Metabolic Adaptation during Fat Loss
Metabolic Adaptation, a complex set of physiologic changes, tends to be the reason most people stall in their weight-loss journey.
As a caloric deficit becomes more severe and/or prolonged, Total Daily Energy Expenditure (TDEE) decreases due to various mechanisms:
- The Thermic Effect of Food (TEF) decreases.
- Non-Exercise Activity Thermogenesis (NEAT) decreases.
- Resting Metabolic Rate (RMR) decreases.
- Thyroid hormones, Insulin, Testosterone, and Leptin become negatively affected.
- Mitochondria, and the body as a whole, become more efficient in conserving energy.
All of these factors lead to the eventual “stall”, although cortisol increase is negligible unless completely fasting, and the decrease in Resting Metabolic Rate tends to be much less than people assume. On average, RMR decreases only in the low hundreds of kcal/day. Therefore “metabolic damage”, as described by various fitness gurus, is largely a myth.
The main culprits for the “stall” are a decrease in activity levels and increased appetite/food intake. These are the body’s compensatory mechanisms for preventing starvation and thus a return to a weight settling point. They are the largest hurdles when trying to prevent weight-regain after a successful diet cycle.
For continued fat loss progress, two primary interventions are then required in isolation or in combination to counter these effects: increasing activity and/or decreasing calorie intake.