Over the past few years, several big-data studies have concluded that, when it comes to hitting the wall in a marathon, men crash harder than women. At any given fitness level or finishing time, men slow more than women in the late stages of the race.
As I discussed in a post last summer, there are conflicting views on why this happens. Does it reflect evolutionary differences in competitiveness and aggression? Social influences? Or straightforward physiological differences? There are reasonable arguments for all of these explanations, and it’s unlikely that there’s one simple answer.
At the American College of Sports Medicine annual meeting last month, researchers from Eric Snyder’s group at the University of Minnesota presented some data testing one of the more popular theories, which is that at any given level of intensity women burn more fat and less carbohydrate than men. Because carbohydrate stores are limited and fat stores are effectively infinite, this should mean that men deplete their carbohydrate stores - and hit the wall, or at least are forced to adjust their pace - sooner.
To test the idea, the researchers recruited 91 novice marathoners (72 women, 19 men), and tested their fitness a few weeks before a marathon. To assess their fat-burning, they measured “respiratory exchange ratio” - the ratio of carbon dioxide produced to oxygen consumed when you breathe, from which you can calculate the ratio of fat to carbohydrate being burned - during a six-minute run at 75 percent of the speed the runners could maintain in a two-mile time trial. The speed used in the six-minute run is close to predicted marathon pace.
As expected, the women burned more fat than the men at the same relative running intensity. The women had a respiratory exchange ratio (RER) of 0.86, which corresponds to getting about 54 percent of your energy from carbohydrate and 46 percent from fat. The men had an RER of 0.89, which corresponds to 64 percent carbohydrate and 36 percent fat.
Also as expected, the women slowed less than the men during their marathon, running their second half an average of 20.5 percent slower than their first half, compared to 29.1 percent slower for the men.
So, on the surface, the hypothesis seems promising. But when the researchers analyzed the full dataset, it turned out there was no significant correlation between RER (as a proxy of fat-burning) and marathon slowing among either the men or the women. Knowing how efficient a fat-burner someone is doesn’t allow you to predict how much they’ll slow in a marathon.
Is this the final word on the topic? Probably not. You’d need a massive study with hundreds or perhaps thousands of people to really know for sure that fat-burning ability has no influence on marathon pacing. In practical terms, there are good reasons to think that it does have at least some effect.
Still, the study suggests that fat-burning isn’t what drives these big-picture trends in which men consistently crash harder than women. Or to put it another way: if you’re a guy and you keep hitting the wall, don’t blame your metabolism - blame your optimism.