Calorie deficit

A calorie deficit occurs when calorie intake is lower than expenditure, creating negative energy balance and producing weight loss.^{1International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 464.} It can come from eating less, exercising more or both.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 131.} A pound (about 0.5 kg) of fat tissue holds around 3,500 calories, so a 500-calorie daily deficit is a rough guide to losing a pound a week if most of the loss is fat.^{3Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 54.}

A deficit also makes muscle harder to build: one study found that a deficit of about 20% cut muscle protein synthesis by roughly 19%, while another found a 27% reduction after five days at a 500-calorie deficit.^{4Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 219.}

In practice

• Aim for a loss of 0.5 to 1% of body weight per week, a common NSCA guideline that Helms and colleagues also use to limit muscle and strength loss.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 131.},^{3Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 54.}
• Rapid loss can cause dehydration and poorer vitamin and mineral status; large deficits can also cost performance and substantial lean mass.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 131.}
• A smaller deficit takes longer but may work better overall because clients find it easier to sustain.^{5International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 468.}
• Keep protein at 1.6 g/kg of body weight a day, possibly up to 2.2 g/kg, to support satiety and muscle retention.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 132.}
• Adjust as the client diets: energy expenditure changes, so the initial calorie target is a starting point.^{7Helms et al., The Muscle & Strength Pyramid: Nutrition, 2nd ed., p. 55.}