What is metabolism: your basal metabolism, how it changes with age and when it points to a disease

What is metabolism

In this article you’ll read what metabolism is, what your basal metabolism is, how both change with age and when a slow or fast metabolism can point to a disease. That matters, because the word is used loosely, and knowing the parts tells you which part can be influenced and which signs mean you’d do better to see a doctor. Metabolism, what is it in plain words? The energy your body uses in a day. Researchers split that energy into three parts: your basal metabolism, the energy it takes to digest food, and the energy of movement.⁠⁴ The BioProfile metabolism scan gives you an indication of where you stand today in this area. It’s self-insight, meant to help you make better choices in your diet and lifestyle. It is not a diagnosis. Only a doctor can make one.

What is basal metabolism

Your basal metabolism is the energy your body uses at rest, just to keep running. That’s about two thirds of the energy you use in a day.⁠¹ How high it is depends on your weight, age, height and sex, and muscle tissue uses far more energy than fat tissue.⁠¹

A Scottish study measured basal metabolism in 150 adults and looked at what explained the differences between them. Fat-free mass, meaning everything in the body except fat, explained by far the largest share. Fat mass and age explained only a small part. Once body composition was taken into account, sex made no clear difference.⁠⁵ In men, the thyroid hormone thyroxine in the blood explained part of what remained. In women, thyroxine made no clear difference.⁠⁵

What this means for you: two people with the same weight can use quite different amounts of energy at rest, and the main reason is how much of that weight is muscle and organs. At the same body weight, women have about 10% more fat tissue than men, which is why their basal metabolism is lower.⁠¹

What is metabolism, age by age

A large study pooled measurements of daily energy expenditure in people aged 8 days to 95 years. It used doubly labelled water, a method that measures energy expenditure during normal everyday life. After adjusting for fat-free mass, energy expenditure rose to about 50% above adult levels around the first birthday. After that it fell slowly, reaching adult levels at about 20. From 20 to 60 it stayed stable, and after 60 it declined.⁠²

Put simply: once you account for body size, your metabolism in your forties runs largely the way it did in your twenties. The real slowdown only comes after 60.⁠² What does change earlier is what the body is made of. As you get older, fat tissue increases and muscle tissue decreases, and that lowers your basal metabolism.⁠¹

What raises and lowers your metabolism

Processing food takes energy. On a mixed diet, that is 5 to 15% of the day’s energy expenditure. It is higher when a meal contains relatively a lot of protein or alcohol, and lower when it contains a lot of fat. Protein also helps you feel full.⁠⁴ In practice, this means a meal with more protein takes a little more energy to process and keeps you full for longer. It is a small part of the daily total.

Your weight plays a part as well. In a study of 18 adults with obesity and 23 who had never had obesity, researchers held each person’s weight 10% below their starting weight. Daily energy expenditure then fell clearly in both groups, even when calculated per kilo of fat-free mass. When weight was held 10% above the starting weight, energy expenditure rose. The authors conclude that the body adjusts its energy expenditure to resist a weight that differs from its usual one.⁠⁶ For you, this means your metabolism adapts in both directions, which helps explain why holding on to a new weight takes effort.

What is metabolism, illness and the thyroid: when it points to a disease

Some conditions change your metabolism in a way you may notice, and the thyroid is one of them. When the thyroid works too slowly, Thuisarts lists feeling cold easily, gaining weight, feeling tired or sluggish, hard stools, dry hair with more hair loss, and dry skin.⁠⁷ When the thyroid works too fast, the signs are: your weight dropping while you eat as you normally do, restlessness, anxiety or irritability, a fast heartbeat, sweating a lot, sleeping badly, trembling, diarrhoea, a swelling in the neck and problems with the eyes.⁠⁸ You can read how a slow metabolism feels in everyday life in slow metabolism: symptoms and what a scan measures.

Thyroid problems are common in the Netherlands. Of the 152,180 people taking part in the Dutch Lifelines study, 3.1% were taking thyroid medication. Of those who were not, 10.8% had an abnormal TSH, the blood value that shows how hard the body is driving the thyroid. The researchers report many thyroid conditions that had gone unnoticed.⁠³ In short, about one in ten adults may have a thyroid value outside the normal range without knowing it.

Metabolic syndrome is the other metabolic condition that the Dutch studies count. Of 74,857 people taking part in Lifelines, 17.5% of men and 10.6% of women had metabolic syndrome at the start. Over about four years, it newly developed in 7.8% of men and 13.2% of women, and how often it developed differed by occupation.⁠⁹ The thyroid and metabolic syndrome are linked too: among 26,719 participants with normal thyroid function, people with the highest levels of the thyroid hormone FT3 had metabolic syndrome more often than people with the lowest, even within the normal range.⁠¹⁰ What this means for you: metabolic syndrome is common, it can develop in adulthood, and thyroid hormones and metabolic health go hand in hand.

If you recognise several of these signs, your GP can measure free T4 and TSH in your blood. If your thyroid works too fast, your GP refers you to the hospital, where an internist looks into it further.⁠⁸ If you already know your thyroid works too slowly, Thuisarts advises contacting your GP if your symptoms get worse or come back.⁠⁷

What the scan measures and what it does not

The metabolism scan gives you a score for where you stand today and a baseline to compare against later. What the scan asks and what it is based on is set out on the scan page. The scan doesn’t take blood, so it doesn’t measure thyroid hormones or TSH. For that you need a blood test.⁠⁸ The scan is not a diagnosis.

In summary

What your metabolism is: the energy your body uses in a day, made up of your basal metabolism, the cost of digesting food and the energy of movement. Your basal metabolism is the largest part of it and depends mainly on how much muscle and other fat-free mass you carry. Once you account for body size, metabolism stays stable from 20 to 60 and declines after that. Food and weight change it to a modest degree. A thyroid that works too slowly or too fast changes it in a way you may notice, and a blood test at your GP shows whether that is the case.