It’s a common refrain in our apartment after my boyfriend comes home from a particularly hard, five-hour mountain bike ride that involved climbing Apex twice and I’ve asked, “How was it?”

“Great. Brutal. I tasted blood.”

I’ve never understood this. An exertion so difficult that you can taste iron, an output of exhaustion and truly pushing your body to its end. I pushed myself that hard when I was a competitive runner. I ran eight-mile cut-downs that finished with a sub-six-minute mile and survived repeat 400-meter intervals at a sub-60 pace. My legs were too wasted afterwards to walk the 200 feet to the locker room. But I’ve never tasted blood. I’ve thrown up, sure, but the whole blood thing has alluded me. My question for Randy Wilber, senior sport physiologist at the Olympic Training Center in Colorado Springs: Why?

Genetics, our body’s response to exercise’s affect on pulmonary arterial pressure, and the structural makeup of our lung’s capillaries, says Wilber. His analogy is that some people are more prone to certain types of allergens (e.g., cat hair), whereas other are not, thanks to our parents and how we’re affected by the environment in which we live.

Live at a moderate or high altitude? That betters your chance for the phenomenon, due to an increase in pulmonary arterial pressure from the thinness of the air.

Work out in cold-dry air? That can affect the structural “shear” stress on the capillaries, which makes it more likely to damage them and release the iron.

Destroying your capillaries doesn’t usually happen on an everyday run. It’s a result of high-intensity exercise, like the effort at the beginning of a race or on a difficult hill climb. And there’s no danger to tasting blood as long as you don’t have a mouth full of it. (Interesting fact from Wilber: “The Fe [iron] residue from this capillary bed ‘bleeding’ moves upward toward the mouth in humans, thus the metallic taste,” he says. “In thoroughbred race horses, the Fe residue moves downward into the lungs, which leads many racehorse trainers to use bronchodilatory drugs pre-race to prevent obstructive airway problems with the horse during the race.”)

More than anything, I’m happy to know that my boyfriend is not, as he thinks, tougher than me for tasting blood. His capillary structures are weaker. And that’s a win I’ll take.

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