As we increase the number of days of exercise, our ability to maintain extremely high levels of energy expenditure declines. Picture: CHRIS VAN LENNEP/123RF
As we increase the number of days of exercise, our ability to maintain extremely high levels of energy expenditure declines. Picture: CHRIS VAN LENNEP/123RF

The idea that to lose weight and boost health we just need to “eat less and move more” is a staple of medical and dietetic advice worldwide.

It is the so-called Cico (calories-in, calories-out) theory of energy metabolism, the foundation on which weight loss and health advice rests.

According to the theory, weight-loss benefits accrue for hours even after physical activity ceases. That’s courtesy of the energy-burning fire of metabolism that physical activity generates.

It’s why so many of us run marathons and ultra-endurance events, such as the Two Oceans and Comrades. We believe that the more we exercise, the more calories we burn.

A small new US study in Science Advances challenges that paradigm in its entirety.

The study defines the limits of “the realm of what’s possible for humans”, senior study author Herman Pontzer, an associate professor of evolutionary anthropology at Duke University, said in a media release.

It shows that we may all have a finite pool from which we can draw energy, and that the limits of human performance may rest in the gut – in particular, how many calories the gut can process.

The study, titled “Extreme events reveal an alimentary limit on sustained maximal human energy expenditure”, builds on a small but growing body of research that overturns conventional thinking about metabolism, calories and energy use.

The study authors measured daily calories burnt by six runners who ran six marathons a week for five months. The runners took part in the 2015 Race Across the USA, a 4,828km once-off event from California to Washington, DC.

The researchers compared their data to other feats of human endurance that require extreme amounts of energy, including punishing 161km trail races and pregnancy.

During the study period, the researchers found that the runners could burn calories only up to 2.5 times their resting metabolic rate. This is similar to the other extreme endurance events, showing that as we increase the number of days of exercise, our ability to maintain extremely high levels of energy expenditure declines, plateauing at about 2.5 times the resting metabolic rate.

Athletes who hauled 226kg sleds across Antarctica for days in sub-freezing temperatures or cycled the Tour de France, for example, all had the same response. This means that the longer and harder the exercise, the fewer calories the body may be able to dedicate to exercise.

Interestingly, the researchers reported that the maximum sustainable energy expenditure found among endurance athletes was only slightly higher than the metabolic rates women sustain during pregnancy.

This suggests that the same physiological limits that keep, for example, Ironman triathletes from shattering speed records may also constrain other aspects of life. These include how big babies can grow in the womb, say the researchers.

Thus, they say the limiting factor for extreme endurance events may be the digestive tract’s ability to break down food.

Study co-author Lara Dugas is an associate professor at Loyola University Chicago in the department of public health and a University of Cape Town (UCT) graduate in exercise physiology from the Research Unit for Exercise Science and Sports Medicine.

Her UCT PhD thesis looked at the accuracy of wearable monitors to estimate how many calories people burn each day. Since then, her research has evolved to looking at metabolism, diet and exercise and how gut microorganisms play a role in obesity.

Dugas said via e-mail that the popular belief for weight loss or treating the obesity epidemic largely remains that people need to exercise more, because more exercise — or calories out — means more weight loss. She disagrees with it because, to date, it has “not worked out”.

Despite the fact that more people than ever are self-reporting higher levels of recreational physical activity, obesity numbers continue to increase, Dugas said.

“This indicates that simply increasing one’s level of physical activity may not be sufficient for weight loss because of other unintended consequences, like increased hunger.”

The study data do not support the idea that we all have an infinite amount of energy to keep burning and that if we just keep exercising, we'll keep increasing our metabolic rate, she said.

The study shows that total energy expenditure is, in fact, not “additive”. Therefore, we do not just keep adding on the calories as we move around more.

Instead, the study shows that our total energy expenditure may be “constrained”, Dugas said. It also shows competition from our pool of energy for the energy required for physical activity.

Simply increasing one’s level of physical activity may not be sufficient for weight loss because of other unintended consequences
Dr Lara Dugas

“The competition may come from our normal daily requirements [or resting metabolic rate], or from the immune system if we are sick, or from normal cellular growth and repair,” she said

What happens in these extreme events, and what the researchers noticed among the athletes they studied is that, anecdotally, their enthusiasm level significantly dropped off after the event.

“Towards the end, the athletes were more docile and lethargic,” Dugas said. “It was if they were saving their energy for their daily marathon.”

The study has limitations, of which the small sample size of six is one — and no surprise. “It’s hard to find a large group of people willing to run a marathon a day for 140 days,” Dugas said.

And as with any research project, researchers must take care not to oversimplify the data and make sweeping generalisations. The researchers observed those requirements.

“We can’t generalise our findings to a broader population,” Dugas said. “These data are from endurance athletes running a marathon a day for 140 days. That lends support to the constrained model of energy expenditure but is not definitive and needs to be replicated.”

The researchers were also careful with measurements and tried to reduce as much bias as possible. For example, they used the same metabolic equipment for all measurements across the 140 days.

UCT emeritus professor of sports science Tim Noakes said the study finding that energy expenditure drops in the second week “seems robust”. It implies that the body can adjust its “efficiency”, becoming more efficient as the duration of exercise becomes extreme, he said.

How this happens is difficult to explain, Noakes said, because it threatens the laws of thermodynamics: “How do you get the same amount of mechanical work done by burning less fuel?”

On the other hand, scientists have long known that as people spend more time exercising every day, they tend to rest more during the day. As the athletes were “only running about 42km a day, they had plenty of time to rest during the rest of the day”, said Noakes. That would have been easy to control in a laboratory study but less easy in this study.

Noakes did not find the argument that the gut limits exercise performance by determining how much we can eat each day to be compelling. Instead, he argued for the need for recovery that sets the limit of how much athletes can do daily during prolonged events.

“Having worked with athletes doing these long events, we find that to be generally successful, they need to exercise for about 10 hours and rest for 14 hours a day.

“Without 14 hours’ rest daily, they risk becoming progressively more exhausted.”

The US researchers are continuing their research. As far as they know, no-one has ever sustained levels beyond the limit that shows up in their latest research.

“So I guess it’s a challenge to elite endurance athletes,” Pontzer said.

“Science works when you’re proven wrong. Maybe someone will break through that ceiling some day and show us what we’re missing.”