What it means
The old rule says a 3,500-calorie deficit equals one pound of fat. It is a rough guide for the first weeks, but it overstates long-term loss because your burn drops as you get smaller.
A deficit of about 500 calories a day is a common starting point. Larger deficits can speed early loss but are harder to sustain and raise the risk of losing muscle.
GLP-1 medicines mostly work by making a deficit easier: you feel full sooner and think about food less. The deficit itself is still what drives the loss.
Very low intake, well under about 1,200 calories a day for many women or 1,500 for many men, can make it hard to meet protein and nutrient needs. Those cutoffs are general guides, not limits for every person.
Why it matters
Every weight-loss target and timeline on Sunny Weigh comes down to the size of your deficit.