Metabolism

Metabolism is the whole set of chemical reactions a body uses to turn food and stored fuel into energy, and to build and repair its own tissues. It has two halves that run at the same time: catabolism, which breaks molecules down and releases energy, and anabolism, which spends that energy building new ones. Enzymes carry out the reactions, and hormones — thyroid hormone and insulin above all — set the pace. A fuller reference on how these pieces fit together is the entry on Metabolism.

What it measures

Metabolism is a process, not a test result, so nothing reports it as one number. What laboratories and clinics actually measure are pieces of it. Energy turnover is measured as oxygen consumed and carbon dioxide produced — indirect calorimetry, done with a mask or hood, or over days with doubly labeled water. That gives energy expenditure in kilocalories per day, at rest or across a whole day, and it is also estimated from prediction equations using height, weight, age and sex; see Basal metabolic rate (BMR) for how that figure is defined.

The chemical side is read from blood: fasting glucose and HbA1c for how sugar is handled, insulin for how hard the pancreas is working, triglycerides and HDL for fat handling, and TSH with free T4 for thyroid drive. There is no single reference range for “metabolism” itself — each of those individual markers has its own, and each one differs by laboratory, assay, sex and age.

Why it matters for longevity

When several of these markers drift together — high fasting glucose, high triglycerides, low HDL, raised blood pressure and a large waist — the cluster is called Metabolic syndrome, and in observational studies it is associated with higher rates of type 2 diabetes, cardiovascular disease and all-cause mortality. Insulin resistance is associated with the same outcomes, often years before glucose looks abnormal.

The nutrient-sensing pathways that read metabolic state — insulin and IGF-1 signaling, mTOR, AMPK — change lifespan most reliably when they are manipulated in laboratory animals. In humans the evidence is far thinner and mostly runs through risk markers rather than lifespan itself. The old assumption that metabolic rate simply slides downward from early adulthood has also softened: large pooled analyses of doubly labeled water data suggest that, once body size and composition are accounted for, energy expenditure stays fairly steady through middle adulthood and falls later in life. Much of what people experience as a “slowing metabolism” is lost muscle and less movement rather than the cells themselves changing gear.

What changes it

Body size and body composition do most of the work. A bigger body burns more at rest, and lean tissue burns more per kilogram than fat, so resistance training and enough protein protect the part of the total you can defend as you age. Daily movement — walking, standing, fidgeting, chores — moves total expenditure more than a training session does for most people.

Food changes the chemistry more than the rate. Digesting protein costs more energy than digesting fat or carbohydrate, and caffeine lifts expenditure slightly and briefly, but neither is large enough to outrun intake. Sustained heavy dieting works the other way: resting expenditure falls somewhat more than the weight loss alone predicts, and part of that drop persists. Short sleep and chronic stress worsen insulin sensitivity without changing the number of calories you burn.

Two things genuinely shift the rate itself and are worth ruling out: an underactive or overactive thyroid, diagnosed with a blood test and treatable, and some medications. Age and genetics you do not control — which is why the levers that matter are muscle, movement, sleep and what you eat.

Related reading

Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.