Heat acclimation makes you a better thermoregulator, but it does not make you better at burning the carbohydrate you drink. A study published in The Journal of Physiology this week shows exogenous glucose oxidation drops by about 27% at 40°C, and stays down after nine days of heat acclimation.

The timing matters. Elite triathlon is in the Gulf right now: WTCS Abu Dhabi closed the Championship Series era on 13 November, the Dubai T100 follows today, and the Qatar T100 World Championship Final comes to Doha on 11 December.

The standard playbook for hot races is: acclimate, then fuel as you would anywhere else. This paper says the second half of that sentence needs rewriting.

What the study did and found

Tim Podlogar, Gareth Wallis and colleagues at Birmingham, Exeter and USARIEM tested 17 endurance-trained adults (10 men, 7 women). Each cycled for 90 minutes at a moderate 45% of maximal power while drinking glucose at 90 g/h.

They did this three times: at 20°C, at 40°C, and at 40°C again after nine days of isothermic heat acclimation spread over ten days. Relative humidity was 20% throughout. Dual stable-isotope tracers, indirect calorimetry and muscle biopsies tracked where the glucose went.

The acclimation worked on the usual markers: the authors report clear thermoregulatory adaptation. Yet the oxidation of ingested glucose did not move back towards the temperate value.

The mechanism is the interesting part. Glucose still entered the blood from the gut at the same rate in every condition. What fell in the heat was glucose clearance and the oxidation of plasma glucose. The bottleneck is not absorption; it is uptake and use by the working tissues.

It is not a dehydration story

The obvious objection is that heat studies often confound temperature with fluid loss. Two strands of evidence argue against that explanation.

First, a Loughborough study last year kept runners euhydrated. Ten trained runners ran 100 minutes at about 65% of VO₂max, at 34°C or 19°C, replacing about 90% of sweat losses and drinking 60 g/h of glucose. Average exogenous oxidation was still about 20% lower in the heat, and the body leaned harder on its own carbohydrate stores to make up the difference.

Second, the new study points to a peripheral limit. Gut-to-blood delivery was unchanged, so the effect sits downstream, in how muscle takes up and oxidises circulating glucose. Two labs, two exercise modes, two temperatures: the direction and rough size of the effect (20–27%) agree.

Why it matters: more grams in is not more grams used

The practical consequence is a larger draw on glycogen for the same intake. In the new study the deficit was about 0.14 g/min, roughly 8 g of ingested glucose per hour that is not oxidised.

That sounds small, but it compounds. As a rough extrapolation, not a measured result, a T100 lasting around 3.5 hours at that deficit means about 30 g less fuel from the bottle. In the Loughborough study data, endogenous carbohydrate use rose by about 0.24 g/min in the heat, which over a long race is a meaningful extra withdrawal from muscle and liver glycogen.

This collides with the current push towards 100–120 g/h intakes. If oxidation is capped lower in the heat, pushing intake harder may simply load the gut, exactly when heat is already making gastrointestinal problems more likely.

Three caveats keep this honest:

• Glucose only. The new study used glucose alone. An older Birmingham study found a glucose–fructose mix oxidised about 36% faster than glucose in the last hour of cycling at about 32°C. Whether multiple transportable carbohydrates escape the heat penalty is the obvious next experiment.
• Moderate intensity. 45% of maximal power is well below race pace. At race intensity, carbohydrate demand and heat production are both higher.
• Performance is not oxidation. A 2026 systematic review found carbohydrate’s benefit in the heat equivocal: five of nine trials showed gains, four did not, and seven of nine never measured gut symptoms.

What the T100 race data shows

If heat erodes fuel availability late in a race, run splits should suffer more in hot races than in cool ones. I tested that on every T100 result from 2024 to 2026 on PTO Stats: 37 race-days and 555 finishes by 91 athletes who raced at least three times.

Each split is expressed against that athlete’s own median, so +5% means five per cent slower than their usual T100 run. Heat is the simplified wet-bulb globe temperature (sWBGT) at the day’s peak, calculated from hourly temperature and dew point in the Open-Meteo historical weather archive.

Most race-days sit within about 5% of zero whatever the weather. The exception is Singapore: all five editions, men and women, ran 5–10% slower than usual, roughly three to six minutes on an hour-long run.

Across all races, each extra degree of sWBGT was associated with a 0.48% slower run (95% CI 0.19 to 0.76; athlete fixed effects, errors clustered by race). The bike showed no such link (−0.29% per degree, CI −0.70 to 0.12).

