Runners on the 3,080 mile Race Across the USA in 2015. Photo: Bryce Carlson/Purdue University

A small study of endurance athletes shows that even the most fit human body can only sustain 2.5 times the person's resting rate of metabolism (RMR), with researchers saying they suspect this is due to the capability of the body to digest food, according to a study published in Science Advances Wednesday.

Why it matters: Human energy expenditure is important because a person's limits affect all their body's systems, including reproduction, thermoregulation and physical activity.

What they did: The researchers tracked data from 5 athletes who completed the grueling Race Across America (RAUSA) event, where they run from coast-to-coast, traveling roughly a marathon a day, 6 days a week, for 14-20 weeks.

  • They also compiled data from others endurance events, like marathons, the Tour de France, swimming, arctic trekking and pregnancy.

What they found: While metabolic rates varied based on the endurance event length, each person had the same metabolic ceiling of 2.5 times their rate of metabolism at rest.

  • Study co-author John Speakman tells Axios this means "you can't expend more than 2.5x RMR without burning fat because you can't get energy from food at a faster rate than that."
  • "It is important because it tells us about limits in very different scenarios: military ops, endurance sport, reproduction, etc.," says Speakman, a biologist at the University of Aberdeen.

Separately, a study published in Diabetes Care on Wednesday links metabolic disorders like obesity, high cholesterol and hypertension to irregular sleep patterns — going a step past prior studies showing lack of sleep may play a role in causing those disorders.

  • What they found: For every hour of variability in sleep time, a person may have "up to a 27% greater chance of experiencing a metabolic abnormality," the National Institutes of Health, which helped fund the study, said in a statement.
  • "[E]ven after considering the amount of sleep a person gets and other lifestyle factors, every one-hour night-to-night difference in the time to bed or the duration of a night’s sleep multiplies the adverse metabolic effect," said study co-author Tianyi Huang of Brigham and Women’s Hospital, per the statement.

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