Metabolic Rate

Metabolic rate is the rate of metabolism, the amount of energy used by an animal per unit of time. The metabolic rate of the body is the overall rate of tissue oxidation of fuels by all the body’s organs. Basal metabolic rate (BMR) is the amount of energy used daily by animals at rest. Basal metabolic rate is the number of calories your body needs to accomplish its most basic (basal) life-sustaining functions.

Energy expenditure breakdown

  • Liver 27%
  • Brain 19%
  • Heart 7%
  • Kidneys 10%
  • Skeletal muscle 18%
  • Other organs 19%

About 70% of a human’s total energy use is due to the basal life processes within the organs of the body (see table). About 20% of one’s energy use comes from physical activity and another 10% from the digestion of food after eating. Even when resting, your body burns calories by performing basic functions to sustain life, such as: breathing, circulation, nutrient processing, cell production, etc.

All of these processes require an intake of oxygen to provide energy for survival, usually from macronutrients like carbohydrates, fats, and proteins. The Krebs cycle produces energy-rich ATP molecules and gives off carbon dioxide.

Individual differences

The metabolic rate varies between individuals. One study of 150 adults representative of the population in Scotland reported basal metabolic rates from as low as 1027 kcal per day (4301 kJ) to as high as 2499 kcal (10455 kJ). The average was 1500 kcal (6279 kJ) per day.

One popular way to estimate BMR is through the Harris-Benedict formula, which takes into account weight, height, age, and gender.

Women: BMR = 655 + (9.6 × weight in kg) + (1.8 × height in cm) – (4.7 × age in years)

Men: BMR = 66 + (13.7 × weight in kg) + (5 × height in cm) – (6.8 × age in years)

The researchers calculated that 62.3% of this variation was explained by differences in the mass (weight) minus the fat reserves. Other factors were the amount of fat (6.7%), age (1.7%), and experimental error including within-subject difference (2%). The rest of the variation (26.7%) was unexplained.

So there are differences in BMR even when comparing two subjects with the same lean body mass. The top 5% of people are metabolizing energy 28-32% faster than individuals with the lowest 5% BMR. For instance, one study reported an extreme case where two individuals with the same lean body mass of 43 kg had BMRs of 1075 kcal/day (4.5 MJ) and 1790 kcal/day (7.5 MJ). This difference of 715 kcal (67%) is equivalent to one of the individuals completing a 10-kilometer run every day.

Scaling effects

Metabolic rates vary according to animal size, and this has been discussed for over a century.

The graphs show that :

  • The metabolic rate of mammals is a regular function of their body size, and
  • The function is significantly different from a direct function of their body surfaces.
  • On a log-log scale, the metabolism of mammals as related to their body size forms a straight line with a slope of about 0.75.
  • Later research showed that similar relationships hold for ‘cold-blooded’ animals and protists.