This is the time of year, Alex Hutchinson reminds us, when fitness journalists write articles about how the miserable heat that’s ruining your workouts is actually doing you a big favor:
You’re lucky to be dripping buckets of sweat and chafing up a storm, because heat is the “poor man’s altitude,” ramping up the physiological demands of your workout and triggering a series of adaptations that enhance your endurance.
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Heat training works differently [from altitude training]. The most notable change, after just a few days, is a dramatic increase—of up to 20 percent—in the volume of plasma coursing through your veins. That’s the part of the blood that doesn’t include hemoglobin-rich red blood cells, so it’s not immediately obvious whether more plasma will enhance your endurance under moderate weather conditions.
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If heat training causes your plasma volume to increase, that will lower your hematocrit.
Lundby’s hypothesis is based on the idea that your kidneys are constantly monitoring hematocrit, trying to keep it in a normal range. If your hematocrit has a sustained decrease, the kidney responds by producing EPO to trigger the production of more hemoglobin-rich red blood cells. Unlike the rapid increase in plasma volume, this is a slower process. Lundby and his colleagues figure it could take about five weeks.
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The 11 cyclists in the heat group did those sessions in about 100 degrees and 65 percent humidity; the 12 cyclists in the control group did the same sessions at 60 degrees and 25 percent humidity, aiming for the same subjective effort level. During the heat sessions, the cyclists were limited to half a liter of water to ensure mild dehydration, which is thought to be one of the triggers for plasma volume expansion.
The key outcome measure: total hemoglobin mass increased 893 to 935 grams in the heat group, a significant 4.7 percent increase. In the control group, hemoglobin mass stayed essentially unchanged, edging up by just 0.5 percent.