What is a calorie deficit?
A calorie deficit happens when you eat fewer calories than your body burns. That's it. No magic, no special foods. Your body covers that gap by burning stored energy — primarily body fat.
This is the actual mechanism behind every successful weight loss method ever studied. Low-carb, low-fat, intermittent fasting, keto — they all work, when they work, because they create a calorie deficit.
Three deficit options
Mild (250 cal): ~0.5 lb/week. Easiest to sustain. Recommended for most people, especially those with 10-30 pounds to lose.
Moderate (500 cal): ~1 lb/week. The classic. Works well with strong baseline habits.
Aggressive (750 cal): ~1.5 lb/week. Physically uncomfortable. Not recommended for more than 4-6 weeks.
Why most deficits fail
Hunger increases. Metabolism adapts. NEAT decreases without you noticing. These aren't moral failures — they're predictable responses to caloric restriction.
How GLP-1 medications change the equation
GLP-1s work directly on biological pushback mechanisms. They reduce appetite at the brain level, slow gastric emptying, and quiet "food noise." In clinical trials, people on semaglutide ate roughly 30% fewer calories without being told to restrict and lost 15% of body weight on average over 68 weeks.