What is the Basal Metabolic Rate?

From the point of view of physics, basal metabolism is power, which is measured in Watts (W). The body’s surface area is used as a reference parameter as there is a significantly stronger correlation between this and the body weight, for example. This is primarily due to heat emission, which is proportional to body surface area.

An average young adult with a weight of 70 kg has a body surface area of 1.73 m² and a basal metabolism of 80 Watts, which is 80 Joules per second. This gives a daily metabolism of 6.9 million Joules or 6900 kJ. One Joule is equivalent to about 0.24 calories – so the average basal metabolism is around 1650 kcal.

A fifth of the daily basal metabolism is used by the brain, even though at less than one and a half kilograms, it is only a fraction of the body weight. Its energy requirements remain fairly constant and do not change significantly with mental activity.

The liver and skeletal-muscular system are also among the most significant consumers of energy in the body, at 25% each.

A person’s basal metabolism depends on various factors. As well as age, these primarily include body composition and gender.

The basal metabolism is at its highest in the growth stages of childhood and puberty. It remains constant in adults until they reach their 30s and gradually declines over the rest of their lives. At every age, the proportion of skeletal-muscular and fat masses play a central role, as these tissues have different specific metabolic rates. Whilst 1 kg of muscle mass requires approximately 13 kcal a day, it is only about 4.5 kcal for fat tissue. Gender-specific differences can be derived from this, as women generally have lower energy requirements than men, due to their higher proportion of fat and lower proportion of muscle.

A physically active person with a higher proportion of muscle also needs more energy at rest than a person of the same weight, but a higher proportion of fat. In relation to the overall basal metabolism, this difference only becomes significant at a higher muscle mass. For example, if 1 kg of muscle is put on and at the same time the same mass of fat is lost, the daily basal metabolism increases by only around 10 kcal.

Hormones, pregnancy or illness also influence the basal metabolism. In the case of a fever, the basal metabolism rises by around 14% per degree Celsius. Conversely, the basal metabolism can also fall. If the energy supply reduces through long-term starvation or fasting, the body can reduce its basal metabolism by up to 40%.

Overall, there are only small variations in basal metabolism between people. An individual’s level of activity is of more importance in daily overall energy requirements. If you want to lose weight in the long-term, you should not expect a great effect or increase in your basal metabolic rate. Similarly, extreme diets are not recommended, as they might promise short-term effects, but can lead to weight gain in the medium or long term. The key to permanent and healthy weight loss is a moderate reduction in calories and regular physical activity.

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