The metal hatch door clangs open and a tray of food is pushed towards me. It closes with a thwunk, and I am permitted to collect my meal.

No, I’m not in prison – welcome to lunchtime in a human metabolism research unit. I’m here to have my metabolism, that is, the rate at which I burn calories, measured. 

 Your metabolic rate, in simple terms, is how much energy your body burns – and calories are a unit of energy. 

If you’ve ever wondered whether a sluggish metabolism is why you can’t lose weight, or why friends can seemingly eat whatever they want and never gain an inch – that’s what I’m here to find out.

While there are formulas to estimate how many calories you need based on your sex, height and weight and age, I’m at the University Hospital Coventry, which has a chamber to measure this in a more accurate way. 

The chamber, or the whole body calorimeter to give it its proper name, takes readings every two minutes to work out how many calories someone is burning at any given moment: be they eating, exercising or sleeping.

A joint venture between the University of Warwick and University Hospitals Coventry and Warwickshire NHS Trust, the chamber was developed to allow medical researchers to study human metabolism in circumstances as close to real life as possible. 

There are only a handful of similar facilities in the world.

I’m going into one of two whole body calorimeter rooms, while fellow guinea pig, serial dieter Andy Leeks, 36, from Kent, goes into the other.

As well as looking at our metabolic rates, the idea is to compare our results to give an idea of the differences between men and women’s metabolism.

Andy, who lost three stone last year after tipping the scales at 16st, is keen to know if a sluggish metabolism could be behind his battle with his weight.

In order to answer these questions, we have to stay in our individual sealed chambers for 24 hours. Each room is about three metres long by two metres wide, with a fold-down bed, sink, desk, chair and loo.

No one else is allowed in to the chamber as this would distort the results. Calorie-counted meals are provided at strict intervals, lights out is at 11pm and caffeine is forbidden, as some research has suggested it can speed up metabolism.

My time in the chamber revealed surprising truths about the way metabolism works – and what it could mean for your weight and health…

HOW THE CALORIE ROOM WORKS

So what is the room measuring? Metabolism is the process by which food and drink is converted into energy to fuel all the processes in our body. Your metabolism never switches off, it is integral to keeping us functioning.

Your metabolic rate is slightly different – this is how much energy all those processes are using up at any one time.

‘To use energy from food, we break it down and convert it into a molecule called ATP [adenosine triphosphate], which fuels all the processes in our cells,’ says Dr Thomas Barber, an associate professor of clinical endocrinology at the University of Warwick and UHCW NHS Trust.

The by-products of converting energy from food into ATP are carbon dioxide and water. The calorimeter room works by measuring the levels of carbon dioxide in the chamber. So more carbon dioxide in the air means you’re burning more energy.

Of course if you eat more than your body needs you store this excess energy as fat.

‘If the body can’t make enough ATP from food, and it’s used up all the glycogen stores [a form of sugar] in the liver, it will start converting its fat stores into ATP instead,’ says Dr Barber.

WHAT REALLY BURNS CALORIES

Before we start the experiment, Andy and I sit inside a machine called the BodPod to work out how much body fat we have. It looks like an egg-shaped space capsule, and it records how much air you displace while inside it. It uses this measurement together with your weight and height to estimate body fat percentage.

This is important because, Dr Barber explains, the biggest factor in determining how many calories we need is how much of our body is lean tissue.

Lean tissue is essentially everything in our body that isn’t fat – from organs such as the brain, heart and lungs to bones, muscles and blood vessels.

Your liver is the main guzzler of energy, accounting for 27 per cent of our basic calorie needs, while the brain accounts for 19 per cent. The heart by comparison uses only about 7 per cent.

The more of this sort of tissue you have – because your organs are bigger or you’re muscly – the more energy you need. Having more fat doesn’t make as much difference as fat tissue needs relatively little energy to maintain itself. It is less ‘metabolically active’.

