Imagine you’re running a race. But this is no normal race—this is a 140-day trek from Huntington Beach, CA to Washington, D.C. Welcome to Race Across the USA, a seemingly-superhuman feat that provides the perfect laboratory to study human endurance. After all, extreme athletes push their bodies to the proverbial “limit”—but what, exactly, is this limit? That’s what a new study published in Science Advances tries to figure out.

The researchers studied six runners over the course of the race and estimated energy usage by incorporating information about food intake, running speed, and even specially-modified water that allowed them to track its excretion. Each subject’s metabolic activity was measured relative to their baseline energy use at rest, a metric called “metabolic scope.” A higher metabolic scope means higher energy usage. In terms of finding physical limits, what is the highest metabolic scope athletes can generate and sustain? Is that number consistent across various athletes?

The numbers showed, as expected, that runners didn’t maintain their initial energy usage throughout the race. During the first week, they had a super high metabolic scope, but by the twentieth week, they were using 20% less energy than in week 1 (around 2.5). To see if this trend was specific to the runners, the scientists compared the runners’ data to measurements of metabolic scope from people carrying out shorter activities, such as Arctic treks and pregnancy. These activities showed the same metabolic limitations. It appears that the highest sustainable metabolic scope is 2.5.

But what makes human endurance finite? That is, why do athletes start off with a super high metabolic scope but drop off for the rest of the race? One explanation is that while we can literally eat enough calories to theoretically fuel a higher metabolic scope, our bodies simply cannot convert the food to energy quickly enough. Previous studies tried overfeeding subjects and observed that the sustained metabolic scope capped at 2.5, or an energy expenditure over twice as high as the resting level. This was very similar to the metabolic scope that the runners approached over the course of the race. So, while these extreme athletes may seem to have superhuman endurance, their bodies are still subject to human metabolic limits.

Managing Correspondent: Aparna Nathan

Original article: Extreme events reveal an alimentary limit on sustained maximal human energy expenditure – Science Advances

Media coverage: Have We Really Found the Limit of Human Endurance? – Outside

Pushing the Limits of Human Endurance – New York Times

Image credit: skeeze/Pixabay