Researchers studying the range of athletic endurance have quantified the maximum energy output that humans can sustain, finding it lower than previous estimates.
This "energy ceiling," which the scientists found to be common across all endurance activities they studied, is 2.5 times a human's rate of metabolism at rest, or roughly 4,000 kilocalories. The findings are published in the June 5 issue of Science Advances.
"At any workload above this ceiling," said study author Herman Pontzer, a professor of evolutionary anthropology and global health at Duke University, "we can't replenish the fuels burned each day, and we lose weight. We can do that for a while, but not indefinitely."
At workloads above this ceiling, humans start burning critical fat stores that they cannot replace in a timely way.
"If you aren't replenishing your fuel stores each day you are slowly starving," said study co-author John Speakman, a professor at the Chinese Academy of Sciences in Beijing and at the University of Aberdeen in Scotland.
While there has been considerable research interest in the limits of sustained energy expenditure for humans, studies that have explored this vary in their results, and few studies have explicitly explored how energy expenditure varies with event duration.
"The previous thought was that Tour de France cyclists reflected the sustainable [maximum energy output] limit, but those studies didn't look across all the available data, didn't quantify weight loss versus endurance and didn't look at events lasting longer than the 20-some days of the Tour," Pontzer said. "This is the first study to look at the relationship between maximum endurance and duration in a rigorous way, and to use those measurements to define the limits of human endurance."
"Studying extreme events is hard," said Pontzer. "And it's difficult to get lots of subjects who are pushing the limits."
His team's study took a different approach than past work, aggregating a number of small studies to provide a much larger dataset for evaluation.
Pontzer and colleagues compiled published measurements of average energy expenditure and metabolic rate for human endurance events including marathons, the Tour de France, swimming, arctic trekking and pregnancy.
They also incorporated original data from the Race Across the USA (RAUSA), the longest event to date from which metabolic measurements have been recorded. In this grueling event, athletes run from Huntington Beach, California, to Washington, D.C., covering approximately a marathon per day, six days per week, for 14 to 20 weeks.
"By incorporating all the previous work along with our own measures," Pontzer said, "we were able to see more."
The group also accounted for something called "metabolic scope," which addresses the naturally greater maximum energy expenditure of heavier individuals.
"The way [to get around this is] to measure how much an individual expends when they are resting — which is also bigger in larger people — and then divide how much they expend when they are working at maximum by the resting number," said Speakman.
Studying the metabolic scope helped his team better understand when weight loss in individuals competing in endurance events was related to burning body fat, which is not sustainable day after day.
Previously, scientists have suggested human energy output thresholds are as high as four to five times a person's metabolism at rest. But "as we showed, [these] individuals are slowly losing body fat at this rate of expenditure, and hence this rate is not sustainable," said Speakman.
According to the new results of Pontzer, Speakman and colleagues, humans in all endurance events share the same energy expenditure maximum: 2.5 times their metabolic activity at rest.
The authors say this shared ceiling suggests that natural selection that enhanced one endurance ability, such as running, may have consequently benefited many other human activities, like gestation length.
These activities involve different muscle groups and organ systems but appear to be united by the same energy intake controls.
"Evolving a greater capacity for endurance would have raised the metabolic limits for other important tasks like pregnancy," Pontzer said. "Alternatively, it's possible that evolutionary pressures to increase investment during pregnancy could have raised endurance capabilities."
Pontzer said these connections were unexpected but "a fun discovery and an exciting direction for future research."
The researchers found that individuals' metabolic rates varied greatly based on the length of the endurance event, with high metabolic rates accompanied by decreases in total energy expenditure. This phenomenon aided the RAUSA athletes, in particular, in completing their particularly long and grueling event.
The bodies of the RAUSA athletes appear to have adjusted energy budgets, said Speakman. "It's like if you have a salary of $100 a day, you just can't spend $120," he explained. "If your household essentials come to $75, your extra scope for entertainment is $25. If you want to spend $50 on entertainment, you can only do that by cutting into the $75 for household essentials."
"That's what the RAUSA people did. They cut down their other expenditures by about 600 kilocalories so they could keep running within the limit," Speakman added. "At present we don't know what the consequences of doing that are."
[Credit for associated image: Bryce Carlson]