"It wouldn't do anything"

The central belief is that an amount of exercise below the threshold is physiologically pointless. The research answers this one fairly directly.

Hannah Arem and colleagues pooled six large studies from the United States and Europe, covering 661,137 adults followed for a median of about fourteen years. They sorted people by how much leisure-time physical activity they reported, measured against the recommended minimum of 150 minutes a week at moderate intensity. Then they compared death rates. People who did the recommended amount, or up to twice as much, had a 31 per cent lower risk of dying during the study than people who did nothing. That's the group the guidelines are aimed at.

The group that matters for this book is the one below it. These people did some activity but less than the recommended minimum. They're the ones who, by the logic of the threshold, were wasting their time. Their risk of dying was 20 per cent lower than that of the people who did nothing.

Put those two numbers side by side. Going from nothing to something, but not enough, was associated with a 20 per cent reduction. Going from there all the way up to meeting the guidelines added another 11 points. Doing three to five times the guidelines got people to 39 per cent. So the first step, from zero to a little, was the largest single step on the whole scale. The rest of the gain was spread out over the distance between a little and a great deal.

A study from Taiwan makes the same point with a specific amount. Chi Pang Wen and colleagues followed 416,175 people for an average of eight years. The lowest active group exercised for about fifteen minutes a day on average, roughly 92 minutes a week. Compared with the inactive group, they had a 14 per cent lower risk of death and a life expectancy about three years longer. Each further fifteen minutes a day reduced the risk by another 4 per cent.

Both of those studies relied on people reporting their own exercise, which is known to be unreliable. More recent work has used devices worn on the body. Ulf Ekelund and colleagues combined eight studies in which 36,383 people wore accelerometers, and followed them for about six years. They split people into four equal groups by total activity. Compared with the least active quarter, the next quarter up had about half the risk of death. The third and fourth quarters did better still, but by much smaller steps. Any physical activity, at any intensity, was associated with lower risk.

I need to be honest about what these studies can't show. They're observational. Nobody assigned people to exercise or not. They observed what people did and what happened to them. People who are already ill tend to move less, and that inflates the apparent benefit of moving. People who exercise also tend to eat better, smoke less and have more money. The researchers adjust for these things as well as they can, and some studies exclude deaths in the first few years to reduce the illness problem. But no adjustment is perfect, and the true causal effect is probably smaller than the raw figures. What I'd rely on is the shape of the curve, which keeps turning up in study after study, in different countries and with different methods. It rises steeply at the bottom and flattens towards the top.

That shape settles most of this chapter's question. The threshold belief pictures exercise as a switch. Below the line nothing happens, and above it you get the benefit. In the data it looks more like a dimmer, and most of the brightening comes in the first part of the turn.

On that Tuesday you thought you were choosing between a session that counted and a gesture that didn't. What you were offered was the most valuable fifteen minutes on the scale, and you turned it down because it wasn't forty-five.