It requires a caloric deficit of 3,500 calories to lose one pound of body fat.
"if you just look at numbers, it takes 3,500 calories to lose 1 lb of fat. 3,500 calories. So, to lose 2 lb, you're talking a deficit of 7,000 calories." (said at 0:11:39)
The statement refers to the classic "3,500-kcal rule" (derived from Max Wishnofsky's 1958 calculation), which is based on the approximate energy content of one pound of human adipose tissue (~454 grams composed of ~80–87% pure lipid yielding ~3,500 kcal). While this figure is a widely cited theoretical benchmark for the energy density of fat tissue, applying it as a linear formula to predict actual human weight or fat loss is biologically inaccurate. In vivo, caloric restriction triggers dynamic physiological changes, including reductions in basal metabolic rate, changes in non-resting energy expenditure (adaptive thermogenesis), and the concurrent loss of lean body mass and water (which have much lower energy density than fat). Consequently, a static cumulative deficit of 3,500 kcal does not produce a continuous, linear 1-lb weight loss over time, and dynamic energy-balance models have largely replaced this rule in clinical research.
- context: What is the required energy deficit per unit weight loss? (International journal of obesity (2005) 2008)
"The rule of thumb approximately matches the predicted energy density of lost weight in obese subjects with an initial body fat above 30 kg but overestimates the cumulative energy deficit required per unit weight loss for people with lower initial body fat." (abstract, results, passage verified)
pubmedfull study (doi) - context: Can a weight loss of one pound a week be achieved with a 3500-kcal deficit? Commentary on … (International journal of obesity (2005) 2013)
"Despite theoretical evidence that the model commonly referred to as the 3500-kcal rule grossly overestimates actual weight loss, widespread application of the 3500-kcal formula continues to appear in textbooks, on respected government- and health-related websites, and scientific research publications." (abstract, results, passage verified)
pubmedfull study (doi)