This is why men, in general, need more calories – they tend to be bigger, with less fat and more muscle. In keeping with this, the BodPod calculations show Andy has 64.5kg lean tissue while I have 42.4kg.

NO SUCH THING AS A FAST METABOLISM

But which of us burns calories faster? ‘There’s this perception that some people burn off calories more quickly,’ says Dr Barber. ‘But metabolic rate is pretty constant in everyone, unless you have a medical disorder, such as overactive thyroid, which puts all the body’s functions into overdrive.

‘Generally, most people burn around 1 to 1.5 calories a minute at rest. And per unit of lean mass, the rate is pretty constant between individuals. It makes sense, because the essential processes in our bodies are the same.

‘Of course, some people will burn off more calories in total, as they have more lean mass – but they also need to eat more calories. It doesn’t mean they will lose weight faster or are less likely to gain weight if they eat too much.’

The results from our time in the chamber show that Andy does burn more calories in a 24-hour period than I do in total – around 2,098 calories, compared to my 1,638 calories, according to Dr Barber’s estimate. At rest, Andy burns 1.15 calories a minute, while I burn 0.85 a minute.

However per kilogram of lean tissue, we’re burning calories at exactly the same rate – 0.02 calories per minute. What’s more, our metabolic rate responded in almost identical ways to all the activities carried out in the chamber (exercise, eating and so on).

The bottom line? ‘Most people can’t blame their weight on metabolism,’ says Dr Barber. ‘The idea of a fast or slow metabolism is a myth.’

Furthermore, some things make less difference than you think: take age.

Though our metabolic rate does slow down as we get older, it plateaus between the ages of 20 and 50. Metabolic rate is highest in newborns, slowing down throughout childhood.

‘It’s only really when you get to 80 that there’s a drastic difference,’ says Dr Barber.

BUT BEING COLD DOES SPEED IT UP

When it’s colder, the body has to work harder to maintain an acceptable temperature and women are thought to react more strongly to cold than men.

To test this the room is gradually cooled. From 10.30am onwards, the temperature drops from a pleasant 22c by 1.5c every half an hour, until it reaches a positively chilly 13c.

Andy and I are wearing regulation blue medical scrubs. They are fairly thin and, before the temperature is returned to normal in time for lunch, my feet feel like ice and my arms are goose-pimpled, but I’m not quite shivering.

WOMEN REALLY DO FEEL COLD MORE

The results show that the cold boosted my metabolism more than twice as much as it did Andy’s. ‘We would expect to see a greater reaction in women than men – especially as your BMI is lower than Andy’s,’ says Dr Barber.

Researchers from Maastricht University pointed out this summer that the female metabolism reacts differently to cold, which is why women are often too cold in air-conditioned offices, which are calibrated according to what’s comfortable for male metabolism.

One possible explanation is that women tend to carry fat around their bottoms and legs, rather than around the middle, as men do. ‘My own hunch is that fat distribution probably does play a part and that fat carried centrally, as happens in men, is more insulating,’ says Dr Barber.

‘You lose a lot of heat from the trunk, including the abdomen and chest, so it would make sense,’ says Dr Barber. ‘For women, their fat is like wearing an extra pair of trousers against the cold rather than a coat as with men!

‘Also, men tend to have more muscle, which may play a role. And women tend to have larger surface area, proportionate to their body volume, as they tend to be shorter. This may mean a greater degree of heat loss.’

I burn an extra 0.28 calories a minute during the temperature drop. Andy, who didn’t find the cold as uncomfortable, burnt only 0.11 extra calories a minute.

So does this mean turning the thermostat down a couple of notches could give your diet a boost? ‘Yes, in theory,’ says Dr Barber. ‘You could also drink a big glass of cold water before a meal.’

EATING SLOWLY COULD HELP

Another intriguing fact revealed by the calorimeter room is that eating actually burns up calories. Andy and I are given lunch (lasagne, followed by yogurt) at 1.45pm, dinner (a ham sandwich, crisps and banana) at 6.20pm and a snack (glass of milk and a fruit bun) at 9pm. At each meal, both of our metabolic rates speed up.

