What CICO is — and what it is not
Calories in, calories out (CICO) is a statement of the first law of thermodynamics applied to human metabolism: energy cannot be created or destroyed. If you consume more energy than you expend, the excess is stored (primarily as body fat). If you expend more than you consume, stored energy is released.
CICO is not a diet plan. It does not say what to eat, when to eat, or how to achieve a deficit. It is a fundamental physical law that governs weight change. No legitimate study has ever demonstrated sustained weight gain in the absence of a calorie surplus, or sustained weight loss without a calorie deficit.
The "calories out" side is more complex than people think
Critics of CICO often misunderstand what "calories out" means. Total daily energy expenditure is not a fixed number — it has four major components, all of which change based on diet, activity, and physiology:
- BMR (Basal Metabolic Rate): ~60–70% of total expenditure. Decreases as body weight falls.
- TEF (Thermic Effect of Food): ~10% of total expenditure. Protein has 20–30%, carbs 5–10%, fat 0–3%.
- EAT (Exercise Activity Thermogenesis): Highly variable — from near zero to 1,000+ kcal/day.
- NEAT (Non-Exercise Activity Thermogenesis): Can vary by up to 2,000 kcal/day between individuals and changes with calorie intake.
When you diet, NEAT often drops — you unconsciously move less, fidget less, and use less energy. This is why TDEE calculators are estimates, not exact figures, and why real-world weight loss often slows compared to mathematical predictions.
Not all calories are equal for body composition
For total weight change, a calorie is a calorie — 500 kcal of chicken and 500 kcal of cake will produce the same weight change if all other variables are held constant. But for body composition, macronutrient composition matters enormously.
Two people eating 1,700 kcal/day will lose different amounts of fat versus muscle depending on their protein intake. The high-protein dieter preserves more muscle, loses more fat as a proportion of weight lost, has better satiety, and likely sustains the deficit for longer.
Food quality also affects NEAT. Ultra-processed foods may contribute to unconsciously reduced movement and lower metabolic rate over time — an area of active research but consistent with the practical observation that people often feel more energetic on whole-food diets.
Hormones affect both sides of the equation
Hormones influence both calorie intake (hunger and satiety signals) and calorie expenditure (metabolic rate and activity levels). Conditions like hypothyroidism, polycystic ovary syndrome (PCOS), and insulin resistance genuinely affect the CICO equation — but they affect it at the level of the inputs and outputs, not by violating the law of thermodynamics.
A person with hypothyroidism has a lower "calories out" number than predicted by standard equations — meaning they need to consume fewer calories to maintain weight. The solution is still a calorie deficit; the deficit just needs to account for their actual, lower metabolic rate.
How to use CICO practically
Start with a TDEE estimate, create a 300–500 kcal daily deficit, and track calories accurately for 2–4 weeks. Compare actual weight change to predicted change. If weight loss is slower than expected, your actual TDEE may be lower than calculated — reduce calories by another 100–150 kcal. If it matches or exceeds predictions, you have an accurate estimate.
CICO works. But making it work sustainably requires managing hunger (through adequate protein, fibre, and food volume), accounting for adaptive changes (by recalculating as weight drops), and being consistent over months rather than weeks.