"A couple of minutes can't matter"

The rule this book builds towards involves getting up for a minute or two every half hour. Put like that, it sounds too small to do anything. Two minutes of wandering about the office can't offset the twenty-eight minutes either side of it. It sounds like the kind of health tip that fills space in a magazine.

The best answer is an experiment, because here researchers could assign the movement and measure the result, which the big population studies couldn't do.

David Dunstan and colleagues in Melbourne recruited nineteen adults aged between forty-five and sixty-five, all of them overweight. Each person came to the laboratory on three separate days. On one day they sat for seven hours without a break. On another they sat for the same seven hours but got up every twenty minutes and walked slowly on a treadmill for two minutes. On the third they did the same with a brisker walk. On each day they were given a sugary, fatty test drink after the first two hours, and the researchers tracked their blood sugar and insulin for the next five.

On the day of unbroken sitting, blood sugar after the drink rose higher and stayed up longer. On the days with breaks, the rise in blood sugar over those five hours was about a quarter smaller, and the insulin response was lower by about the same amount. Insulin is the hormone your body releases to bring blood sugar down, and needing less of it to handle the same drink is a good sign. The slow walk did about as well as the brisk one.

I should be straight about the limits. It was nineteen people, all overweight, studied over single days in a laboratory. Nobody knows from this whether the effect persists over months, and the experiment can't tell you anything about heart attacks or lifespan. But it shows something the big surveys can't. Two minutes of easy movement changes what your body is doing with its blood sugar in the hours around it.

Why would something so slight have a measurable effect? This is the usual explanation, and I'm giving it in outline. Your biggest muscles are in your legs and backside. When you sit they're switched off almost completely, since the chair is doing their job. Working muscle takes sugar out of the blood, and idle muscle takes very little. Standing up and walking switches on the largest muscles you have, and even at a stroll that's a lot of tissue. The sensors and the blood tests pick up how long your large muscles were active, and those muscles are active whether you're walking to the kettle or training.

Think back to chapter 3. The body maintains what gets used. A day with fifteen or twenty short breaks in it asks your legs to work fifteen or twenty times. Each request is small, but a day with none of them looks quite different to your body, and that was the Wednesday in chapter 1.

You already know what a break feels like from the inside. When you stood at twenty to seven you were stiff. If you'd stood at six, and at half past, you wouldn't have been. The two-hour sit is what produced the stiffness, and it needs two unbroken hours to do it.

One piece of arithmetic is worth doing. Two minutes every half hour across a nine-hour working day comes to thirty-six minutes on your feet. Carry on through the evening and it passes fifty. Each break on its own is too short to feel like anything, but the total is a good part of the distance between three and a half thousand steps and six thousand.