Ever since reading James Nestor’s 2014 book Deep, I’ve been fascinated by the scarcely believable feats of freedivers. Plunging 335 feet below the surface of the ocean and making it back on a single breath, or simply holding your breath for 11 minutes and 35 seconds, clearly requires a very special set of skills and traits.
But until a recent conference talk, I’d never considered whether those same characteristics might be useful in other settings where oxygen is scarce—such as the thin air of high-altitude trekking and mountaineering. At the Medicine in Extremes conference in Amsterdam last month, Erika Schagatay of Mid Sweden University gave a presentation that summed up more than two decades of freediving research. The twist that caught my attention: understanding what makes a good freediver could be useful for predicting and perhaps even mitigating altitude sickness.
Schagatay’s initial research interest was in what she calls “professional” freedivers, as opposed to recreational or competitive freedivers. These are people who dive for fish and shellfish, just as their ancestors have for uncountable generations: like the Ama pearl divers in Japan, and the Bajau subsistence fishers in the Philippines and Malaysia. The latter group do repeated dives to about 50 feet, and occasionally go as deep as 130 feet, with such short recoveries that they spend about 60 percent of their time underwater. Over the course of a nine-hour day, they might spend as much as five hours underwater, not breathing.
These diving populations, Schagatay and others have found, share three distinctive characteristics with successful competitive freedivers, who take part in contests around the world sanctioned by AIDA, the international freediving authority:
- Big lungs: In one study of 14 world championship freedivers, vital capacity—the maximal amount of air you can expel from your lungs—was correlated with their competition scores. The three best divers in the group had an average vital capacity of 7.9 liters, while the three worst averaged just 6.7 liters. And it’s not just genetic: Schagatay found that an 11-week program of stretching increased lung volume by nearly half a liter.
- Lots of red blood cells: Divers do tend to have higher levels of hemoglobin, the component of red blood cells that carries oxygen. That’s probably a direct result of their diving. Even if you just do a series of 15 breath holds, you’ll have a surge of natural EPO an hour later, which triggers red blood cell formation.
But there’s a more direct and immediate way of boosting your red blood cell count: squeezing your spleen, which can store about 300 milliliters of concentrated red blood cells. Seals, who are among the animal kingdom’s most impressive divers, actually store about half their red blood cells in their spleens, so they don’t waste energy pumping all that extra blood around when it’s not needed. When you hold your breath (or even just do a hard workout), your spleen contracts and sends extra oxygen-rich blood into circulation. Not surprisingly, spleen size is correlated with freediving performance.
- A robust “mammalian diving response”: When you hold your breath, your heart rate drops by about 10 percent, on average. Submerge your face in water, and it will drop by about 20 percent. Your peripheral blood vessels will also constrict, shunting precious oxygen to the brain and heart. Together, these oxygen-conserving reflexes are known as the mammalian diving response—and once again, the strength of this response is correlated with competitive diving performance.
These three factors help you deal with a complete cessation of breathing for a few minutes. Do they have any relevance to prolonged exposure to a mild decrease in oxygen, like you experience in the mountains? That’s what Schagatay and her colleagues have been exploring in a series of studies involving Sherpas, trekkers, and Everest summiters in Nepal.
In a study published last year, they followed 18 trekkers to Everest Base Camp at 17,500 feet (5,360 meters). Sure enough, the trekkers with the biggest lungs, the biggest spleens, and the biggest reduction in heart rate during a breath-hold were the least likely to develop symptoms of acute mountain sickness.
The size of the spleen isn’t the only thing that matters—its benefits depend on a strong squeezing response to get all the red blood cells out. In a 2014 study of eight Everest summiters, they found that three repeated breath holds prior to the ascent caused spleen volume to squeeze, on average, from 213 milliliters to 184 milliliters. After the ascent, the same three breath holds caused the spleen to squeeze down to 132 milliliters. Prolonged exposure to altitude had strengthened the spleen’s diving response. In fact, there’s also evidence that simply arriving at moderate altitude will cause a sustained mild spleen contraction, as your body struggles to cope with the oxygen-poor air.
Some of these adaptations are clearly genetic. Both Sherpas and Bajau freedivers have bigger spleens than other closely related populations, presumably thanks to generations spent either high in the mountains or underwater. But Schagatay doesn’t believe it’s all genetic. After all, Sherpas who no longer live at altitude have bigger spleens than Nepalese lowlanders, but not as big as Sherpas who still live at altitude. Along with other traits like the diving reflex, it’s something that improves with training, she believes.
What can you do with this information in practice? Here’s some data from the Everest Base Camp study, showing the percent decrease in heart rate during a one-minute breath-hold. The participants are divided into three groups, based on their Lake Louise Questionnaire (LLQ) scores, a measure of acute mountain sickness during the trek. Those with the highest scores—the sickest, in other words—barely have any reduction in heart rate; those with the lowest scores averaged about 18 percent lower:
To test your own heart-rate decrease during a one-minute breath hold, you’d need a proper heart-rate monitor, since the relevant data point is the lowest instantaneous rate you reach by the end of the minute. It’s just one factor among many, but it might give you some indication of whether you’re likely to suffer from altitude illness on a trek, which could help inform your decision about how aggressive an itinerary to follow or whether you want to take Diamox prophylactically. (This particular study was done in Kathmandu, at 4,800 feet, so it’s possible that the predictions would be different at sea level—grist for a future study.)
Even more intriguing is the possibility that you can train these responses. For example, in a 2013 study, Schagatay and her colleagues found that two weeks of 10 maximal breath holds per day strengthened the diving response, producing a quicker and more pronounced drop in heart rate. The next step: figuring out whether this type of improvement would make any practical difference to trekkers.
The bigger takeaway, for me, is the idea that freediving isn’t as crazy and unnatural a pastime as I initially thought when I first read Deep. The mammalian dive reflex originates way back in our evolutionary history—it’s what Per Scholander, one of the first scientists to study it, called “the master switch of life.” And if Schagatay is right, the circuitry that enables us to go deep is also what enables us to make it to the top of Mount Everest—because, as she puts it, we were born to dive.
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