This is the season when fitness journalists write articles about how the miserable heat that ruins your workout is actually doing you a great favor. You're lucky enough to drip buckets of sweat and scrub a storm, because heat is the height of the "poor man" that increases the physiological demands of your workout and triggers a series of adjustments that improve your endurance.
Here is the version of this story I wrote two summers ago, and I'm sticking to it. But I may need to update the reasons for the great heat based on a new study in Experimental Physiology. According to a research team led by Carsten Lundby and Bent Ronnestad from the Inland University of Applied Sciences in Norway, heat increases the amount of oxygen-bearing hemoglobin in your blood, just like during altitude training – but it's not a quick fix.
One of the key factors in endurance performance is how quickly you can get oxygen from your lungs to your muscles through your blood. In particular, it is the hemoglobin in your red blood cells that absorbs oxygen. Spend a few weeks at high altitude where the air is thin and your body responds by producing more hemoglobin. That is why the vast majority of elite endurance athletes do altitude training.
Heat training works differently. The most notable change after just a few days is a dramatic increase in the volume of plasma flowing through your veins by up to 20 percent. This is the part of the blood that does not contain hemoglobin-rich red blood cells. It is therefore not immediately apparent whether more plasma improves your endurance in moderate weather conditions. In fact, there is an ongoing debate among scientists on this very question. For example, one theory states that the additional plasma dilutes the lactate accumulation during hard exertion. (There is no doubt, however, that it increases performance in hot conditions: the additional plasma volume helps, among other things, to transfer excess heat to your skin.)
When the Journal of Physiology held a debate a few years ago about whether heat training improves performance in moderate conditions, the paper's co-author who argued against the proposal was none other than Carsten Lundby. He's not buying the claim that more plasma is useful on its own.
But in recent years, Lundby and his colleagues have considered another option. The extra volume of plasma causes a thinning of the concentration of red blood cells in your blood, an amount called hematocrit. If your total blood is 45 percent red blood cells, your hematocrit is 45. If the plasma volume increases as a result of heat training, your hematocrit is reduced.
Lundby's hypothesis is based on the idea that your kidneys are constantly monitoring the hematocrit and trying to keep it in a normal range. If your hematocrit decreases steadily, the kidney will produce EPO to trigger the production of more hemoglobin-rich red blood cells. In contrast to the rapid increase in plasma volume, this is a slower process. Lundby and his colleagues assume that it could take about five weeks.
He and his colleagues published their first results in Frontiers in Physiology in November. After five and a half weeks, 12 trained cyclists who did one hour of heat training five days a week (integrated into their regular training) actually showed a slight increase in hemoglobin compared to a group of nine cyclists who did the same training in cooler conditions. However, the results were very different, possibly because the test subjects were not all at the same fitness level.
For the new study, they recruited real elite cyclists with an average VO2 maximum of 76.2 milliliters of oxygen per kilogram of body weight per minute. During the five-week course, they trained about ten hours a week, and this program included five afternoon lessons with 50 minutes of “light exercise” on a stationary bike. The 11 cyclists in the warming group completed these sessions at around 100 degrees and 65 percent humidity. The 12 cyclists in the control group completed the same sessions at 60 degrees and 25 percent humidity to achieve the same subjective level of performance. During the heating sessions, cyclists were limited to half a liter of water to ensure mild dehydration, which is considered one of the triggers for the expansion of the plasma volume.
The most important result measure: The total hemoglobin mass in the heat group rose by 893 to 935 grams, which corresponds to a significant increase of 4.7 percent. In the control group, the hemoglobin mass remained essentially unchanged and only increased by 0.5 percent. The individual results looked like this:
(Figure: Experimental physiology)
The study also included a number of physiology and performance tests, including VO2 max, lactate threshold, and a 15-minute time trial. There were no statistically significant differences between the groups, but some of the results showed "small to medium effect sizes" that favored the warming group. For example, the heat group increased lactate threshold output by 2.8 percent, while the control group decreased 0.4 percent. In addition, the warming group increased the average performance by 6.9 percent during the 15-minute trial, while the control group improved by 3.4 percent.
All in all, the results are cautiously encouraging. They don't prove that Lundby's hypothesis that diluted blood stimulates more EPO caused the changes, but they do suggest that something good seems to be happening after about five weeks.
This is good news, but in a way it is also a problem. One of the reasons why heat training has attracted so much attention in recent years is that it is relatively practical and accessible. Only a tiny fraction of the world's athletes can spend a month in the Alps before each big race. But many people can do heat training simply by stepping out the front door – or, according to some studies, even lounging in the hot tub or sauna after training.
It's a bigger question, though, to commit to five long weeks of miserable, sticky heat training. Lundby and his colleagues acknowledge this limitation and note that "this type of training in amateur sports may have little relevance." For those looking for every possible benefit, the results will undoubtedly attract attention. And for those who live in places like Texas (or even supposedly cooler parts of the continent like Toronto, where I live, which started the summer with an oppressive streak of heat warnings), it is much needed comfort. You may not have opted for weekly heat training, but at least you could get some hemoglobin from it.
To learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter and read my book Endure: Mind, Body and the Strange Elastic Limits of Human Performance.
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