Basal metabolic rate (BMR)

Basal metabolic rate (BMR) is the energy the body uses at rest to keep breathing, maintain heart function and control temperature.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 121.} The NSCA calls it resting metabolic rate (RMR) and estimates it at 60 to 75% of total daily energy expenditure, except in manual laborers and athletes in heavy training.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 121.} The IPTA textbook gives 50 to 65%.^{2International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 465.} Norton and Baxter call it “existence calories” and describe a much smaller share in very active people.^{3Norton and Baxter, The Complete Reverse Dieting Guide, p. 14.}

BMR and RMR are often used interchangeably, though they can differ by up to 10%.^{2International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 465.} BMR is measured in a lab first thing in the morning after an overnight stay; RMR is measured after the client has woken up and traveled in.^{2International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 465.} BMR is therefore the more controlled measurement, but either can vary with stress, caffeine or fever.^{2International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 465.}

In practice

Most clients can’t get a lab test, so trainers estimate BMR with an equation such as Harris-Benedict, using factors including sex, height and weight.^{4International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 466.} Multiplying by an activity factor from 1.2 to 1.9 gives an estimate of daily needs.^{4International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 466.}

RMR rises with gains in lean tissue and falls with low calorie intake or loss of lean tissue; normal genetic variation can produce differences of up to 20% between people.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 121.} Building lean mass may raise BMR.^{4International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 466.} Dieting also causes metabolic adaptation: Norton and Baxter report that BMR averages about 15% below what weight loss alone would predict.^{5Norton and Baxter, The Complete Reverse Dieting Guide, p. 18.}