The number of calories burnt starts to climb at the start of each meal, peaking when we finish eating and then remain higher for up to two hours after.

‘This is known as diet-induced thermogenesis,’ says Dr Barber. ‘That is, the extra energy required for digestion.

‘In theory at least, this would mean if you were to eat exclusively very low-calorie, foods such as celery and cucumber, which contain mostly water, you may end up burning more calories through digestion than you consume.

‘We’ve also published a study that showed eating more slowly increased post-prandial calorie burn.

‘We don’t know why this should be, but we found eating the same meal in 40 minutes compared to eating it in ten minutes burned around 50 more calories in the hours after the meal.’

DOING NOTHING BURNS CALORIES

Exercise boosts metabolism, as we’re so often told. To see just how much difference it makes, Andy and I are asked to do sets of a stepping exercise. Using a low step (the sort you’d use in a step aerobics class) and a metronome set on our computers to keep time, we step up and down for 15 minutes each time. It was about the pace of a brisk walk, enough to raise the heart rate a little.

Even so, our metabolic rates climbed steadily throughout the hour in which we exercised and stayed raised for two hours after.

Andy’s average calorie burn per minute during exercise increased from 1.5 to 3.7 calories, while mine went up from 1.1 to 2.7 calories per minute, meaning exercise increased our metabolic rates 2.5 fold.

‘Just by moving about more, you’d be boosting your metabolism. It doesn’t have to strenuous,’ says Dr Barber.

Finally, at 11pm, it’s time for bed. Metabolic rate drops while we sleep simply because we’re not doing as much physical activity – though our brain and other organs are still working.

From our metabolic rates while we’re sleeping, Dr Barber gets a precise reading of our Basal Metabolic Rate (BMR) – this is the minimum number of calories we would need per day if we were to do absolutely nothing.

My BMR is 1,224 calories, while Andy’s is 1,656 calories. This, Dr Barber explains, is fairly fixed.

‘That said, you do increase your BMR slightly after exercise – known as afterburn,’ he says.

For Andy and me, our metabolic rate stayed raised for a few hours after exercise. It’s possible that very strenuous exercise could see your BMR stay raised for many hours afterwards.

And if you were to dramatically boost your muscle mass through exercise you would also increase the total calories you burnt in a day.

DOES DIETING SLOW IT DOWN?

Andy was keen to know whether his history of yo-yo dieting – not least doing ten diets in 50 days, as he did for his book Minimize Me – had affected his metabolism.

‘My weight has fluctuated between 13st and 18½st, and people often say that dieting wrecks metabolism, so I was curious,’ he says.

In fact, his dieting past is unlikely to have made a difference, according to Dr Barber. ‘Yo-yo dieting won’t do anything in particular. However, weight loss can sometimes end up reducing BMR because as well as losing fat, lean mass is often also reduced.’

Exercise is the best way to counteract this, he says.

HOW SPICY FOOD HELPS YOU BORN CALORIES

‘It could be that different foods could have different effects on metabolism,’ says Dr Thomas Barber, an obesity researcher and associate professor of clinical endocrinology. ‘The main one we have evidence for is chilli.’

Research has suggested eating chilli can help you burn perhaps an extra 50 or 60 calories in the hours immediately after a meal, as chilli effectively stresses the body. ‘When you eat chilli it affects a particular receptor in the mouth,’ says Dr Barber. ‘This then acts on the sympathetic nervous system, which controls our fight or flight response in stressful situations.

‘In some people, this activates brown fat (which burns calories, unlike normal ‘white’ fat), boosting metabolism.

‘However, not all of us have active brown fat, so it won’t necessarily boost everyone’s metabolism.’

Originally published by the Daily Mail, Good Health, in December 2015.