Body mass index (BMI)

Body mass index (BMI) is weight in kilograms divided by height in meters squared.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 177.} The usual cut points are below 18.5 for underweight, 18.5 to 24.9 for normal, 25 to 29.9 for overweight and 30 or more for obese.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 177.} ACSM recommends lower cut points of 23 and 25 for overweight and obesity in Asian populations, who develop health problems at lower BMIs.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 177.}

BMI correlates significantly with total body fat, but it doesn’t measure body composition: it can’t distinguish fat from muscle or bone.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 532.},^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 234.} It overestimates fatness in muscular clients and underestimates it in older adults who have lost muscle.^{2Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 532.} Apart from people carrying a lot of muscle, a BMI of 30 or more indicates excess body fat.^{1Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 177.}

In practice

Use BMI as an initial screen, not a diagnosis.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 234.} It shouldn’t be the only measure of weight loss or program success: an athlete with low body fat can fall in the overweight category, while a client with little muscle can fall in the normal category despite increased risk.^{4Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 219.}

Add a method that estimates body fat percentage.^{5Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 179.} Measure waist circumference too, since a large waist can flag increased risk even at a normal BMI.^{6Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 178.}