Holmer says heat training does not fix carb burning
Lower heart rate, lower core temperature, more plasma volume, and still a worse return on the gels you swallow.
Heat acclimation does a lot of good things, but fixing your fueling is not one of them. Brady Holmer laid out the adaptations everyone trains for, a lower heart rate, a lower core temperature, and higher plasma volume (more fluid in the blood). Then he asked whether those changes also restore your ability to burn the carbohydrate you ingest, and answered his own question.
The question is whether those adaptations also restore your ability to burn the carbohydrate you ingest.
"It turns out, they don't."
What the argument was about
Tim Noakes pushed back with a different explanation. He argued that exogenous carbohydrate oxidation (burning the carbs you swallow during exercise, rather than the ones already stored in your body) exists mainly to stop blood glucose from climbing too high. In the study, he said, blood glucose concentrations were lower during exercise in the heat, so there was less pressure on the body to burn the incoming sugar. He also offered a mechanism he was open about not having proven, writing that the rate of intestinal glucose absorption may be reduced during exercise in the heat and calling it his guess.
The rate of intestinal glucose absorption may be reduced during exercise in the heat; that's my guess.
Holmer disagreed on the detail. He replied that plasma glucose was actually elevated in hot and hot acclimated exercise compared with cool conditions in that study, that the researchers looked at intestinal absorption and it was not suppressed by heat, and that the limiting factor was metabolic clearance of glucose.
What it actually means
The two of them are arguing about mechanism, not about the headline finding. Both are reading the same study, and both accept that less of the carbohydrate you take on gets oxidised when it is hot. Where they split is whether the gut is the bottleneck, which is the part a runner would most want to fix, or whether the body simply is not clearing glucose at the same rate once core temperature is up.
For anyone with a hot goal race, the practical read is narrow. Heat acclimation is still worth doing for the cardiovascular side, the lower heart rate and core temperature at the same effort. But if these results hold, a hot day puts a ceiling on what your fueling plan can actually deliver, and no amount of sauna sessions or hot runs lifts it. Holmer has previously argued that fuelling questions get framed too simply, including on carb loading, and this is another case where the obvious fix does not do the job people assume it does.
They also looked at intestinal absorption and that wasn't suppressed in heat. Limit was metabolic clearance of glucose.
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