The Fundamental Rule of Weight Management
If you have ever looked into weight loss or weight gain, you have probably heard the phrase "Calories In vs. Calories Out" (CICO). While human metabolism is complex, CICO represents the fundamental physical law of thermodynamics applied to the human body: energy balance.
Understanding the Equation
The concept is simple:
- Calories In: The total amount of energy you consume through food and beverages.
- Calories Out: The total amount of energy your body burns. This includes your basal metabolic rate (BMR), the thermic effect of food (the energy used to digest food), and physical activity.
The Three States of Energy Balance
Depending on how your "Calories In" compares to your "Calories Out", your body will be in one of three states:
1. The Caloric Deficit (Weight Loss)
Calories In < Calories Out
If you consume fewer calories than your body burns, your body must find energy from somewhere else to keep functioning. It does this by breaking down stored energy—primarily body fat. Over time, a caloric deficit results in weight loss.
2. Maintenance (Weight Maintenance)
Calories In = Calories Out
If you consume exactly the same amount of calories that your body burns, your weight will remain stable. The exact number of calories this requires is known as your Total Daily Energy Expenditure (TDEE).
3. The Caloric Surplus (Weight Gain)
Calories In > Calories Out
If you consume more calories than your body burns, the excess energy is stored. Depending on your activity level (such as strength training) and genetics, this excess energy will be stored as muscle tissue or body fat.
Is It Really That Simple?
From a pure weight-change perspective, yes. To lose weight, you must be in a deficit. However, health and body composition are more complex than just CICO.
For example, 1,500 calories of junk food and 1,500 calories of lean protein and vegetables will have the same effect on your scale weight, but they will have vastly different effects on your hunger levels, hormone regulation, energy, and muscle retention. This is why balancing your macronutrients is highly recommended.
Next Steps
To put CICO into practice, you first need to estimate your "Calories Out." You can do this using our TDEE Calculator. Once you know your TDEE, you can adjust your "Calories In" to match your goals.