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What Is Resting Metabolic Rate?

Health · November 8, 2024 · Dr. Nadia Okoro · 5 min

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Resting metabolic rate is the energy your body uses just to keep you alive at rest. Here is what it is, what drives it, why crash diets can lower it, and how it fits into the bigger calorie picture.

Even if you spent an entire day lying perfectly still, your body would burn a substantial number of calories. Your heart would keep pumping, your lungs would keep breathing, your cells would keep repairing themselves, and your brain would keep humming away. The energy needed for all that quiet, essential work has a name: your resting metabolic rate. Understanding it cuts through a great deal of confusion about diet, weight and the much-misused word "metabolism". This article is general information, not personal medical or dietary advice; speak to a clinician or registered dietitian about your own needs.

What resting metabolic rate is

Resting metabolic rate (RMR) is the number of calories your body uses at rest to keep its basic functions running. That includes breathing, circulating blood, maintaining body temperature, sending nerve signals and continuously repairing and replacing cells. It is the energy cost of simply being alive, before you stand up, walk anywhere or eat anything.

You will also see the term basal metabolic rate (BMR). The two are nearly the same; BMR is measured under stricter laboratory conditions, while RMR is measured a little more loosely, so RMR tends to come out marginally higher. For practical, everyday purposes you can treat them as the same concept.

How it fits into total energy use

Your total daily energy expenditure, the full set of calories you burn in a day, has three main parts. RMR is by far the largest of them.

• Resting metabolic rate — the energy to stay alive at rest. For most people this is roughly 60 to 75 per cent of the daily total.
• Physical activity — everything from formal exercise to fidgeting, walking to the kitchen and standing up. This is the most variable part and the one you can most readily change, which is why it pays to stay active even at a desk.
• The thermic effect of food — the energy used to digest, absorb and process what you eat, usually around 10 per cent of intake.

The headline point is that the body's background "tick-over" dwarfs the calories burned by a single workout. That does not make exercise pointless, far from it, but it explains why you cannot out-train a poor diet, and why activity across the whole day matters as much as one gym session.

What determines your RMR

People often assume metabolism is mostly a matter of luck, with some lucky souls blessed with a "fast" one. Genetics do play a part, but the largest, most predictable driver is simpler: body size, and especially the amount of lean tissue you carry.

• Body and muscle mass. Larger bodies and, in particular, more muscle require more energy to maintain. Muscle is metabolically active tissue, so people with more of it tend to have a higher RMR. This is the main reason men, on average, have a higher RMR than women: they typically carry more muscle.
• Age. RMR tends to decline gradually with age, partly because people often lose muscle as they get older, an effect that staying active can slow.
• Sex and genetics. Hormonal differences and inherited factors nudge the figure up or down.
• Other factors. Thyroid function, illness, certain medications and even temperature can affect it.

Can you boost it?

This is where myth thrives. Claims that particular foods, drinks or supplements will "rev up your metabolism" are mostly overblown. Some, such as caffeine or chilli, produce a tiny, short-lived bump that makes no meaningful difference to weight. There is no pill or superfood that meaningfully raises RMR.

What genuinely helps your total energy use is more reliable and less glamorous:

1. Build and keep muscle. Resistance exercise increases lean tissue, which raises RMR a little and, more importantly, protects it as you age.
2. Move more across the day. The activity component is where most real-world variation lies. A balanced approach to eating plus regular movement does far more than any "fat-burning" trick.
3. Be wary of crash diets. Severe, prolonged calorie restriction prompts the body to lower its resting energy use to conserve fuel, an effect sometimes called adaptive thermogenesis, and you also end up with a smaller body that simply needs less. Both make rapid weight loss hard to sustain and weight regain common.

The reliable lever is not speeding the metabolism up artificially, but protecting muscle and staying active so your body uses more energy overall.

Why it matters for weight

Weight change ultimately reflects the balance between energy taken in and energy used. Because RMR is the biggest piece of energy out, it sets the baseline around which everything else moves. Two people of similar weight can have noticeably different RMRs because of differences in muscle, age and genetics, which is one reason identical diets produce different results.

It also explains a frustrating reality of dieting. As you lose weight, your RMR falls, because there is less of you to maintain, so the calorie deficit that worked at the start gradually shrinks. This is normal physiology, not a personal failing, and understanding it makes gradual, sustainable change far more sensible than crash dieting. Quality sleep and managing stress support healthy appetite regulation alongside all of this.

The bottom line

Resting metabolic rate is the energy your body burns just to keep you alive at rest, and it is the largest part of the calories you use each day. It is driven mainly by your body size and lean tissue, along with age, sex and genetics. You cannot meaningfully "boost" it with foods or supplements, but you can raise your overall energy use by building muscle and moving more, while very restrictive diets tend to push it down. Treat your metabolism not as a switch to be flipped but as a system to be supported, steadily, over time.

Key takeaways

• Resting metabolic rate (RMR) is the number of calories your body burns at rest to power basic functions like breathing, circulation and cell repair.
• RMR is the largest part of most people's daily energy use, typically around 60 to 75 per cent of total calories burned.
• The biggest driver of RMR is body size, especially the amount of lean tissue such as muscle; age, sex and genetics also play a role.
• Very low-calorie 'crash' diets can lower RMR as the body adapts, which is one reason rapid weight loss is hard to maintain.
• You cannot dramatically 'boost' your metabolism with foods or supplements, but building muscle and staying active raise your total energy use.

Sources

• British Nutrition Foundation
• NHS — Understanding calories
• World Health Organization — Healthy diet

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