Calories determine the direction
The simplest useful explanation of calories in and calories out.
The short answer
Over time, a sustained calorie surplus tends to increase body weight, a deficit tends to reduce it and maintenance keeps the average broadly stable.
Energy balance without pretending the body is simple
Calories in are the energy absorbed from food and drink. Calories out include resting metabolism, movement, training and digestion. Both sides vary, but the long-term direction remains useful for changing body composition.
Body composition is not the whole of health
Food quality, fibre, micronutrients and variety matter for health, satiety and how well you feel. They do not cancel energy balance. For appearance and body-weight direction, calories and adequate protein are the first variables to understand; health deserves attention beyond that simple calculation.
Remember: Use calories to set the direction. Use food quality to make that direction healthier and easier to sustain.
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