That slope is really a Singapore effect. Remove the five Singapore races and the run slope falls to zero (−0.06% per degree, CI −0.30 to 0.17). Between roughly 18 and 29°C sWBGT, race-day heat explains very little of how elite athletes run.

The bike works as an internal control. If Singapore were simply a slow venue or a bad day, the bike should slow too.

It didn’t. In the hot band the median bike split matched athletes’ usual, while the run was about 4% slower. The interval for the run crosses zero because Miami, also hot, showed no penalty. A run-specific slowdown confined to the hottest, most humid races fits a heat-and-fuel story, but it cannot prove one.

What this means for Doha. Last year’s final sat in the warm band (sWBGT 26.8°C), and its run splits were normal: +0.7% for women and −0.2% for men against each athlete’s usual. On that evidence, December in Doha does not trigger the run penalty seen in Singapore.

One limitation to state up front: race data cannot separate fuel from thermoregulation, pacing or tactics. A steeper fade in the heat is consistent with this mechanism; it does not prove it. December in Doha is also typically warm rather than extreme, well short of the 40°C used in the lab (hopefully as I will be in the race too, injuries permitting!), so the effect at the final may be smaller.

Three further caveats on the data:

• Singapore is one venue. Its course, its start time and its five editions all travel together, so a slow run course could mimic a heat effect. The normal bike splits there argue against a general slow-venue explanation, but not against a slow run course.
• Weather is approximate. sWBGT uses the day’s peak temperature and humidity, not conditions at race time, and ignores sun and wind.
• One race excluded. Dubai 2025 men’s run splits were around 50–55 minutes, far quicker than any other edition, which suggests a shortened course; it is left out.

What practitioners should do before Doha

Keep heat acclimating; just stop expecting it to solve fuelling. A practical checklist:

1. Acclimate anyway. Lower core temperature, better sweating and a lower heart rate are real gains for performance and safety. The new study confirms they happen; it shows they are not the whole story.
2. Start with full glycogen stores. If more of the race is fuelled from the inside, the tank you start with matters more. Carbohydrate loading deserves attention before hot races, not only long ones.
3. Prefer multiple transportable carbohydrates. Glucose–fructose blends have the best evidence for higher oxidation in the heat, though they have not yet been tested against heat acclimation.
4. Gut-train in the heat, not just at home. Rehearse race intake in hot sessions and record gut symptoms. Tolerance in a cool training block does not transfer automatically.
5. Don’t chase the highest number on the label. If oxidation is capped lower, extra grams mostly add gut risk. Find the highest intake an athlete tolerates in the heat, then hold it.
6. Use cooling to protect pace. Pre-cooling, ice and cold fluids reduce heat strain directly; they may also buy back some of the metabolic penalty, though that is untested.
7. Pace the first half conservatively. A heat-driven increase in glycogen use argues for restraint early, so there is something left for the run.

The bottom line

Heat acclimation fixes how hot you get, not how much of your drink you burn. In the best mechanistic data we have, ingested glucose oxidation runs 20–27% lower in the heat, regardless of hydration and regardless of nine days of acclimation. For athletes racing in the Gulf this winter, that shifts the emphasis towards starting glycogen, carbohydrate type, gut training and pacing, rather than simply drinking more.

The open question I most want answered: does a glucose–fructose blend, in heat-acclimated athletes at race intensity, close the gap?

References

• Podlogar T, Gifford RE, Thomas J, et al. Heat acclimation does not alleviate the reduction in exogenous glucose oxidation that occurs when exercising in hot conditions. J Physiol. 2026. doi:10.1113/JP289304
• Mougin L, Horner M, Edwards D, et al. Heat stress impairs exogenous carbohydrate oxidation during prolonged running when maintaining euhydration. J Appl Physiol. 2025;139(6):1436–1446. doi:10.1152/japplphysiol.00873.2025
• Jentjens RLPG, Underwood K, Achten J, et al. Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat. J Appl Physiol. 2006;100(3):807–816. doi:10.1152/japplphysiol.00322.2005
• Salame A, Brown D, Oueijan K, McCullough D. Carbohydrate supplementation for endurance exercise in the heat: a systematic review with practical recommendations. J Int Soc Sports Nutr. 2026;23(1):2669307. doi:10.1080/15502783.2026.2669307

Race context: TRI247, WTCS Abu Dhabi 2026 results · PTO Stats, Qatar T100 World Championship Final