Enter the calorimeter: a chamber that measures how many calories your body needs

Like a car needs gas, our bodies need food to work — but how much varies from person to person By Leora Eisen, director of Food for Thought

“Wish me luck!” says Tim Spector as a metal door shuts behind him. For the next eight hours, he will be kept inside a metabolism chamber at University Hospitals Coventry and Warwickshire (UHCW). Everything he does — especially how he breathes — will be measured minute by minute.

Spector is writing a new book about diet and wants to know how many calories his body needs to function. He’s come to the right place; the UHCW’s whole body calorimeter, one of two in the human metabolism research unit, is the gold standard of metabolism measurement.

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“It looks a bit like a submarine, doesn’t it?” quips Tom Barber, an associate professor at the University of Warwick who specializes in metabolism-related diseases like obesity and diabetes. “It’s like a small one-bedroom flat that’s hermetically sealed from the outside world.”

As Spector sleeps, reads, exercises and eats, Barber and his colleagues keep a close watch through a window and speak to him via a monitor. In order to accurately measure Spector’s metabolism, no air can enter into or escape out of the chamber.

Your body is powered by food and releases CO2 as a byproduct

Your body is like a car: your muscles and organs won’t work unless you’ve filled up the tank. “You can’t drive your car if there’s no petrol in it, right? You need energy to drive your car,” says Barber.

When your body breaks down the chemicals in food, it releases energy. “Metabolism is essentially a measurement of how much energy your body requires to power all your cells,” Barber explains.

Carbon dioxide is a byproduct of metabolism. By determining how much CO2 Spector is releasing and how much oxygen he’s breathing in, the team can determine Spector’s metabolic rate: “The chamber is really a big, fancy gas analyzer. The greater the metabolic rate — the ‘burning of calories’ — the more oxygen is consumed and the more CO2 is released.”

Spending the day in the calorimeter

First, Spector takes a nap. This allows the researchers to calculate his basal metabolic rate — how many calories he burns while doing nothing (in most people, this is about one calorie per minute).

After a few hours, he does 15 minutes of exercise on a step platform. Finally, after going 24 hours without any food, Spector gets to eat a cheese sandwich. “Not exactly my favourite last meal,” he laughs.

Metabolic rate varies depending on body mass

Determining someone’s metabolic rate is complicated. After Spector leaves the chamber, the team use powerful computers and complex algorithms to analyze the data.

Body mass is a crucial component of metabolism. Some of us have more body fat than others and this influences how many calories our bodies need. “If you have more cells, you require more energy to power those cells. So it’s a proportionate thing,” notes Barber.

Spector has already had his body mass measured in a Bod Pod, a machine that estimates body fat percentage by measuring body volume and mass. Spector has a lean body mass, so just like a small compact car requires less fuel than a large SUV, he needs fewer calories than most to keep his body running smoothly.

Barber tells him the results: his body requires only 1,584 calories a day. Spector is shocked: “But that’s nothing!”

Government guidelines for men suggest eating 2,500 calories daily. “If you had 2,500 calories a day for a year, you would put on a lot of weight,” adds Barber, “because we know that there would be a calorie mismatch.” Based on Spector’s metabolic rate during exercise, Barbar estimates that he would have to do three hours of strenuous exercise every day to burn off those extra calories.

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