Slow metabolism: the symptoms people mean, what your daily energy expenditure depends on and what a scan measures

In this article you read what a slow metabolism is, which symptoms people mean by it, what your daily energy expenditure depends on and what a free scan tells you about your habits. That matters, because the nutrition advice you keep reading leaves out where the biggest measured differences between people of the same size sit: body composition and daily activity. The BioProfile metabolism scan asks a short series of questions about those habits and gives you an indication of where you stand today, as one of the 11 health areas the full scan assesses. This is not a diagnosis. It is insight into where you stand today. What this scan does not do: point to a cause or make a diagnosis. Only a doctor can do that.

What a slow metabolism actually is

Metabolism is not one single button. The basal metabolic rate, the energy your body uses just to keep running, is about two thirds of daily energy expenditure.⁠¹ In 150 adults, fat-free mass, so muscles, organs and bones, explained 63 per cent of the differences in basal metabolic rate between people; fat mass explained a further 6 per cent and age 2 per cent, and circulating thyroxine 25 per cent of what was left, only in the male participants. Sex had no significant effect once those three were in the model.⁠² So the biggest reason one person uses more energy at rest than another of the same size is how much fat-free tissue that person carries.

Slow metabolism symptoms: what people mean by them

Feeling cold quickly, gaining weight, being sluggish or tired, hard stools, dry hair, hair loss and dry skin: those are the complaints Thuisarts names for a thyroid that works too slowly, with too little free T4 alongside a raised TSH in the blood.⁠⁵ Thuisarts also splits the causes of tiredness into life circumstances, thoughts and feelings, lifestyle such as too little activity and poor sleep, and physical causes including hormonal ones, and adds that when tiredness is the only complaint a blood test almost never turns up a cause.⁠⁶ A list of symptoms tells you it is worth looking. The list does not say what you have.

How you would know: what is actually measured

A database of 6421 people, from 8 days to 95 years old, from 29 countries and 64 percent female, measured daily energy expenditure with doubly labelled water. Total daily energy expenditure, basal energy expenditure and fat-free mass were stable from 20 to 60, and segmented regression placed the turning point at 63.0 years. After that, daily energy expenditure corrected for body size fell by about 0.7 percent a year, and in people of 90 and older it was about 26 percent below that of middle-aged adults.⁠³ The slowdown that people in their thirties use as an explanation cannot yet be seen in the measurements. With age, fat tissue increases and muscle tissue decreases, and that is why the basal metabolism falls at a higher age.⁠¹

What moves your daily energy use

16 volunteers without obesity were given 1000 kilocalories a day above their maintenance needs for 8 weeks. Two thirds of the rise in total daily energy expenditure came from non-exercise activity thermogenesis: fidgeting, holding your posture, walking around. Differences in that daily movement accounted for a 10-fold difference in fat storage between the volunteers.⁠⁴ So the part of your energy expenditure with the most room in it is the standing, walking and pottering about of an ordinary day.

The Health Council of the Netherlands advises adults to do at least two and a half hours of moderately intensive exercise a week, with muscle and bone strengthening activities alongside it.⁠⁷ In 2025, 46.1 per cent of people in the Netherlands aged 18 and over met those physical activity guidelines.⁠⁸ The season moves activity more than the total: in 25 young Dutch adults the physical activity level was 1.87 in summer against 1.76 in winter, while total daily energy expenditure was the same in both seasons.⁠⁹

Eating far below what you need for a long time works the other way. Of 16 people in a 30-week competition, 14 were measured again after 6 years: the resting metabolic rate was still 704 kcal a day below the starting value, and 41.0 of the 58.3 kg was back.⁠¹⁰ So a pace you can keep up beats a steep pace.

When to contact your GP

Call your GP if the tiredness lasts a long time, for example longer than a month. Also call if other symptoms come with it: night sweats, less appetite, your weight dropping without you trying, a change in your stools or blood in them, being short of breath at rest, a lot of thirst and passing a lot of urine, palpitations, or feeling very low or very tense.⁠⁶ With a thyroid that works too slowly, you contact your GP if the symptoms get worse or come back.⁠⁵

What the metabolism scan measures

The scan asks about the habits under this area, starting with the two simplest: thirty minutes of movement on most days, and vegetables or fruit every day. Your answers become a score for this area with the reading band the score falls in, and higher is better. A questionnaire does not measure resting energy expenditure; that needs indirect calorimetry or doubly labelled water.⁠^10,3 What the scan gives you is a starting value. It is not a diagnosis.

In summary

Most of your daily energy expenditure goes to the basal metabolic rate, and the biggest measured difference between two people of the same size is how much fat-free tissue each one carries.⁠^1,2 Daily energy expenditure corrected for body size stayed stable from 20 to about 63.⁠³ The symptoms people call a slow metabolism overlap with a thyroid that works too slowly and with ordinary tiredness, and Thuisarts draws the lines for when to call the GP.⁠^5,6 The room to move sits in your daily activity and in the weekly minimum the Gezondheidsraad advises.⁠^4,7 The scan gives you a starting value for your own habits. It is not a diagnosis.