"It has to be in one go"

The old guidelines had a second rule inside them, which was that activity only counted in bouts of ten minutes or more. It's worth knowing where that came from and what became of it, because a lot of people are still living by it.

As far as I can tell, the rule was a product of how the early evidence was gathered and not a finding about the body. The early trials tested sessions of ten minutes and up, so that's what the guidelines could vouch for. Nobody had shown that shorter bouts didn't work. They hadn't been studied.

When the United States updated its guidelines in 2018, the advisory committee reviewed the question formally. John Jakicic and colleagues looked at 29 studies. Their conclusion was that physical activity of any bout duration is associated with improved health outcomes, including lower mortality. They pointed out that the randomised trials had only ever tested bouts of ten minutes or more, which is why the old rule existed. They said the evidence might call for a shift in public health advice, towards counting total activity regardless of how it's broken up.

One study in that review is worth describing. Pedro Saint-Maurice and colleagues took 4,840 American adults aged forty and over who had worn activity monitors for a week, and followed them for about six and a half years. They counted each person's moderate-to-vigorous activity in three ways. One way counted every minute of it, another counted only bouts of five minutes or more, and the third counted only bouts of ten minutes or more. If the bout rule were right, the bouted minutes should have predicted survival better than the scattered ones. They didn't. The associations with mortality were similar however the minutes were counted. People who got their activity in scraps did as well as people who got the same amount in blocks.

As I understand it, the bout requirement has since been dropped from American guidance, and the World Health Organization's 2020 guidelines left it out as well. The WHO authors sum up their message as "some physical activity is better than none". The NHS guidance for adults opens by saying adults should do some type of physical activity every day, and it adds that exercise just once or twice a week can reduce the risk of heart disease or stroke. It also advises breaking up long periods of not moving with some activity.

The research has since gone to even shorter durations. Emmanuel Stamatakis and colleagues used the UK Biobank, where tens of thousands of people wore wrist monitors for a week. They picked out 25,241 people who said they did no exercise or sport at all, and looked for brief bursts of vigorous activity in their ordinary days. A burst could be running for a bus, climbing stairs fast or carrying heavy shopping up a hill, and each lasted a minute or two. Over about seven years, the people who managed three such bursts a day had a risk of death 38 to 40 per cent lower than those with none. The typical total was about four and a half minutes a day.

That is an observational study, with all the cautions from the last chapter, and the size of the effect should be read with some scepticism. People who can run for a bus are healthier to begin with than people who can't. But there's a small randomised experiment that points the same way. Madison Jenkins and colleagues took sedentary young adults and had half of them climb three flights of stairs briskly, three times a day, on three days a week. Each climb took well under a minute. After six weeks the stair climbers' aerobic fitness had improved compared with a control group that did nothing. The improvement was modest, and there were only twelve people in each group. But the whole of the exercise was climbing sixty steps, three times a day, and it still made a difference that could be measured.

So the body doesn't seem to need a continuous block. It responds to the total, and it responds to scraps. The twenty-five minutes you had on Tuesday would have counted. So would three separate eight-minute walks, if that's how the day had offered them to you.