain (Donahoo et al., 2004, Current Opinion in Clinical Nutrition and Metabolic Care).

The more accurate framing: people underestimate food intake and overestimate activity. This has been documented repeatedly in careful metabolic studies. It’s not intentional. Portion estimation is genuinely hard, and calorie counts in our heads for familiar foods are often low.

That said, true metabolic variability does exist between individuals of similar size, driven largely by differences in NEAT. Highly active people unconsciously move more during the day, fidget more, and stand more. Some of this is personality. Some is driven by hormones like leptin. When Levine et al. studied NEAT directly, they found it explained most of the variability in weight gain when people were overfed under controlled conditions.

Adaptive Thermogenesis: What Happens When You Diet

This is the metabolically inconvenient part.

When you cut calories significantly, your body adapts. BMR drops beyond what would be expected from the reduced body mass alone. NEAT often drops even more dramatically: people unconsciously move less when undereating. Together, these reductions are called adaptive thermogenesis (Rosenbaum and Leibel, 2010, International Journal of Obesity).

The practical consequence is that the calorie deficit you calculated based on your starting weight shrinks over time, not just because you’re lighter but because your body is running more efficiently.

This is why aggressive cuts (eating very little, very fast) tend to produce early fast results followed by frustrating stalls. Moderate deficits allow for more muscle preservation, less dramatic NEAT suppression, and a better hormonal environment for fat loss.

The counterintuitive implication: exercising more (especially adding NEAT) is often more effective for sustained weight management than eating dramatically less, because it sidesteps the adaptive thermogenesis response to caloric restriction.

This article is for educational purposes only. It’s not medical advice. Talk to your doctor or a registered dietitian before making significant changes to your diet.