Basal metabolic rate

OVERVIEW

Basal metabolic rate is the body’s caloric consumption during the following conditions:

1. at rest
2. horizontal position – no muscular activity
3. ambient temperature in the thermoneutral zone (ie. 25-30 C)
4. 12 hours after the last meal
5. related to body surface area

Total daily energy expenditure
= sum of basal metabolic rate + thermogenesis (shivering) + physical activity
-> normal per day = 25kcal/kg or 100kJ/kg

MEASUREMENT OF BMR

Direct calorimetry

• amount of heat produced by body over an hour via an insulated Atwater-Benedict chamber

Indirect calorimetry

• O2 consumption measured on the assumption that it is proportional to metabolic rate
• assumed that the respiratory quotient = 0.75 in the fasting state
• respiratory quotient = ratio of CO2 produced : O2 consumed (1.0 when fuel carbohydrates, 0.8 when protein, 0.7 when fat)
• patient breaths in and out of a Benedict-Roth bell spirometer filled with O2 & with a CO2 absorber
• rate of decline of the volume of spirometer = O2 consumption

Calculation of Resting Energy Expenditure

• using the Harris-Benedict equation

REE (males) = 66.5 + (13.7 x body weight in kg) + (5.0 x height in cm) (6.8 x age in years)
REE (females) = 66.5 + (9.6 x body weight in kg) + (1.7 x height in cm) (4.7 x age in years)

• use ideal body weight
• resting energy expenditure in calories
• multiply this by a stress factor to allow for effects of disease (no exercise = 1.2, very heavy exercise twice a day = 1.9)

• more accurate to measure REE by indirect calorimetry