Do skinny people who eat a lot ‘burn it off as heat’?

I used to think that people who stay lean - despite eating a lot - must have “fast metabolisms” or are “burning calories as heat” – like a radiator.

Physiologically, that’s mostly wrong.

The human body is actually very good at not wasting energy.

In healthy people, excess carbohydrate is absorbed, stored, and conserved — not excreted. Heat production does increase slightly when we eat more, but it’s capped and modest.

The real difference between people who gain weight easily and those who don’t isn’t  heat overproduction.

It’s movement.

Specifically NEAT - Non-Exercise Activity Thermogenesis

We all know someone who when sitting their leg is constantly bouncing.

NEAT includes:

• Fidgeting
• Posture changes
• Standing vs sitting
• Pacing
• General restlessness

Key point: NEAT can vary by over 1000–2000 kcal/day between individuals!

Classic overfeeding studies show:

• Some people subconsciously move far more when overfed
• Others become lethargic
• Fat gain tracked inversely with NEAT increase

Skinny “big eaters” are often:

• Constantly moving
• Unable to sit still
• Standing, walking, gesturing more — without noticing

This burns a lot of energy as work, only a small amount as heat.

Levine JA. Non-exercise activity thermogenesis (NEAT). Review on how NEAT contributes to daily energy expenditure and obesity. Mayo Clinic

Levine JA. Nonexercise Activity Thermogenesis (NEAT): Environment and Biology. Endocrinology review describing variability in NEAT and links to energy balance. Mayo Clinic

Non-Exercise Activity Thermogenesis in Human Energy Homeostasis — NEAT varies greatly and may explain differences in obesity susceptibility. PubMed

Overfeeding study showing NEAT increases with surplus calories and predicts fat gain resistance. Active Working

Diet-induced thermogenesis — PubMed review (NIH): overview of how nutrient metabolism increases energy expenditure above basal metabolic rate. PubMed Central

Specific Dynamic Action (thermic effect of food): TEF increases metabolic rate modestly and varies by macronutrient. Wikipedia