What exactly is the basal metabolic rate?
The basal metabolic rate (BMR, also known as basal energy expenditure) describes the amount of energy the body needs at complete rest to maintain its vital functions. These include, among others:
- Breathing and heartbeat
- Maintaining body temperature
- Cellular metabolism and cell renewal
- The function of organs such as the liver, kidneys and brain
Remarkably, depending on lifestyle, the basal metabolic rate can account for 60 to 75 per cent of total daily energy expenditure. This means that even someone who spends the entire day in bed already burns a considerable number of calories – simply so that the body can function.
The basal metabolic rate should be distinguished from the activity-related energy expenditure, i.e. the additional energy we use through movement, work and sport. Taken together, both make up the total daily energy requirement.
Calculating the basal metabolic rate: formulas and their limits
Anyone wishing to calculate their basal metabolic rate can use recognised estimation formulas. One particularly widely used option is the Mifflin-St Jeor equation:
- Men: BMR (kcal/day) = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
- Women: BMR (kcal/day) = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161
For example: a 40-year-old woman weighing 70 kg and 170 cm tall would have a calculated BMR of around 1,402 kcal per day.
However, such formulas only provide approximate values. They take into account neither individual body composition nor hormonal particularities. More precise values can be obtained through indirect calorimetry, which measures oxygen consumption – a procedure carried out in specialised practices or clinics.
Why is energy requirement so individual?
The basal metabolic rate can vary considerably from person to person. The main reasons are:
1. Muscle mass and body composition
Muscle tissue is metabolically more active than fat tissue. One kilogram of muscle uses more energy at rest than one kilogram of fat tissue. People with a higher proportion of muscle therefore tend to have a higher basal metabolic rate – even when they are not currently exercising.
2. Age
With increasing age, the basal metabolic rate decreases, usually slowly but continuously from around the age of 30. One key reason is the age-related loss of muscle mass (sarcopenia). The activity of the cellular power plants, the mitochondria, also declines with age.
3. Sex
Men on average have a higher basal metabolic rate than women, because they usually have more muscle mass and less body fat.
4. Height and weight
The more body mass present, the more energy is used at rest. Tall, heavier people therefore generally have a higher basal metabolic rate than smaller, lighter ones.
5. Hormones
Hormones have a considerable influence on metabolism:
- Thyroid hormones (T3 and T4) largely regulate the metabolic rate. An underactive thyroid can lower the basal metabolic rate, while an overactive thyroid can raise it significantly.
- Insulin, cortisol and sex hormones such as oestrogen and testosterone also affect energy expenditure and body composition.
- During the menopause, hormonal balance changes, which is often accompanied by a decrease in muscle mass and an altered fat distribution pattern.
6. Genetics
Studies show that genetic predisposition also plays a role. Some people naturally have a somewhat higher or lower basal metabolic rate than others with comparable characteristics.
7. Environment and temperature
Cold can slightly increase the basal metabolic rate, as the body needs more energy to regulate temperature. Fever or infections also temporarily increase energy needs.
How does the basal metabolic rate change over the course of life?
- Childhood and adolescence: High BMR per kilogram of body weight, as growth requires a great deal of energy.
- Young adulthood (20–30 years): Stable, usually the highest absolute BMR.
- Middle age (30–60 years): Slow decline, often accompanied by muscle loss where physical activity is lacking.
- Older age (from 60): Marked decrease, particularly where physical activity decreases. More recent research suggests that the decline up to around the age of 60 is less pronounced than long assumed – but becomes more noticeable thereafter.
This development explains why many people gain weight over the years even though their eating habits have barely changed: the body simply needs less energy.
Boosting the metabolism: what really helps
Many myths circulate around the topic of "boosting the metabolism". The following approaches, in particular, are supported by scientific evidence:
Build muscle
Regular strength training is one of the most effective ways to raise the basal metabolic rate in the long term. More muscle mass means greater energy expenditure – including at rest.
Consume sufficient protein
Digesting protein uses more energy than digesting fat or carbohydrates (thermic effect of food). Adequate protein intake also helps preserve muscle mass.
Build movement into everyday life
Not only sport counts. Climbing stairs, going for a walk or gardening also increase total energy expenditure – an effect known in technical terms as non-exercise activity thermogenesis (NEAT).
Get enough sleep and reduce stress
Lack of sleep and chronic stress can have an unfavourable effect on hormone balance and place a strain on the metabolism.
Be cautious with extreme diets
Very low-calorie diets can lower the basal metabolic rate, because the body switches into "energy-saving mode". After the diet, the familiar yo-yo effect may follow. A moderate, balanced diet combined with physical activity is more sustainable.
When should medical assessment take place?
Not every change in weight can be explained by lifestyle alone. A medical assessment can be useful in cases of:
- unexplained weight gain or loss
- persistent fatigue, feeling cold or hair loss
- suspected thyroid disorders
- hormonal changes (e.g. during the menopause)
- existing metabolic conditions such as diabetes
Targeted laboratory tests and, where appropriate, a body composition analysis can help assess whether individual factors are influencing energy needs.
Conclusion
The basal metabolic rate is not a fixed value, but the result of many influencing factors – from age and sex to muscle mass and hormonal balance. Anyone who knows and understands their daily energy requirement can better tailor nutrition and exercise to their own needs. Realistic expectations and a sustainable lifestyle are usually more effective than short-term diets.
This article does not replace medical advice.