22. Energy balance. Influencing factors, pathological changes.
Metabolic rate
Basal metabolic rate (BMR) is the amount of energy per unit time that a person needs to keep the body functioning. It can be measured when several conditions are met:
- Subject must be in a thermoneutral environment
- Measured 12 hours after last meals
- No drugs or coffee
- Measured in an awake, lying down and resting position
- The subject should be without physical and psychological stress
BMR can be measured by measuring the amount of CO2 produced compared to the O2 consumed (respiratory quotient). This method is called indirect calorimetry. It can also be measured by direct calorimetry, where the amount of produced heat is measured.
Most of the energy consumed by the BMR is used to maintain fluid levels in tissues by osmoregulation. Only one tenth is consumed for mechanical work like digestion, heart beat or breathing.
BMR is an individual metric that is mostly constant for each individual person, at least until that person changes their body composition or ages.
The BMR may be increased compared to normal in:
- Hyperthyroidism
- Congestive heart failure
- Fever
- Psychological stress
- People with higher lean body mass (more muscle)
The BMR may be decreased compared to normal in:
- Hypothyroidism
- Problems with the anterior pituitary
- Starvation
- Old age
- Peripheral circulatory failure
The most important determinant of interindividual differences in BMR for healthy people is lean body mass according to a study[1]. If you want to increase your BMR you should gain muscle mass. Another study[2] showed that the age-related decline in BMR is due to age-related decline in lean body mass and not due to aging itself.
Resting metabolic rate (RMR) is the metabolic rate in states in absence of exercise or significant mental activity. It’s always higher than the BMR because it includes non-exercise activity thermogenesis (NEAT) and thermic effect of feeding (TEF). NEAT includes energy spent doing tasks like walking to work, typing, performing yard work and such. TEF is the energy spent to chew, digest and move food through the GI tract.
The total daily energy expenditure (TDEE) is the sum of the BMR + TEF + NEAT + exercise + post-exercise oxygen consumption. It can be calculated by the double labelled water method. This method involves giving the subject water where both the hydrogen and oxygen are labelled as isotopes (as deuterium (D2) and 18O). These isotopes aren’t dangerous and are treated just like normal hydrogen and oxygen in the body. By measuring the rate of emptying of these two isotopes from the body is it possible to calculate the average TDEE over a longer time.
The TDEE of young men doing light work is around 2200 – 2600 kcal/day, and these people should consume the same amount of energy from food. The TDEE is lower in women and children.
Heat
Virtually all our metabolism goes into making heat. If we consider the young man from above can we determine that he uses on average approximately 100 kcal/hour. This man should lose 100kcal equivalent of heat every hour as well to maintain thermal balance. If no heat was lost would this man’s body temperature go up by 1° C every hour!
It’s understandable that it’s essential for the body to continuously change the amount of heat lost as the metabolism changes during the day. Keeping heat loss equal to metabolic rate is important for maintenance of core temperature in the short run. In case of imbalance will there be hypothermia or hyperthermia.
In the long run keeping food calorie intake equal to the metabolic rate will determine the body composition, and imbalances will cause undernutrition or obesity.