In short: metabolism is how your body turns food into energy and building material. Most of the energy you burn each day goes on simply staying alive at rest. Intermittent fasting can help some people eat less, but trials show it works about as well as other ways of cutting calories, and it is not safe for everyone.
At a glance
| Where daily energy goes | About 60–70% at rest (BMR), ~10% digesting food, the rest movement |
|---|---|
| Biggest influence on BMR | Body size and muscle mass, then age and sex |
| Main fuels | Glucose and fat; ketones during long fasts |
| Intermittent fasting | Similar weight loss to daily calorie reduction in trials |
| Do not fast without a doctor | Diabetes on medication, pregnancy, eating disorders, under 18 |
Energy in, energy out
Food, resting needs and movement decide the balance.
Muscle burns more
Strength training helps keep metabolic rate up with age.
See what you eat
A meal photo in the app gives approximate calories next to your other data.
What is metabolism?
Metabolism is the sum of the chemical reactions in your cells. Some break food down into energy (catabolism), others use that energy to build and repair tissue (anabolism). Hormones such as insulin, glucagon, thyroid hormones and cortisol steer which fuel the body uses and when it stores or releases energy.
What sets your metabolic rate?
Mostly body size and the amount of muscle, which uses more energy than fat even at rest. Men usually have a higher rate because of more muscle. Genes, thyroid function and some medicines matter too. A large 2021 study in Science found that metabolic rate adjusted for body size stays fairly stable between 20 and 60 and declines slowly after that, so midlife weight gain is driven more by habits than by a "slowing" metabolism.
What happens in your body during a fast?
In the first hours your body uses glucose from the last meal and from glycogen stored in the liver. After about 12 hours glycogen runs low and fat becomes the main fuel; the liver starts making ketones, which the brain can also use. Insulin falls and the body becomes better at drawing on stored fat. These are normal adaptations, not a detox.
Does intermittent fasting work?
For weight, it works about as well as eating a little less every day. Randomised trials of time-restricted eating (such as 16:8) and alternate-day fasting show modest weight loss and some improvement in blood sugar and blood pressure, mostly explained by eating fewer calories. It suits people who find a fixed eating window easier than counting; it does not suit everyone.
Who should not fast?
People with diabetes treated with insulin or other glucose-lowering medicines (risk of low blood sugar), pregnant or breastfeeding women, children and teenagers, people with a history of eating disorders, and people who are underweight or frail. Anyone with a long-term condition or regular medication should talk to a doctor first.
Key terms
- Metabolism: all chemical reactions that turn food into energy and building material.
- Basal metabolic rate (BMR): the energy your body uses at complete rest.
- TDEE: total daily energy expenditure, BMR plus digestion and movement.
- Glycogen: the stored form of glucose in the liver and muscles.
- Ketones: fuel the liver makes from fat when glucose is scarce.
- Time-restricted eating: eating only within a set window each day, for example 8 hours.
Sources
- Pontzer et al., Daily energy expenditure through the human life course, Science (2021)
- NIDDK: Health risks of overweight and obesity, and healthy eating
- Liu et al., Calorie restriction with or without time-restricted eating, NEJM (2022)
- Trepanowski et al., Alternate-day fasting vs daily calorie restriction, JAMA Internal Medicine (2017)
Frequently asked questions
Can I speed up my metabolism?
Modestly. Building muscle through strength training and staying active raise daily energy use. Foods or supplements marketed as "metabolism boosters" have very small effects.
Does eating late at night slow metabolism?
Total intake matters most. Some studies suggest eating earlier in the day suits the body clock better, but the effect on weight is small compared with how much you eat.
Is 16:8 fasting safe?
For most healthy adults it is safe. It is not suitable for people on glucose-lowering medicines, in pregnancy, for teenagers or for anyone with a history of eating disorders without medical advice.
Does fasting put the body in "starvation mode"?
Long, severe calorie restriction lowers energy use somewhat. Short daily fasting windows do not cause a meaningful drop.
Why do I feel cold or tired when dieting?
Eating much less than you burn lowers some energy-consuming processes. Feeling very tired, dizzy or cold is a sign to eat more and to talk to a doctor if it continues.
Does thyroid disease affect metabolism?
Yes. An underactive thyroid lowers metabolic rate and an overactive one raises it. A simple blood test checks thyroid function; your doctor decides if you need it.
Try it in Anpheros Daily
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