We spend a lot of time talking about watts and pace and nutrition windows. Here’s a change of pace, five things about endurance athletes that are just genuinely fascinating. The kind of stuff you drop at a dinner table and watch people put their forks down. Or in my case, and watch my wife and kids roll their eyes waaaaay back.

Elite athletes have completely different gut bacteria to the rest of us and it actually makes them faster.

Harvard researchers collected stool samples, yes, from runners before and after the Boston Marathon and found significantly higher levels of a bacteria called Veillonella in the runners, barely present in sedentary people at all. But it gets even weirder. When researchers gave Veillonella from a marathon runner to mice, the mice ran 13% longer on a treadmill than the control group. Thirteen percent. From gut bacteria. Haha, what! Your training and your microbiome are apparently in a feedback loop you had no idea about.

Tour de France riders burn more calories in three weeks than most people burn in roughly six months.

A rider burns anywhere from 4,000 calories on a flat stage to 9,000 on a mountain stage, with the full peloton of 176 riders burning a combined 25 million calories across the whole race. Per rider, the three-week total sits north of 100,000 calories.

A trained endurance athlete’s heart is literally a different shape to yours.

Years of sustained training physically restructures the left ventricle, it enlarges and its walls thicken to handle the enormous volumes of blood required during exercise. The result is a resting heart rate that can sit in the 30s or low 40s with each beat pushing so much more blood around that fewer beats are needed. Miguel Indurain, five-time Tour de France winner, had a recorded resting heart rate of 28 beats per minute. Twenty-eight. An ECG from an elite endurance athlete looks so abnormal that a doctor unfamiliar with athlete physiology would think something was seriously wrong. In this case, wrong is just very, very fit.

The human body has a hard metabolic speed limit, and elite athletes are right at the edge of it.

Researchers tracking ultra-runners, cyclists, and triathletes over weeks found that no matter how intense the effort, the body maxes out at about 2.5 times its basal metabolic rate over the long term. Short bursts above that are possible but the body compensates by quietly turning off background energy use to free up fuel. The ceiling exists because digestion can only absorb so many calories per day regardless of how hard you’re working. Elite endurance athletes are essentially the humans who have pushed closest to that wall without going through it.

Endurance training appears to slow down aging at a cellular level.

Telomeres are the protective caps on the ends of your chromosomes. Every time a cell divides they get shorter, and when they run out, cells stop functioning properly. That’s a big part of what aging actually is. Studies consistently find that elite athletes have significantly longer telomeres than inactive people of the same age, and endurance athletes may live 4 to 8 years longer than those who engage in normal physical activity.

So next time someone asks you “watts wrong with you”, you tell them you are basically anti-aging and throw some of these other facts at them! Just don’t feed anyone your Veillonella, please.