Why do people run? It’s a question that continues to fascinate scientists, from psychologists to exercise physiologists. Even though it is predicted that driverless cars will transform society in the coming decade, running still retains an enduring appeal.

In 2014, a survey found there were 10.5 million runners in the UK , whose reasons for running included “maintaining fitness”, “stress relief, “a new challenge” and “competition”.

Psychologists studying ultra-marathon runners recently concluded that one of their main sources of motivation was not muscle-building or chasing PBs, but attaining the state of mind known as “ flow ”. But another study , of 3,500 runners across seven European countries, 40 per cent revealed that they ran for the sole purpose of losing weight.

But while weight-loss is a goal coveted by many runners, it is not always easy to achieve. Theoretically, it seems straightforward. Roughly speaking, losing 0.45kg involves burning off 3,500 calories, the equivalent of about 35 miles of running. But it rarely works out quite so simply.

One of the reasons is that, the more you exercise, the more the body naturally tries to compensate by altering your metabolism through a series of evolutionary-based protective mechanisms – first highlighted around 40 years ago – which are designed to prevent starvation and indefinite weight loss.

"The human body has a pretty good capability for regulating its bodyweight," says Glenn Gaesser, professor of exercise science at Arizona State University. "We all have a set-point range for our weight and, while the average person may consume three-quarters of a million calories per year, from year to year we weigh pretty much the same thing unless something happens that greatly distorts our lifestyle, such as a purposeful weight-loss diet, or some sort of tragic event that changes our behaviour."

Following exercise, a range of small but significant compensatory processes kicks in that resist the intended energy deficit. Changes take place within the body’s tissue that reduce resting metabolic rate, especially during sleep, and as weight decreases, you burn calories less efficiently the next time you exercise. This seems to be particularly the case if a runner cuts their calorific intake at the same time as embarking on a new exercise regime.

In addition, there are certain psychological and behavioural changes that also combine to undermine attempts to shed weight.

Many of us tend to overestimate the number of calories we have actually burned through exercise and increase our energy intake , either consciously or subconsciously.

This is one of the reasons why some people are dismayed to find that they have put on weight despite taking up running. Gaesser says it has been found that people who had watched promotional exercise videos tended to consume more calories than they would normally, despite having not done any exercise themselves.

This is because exercise is thought to influence our eating behaviour by modulating both the pleasure we get from eating and the drive to eat, known as the body’s hedonic response to food.

While, in the long term, exercise is thought to help hone our appetite regulatory system, making it easier to cut down on snacking and instead eat at set times of the day, in the short term vigorous exercise stimulates brain areas associated with reward and dependence. This makes us crave high-fat, energy-dense foods, which can negate the beneficial effects of a running regime.

But perhaps the most common reason people don’t lose always lose the weight they expect to through running, is that, post-run, they simply slow down during the rest of the day. It is a trend many scientists have observed by fitting runners attempting to lose weight with motion sensors or heart-rate monitors.

“People often subconsciously modify their behaviour,” Gaesser says. “There are studies that show that when people take part in a structured programme of 30-40 minutes’ running, three to five days a week, outside of the structured activity, some simply don’t do as much throughout the day as they would normally do. Usually, they would naturally get in around 7,000 steps a day through regular activities, but during the running regime that might fall to 4,000-5,000 steps. And you can see that the people who had the biggest decrease in their spontaneous activity during the rest of the day are the ones who don’t lose as much weight as they were anticipating.”

Gaesser recommends that runners wanting to lose weight invest in an activity monitor so they can check that they are still maintaining the same number of steps and overall activity when they are not running. But it is also possible to boost your chances of burning more calories by adapting your running regime to include more high-intensity training.

Some runners experience an initial boost of weight loss before finding that the rewards plateau over time, despite their best efforts. “It’s not a clear-cut relationship between calories and weight loss,” Gaesser says.

“So if you triple your calorie expenditure by running, you won’t necessarily triple your weight loss. It works to a point, but then there does seem to be an upper ceiling as your body’s metabolism adapts and resists.

“There is recent evidence to suggest that really high-intensity interval training can stimulate the metabolic rate to remain elevated for periods of time after exercise, which encourages weight loss as you continue to burn calories even when resting. But we don’t yet know whether that continues to work over a timespan of months and years.”

However, the sort of high-intensity training Gaesser is referring to requires pushing yourself to up to 80 per cent of your VO2 max, the maximum volume of oxygen you can consume, over a period of at least an hour. This is far harder than many people are willing to push themselves.

But overall, science still indicates that if you want to lose weight, running is one of the better option as it uses a lot of big-muscle groups, key for calorie burning. In stark contrast, swimming tends to perform worst in studies, as the temperature differences in the water mean you lose a lot of heat. The end product is that body fat losses tend to be somewhat neutralised.

But as for how running shapes up to other activities in terms of weight loss, we simply don’t know. “We don’t know whether running is superior to cycling or rowing or even cross-country skiing in terms of optimal body-fat loss because there simply isn’t that data available,” Gaesser says.

“But it’s superior to walking simply because you can run at a higher level of energy expenditure and still have the same level of physiological and perceived effort. So it’s a more efficient way to expend those calories.”

Guardian service