Training In Hot Conditions Makes You Eat More?

Just after sunrise

After all the rainy days during the first 2 weeks of September, the temperatures were nice and cool. This past week it seems like "someone turned the tap off" and it's been way hotter. 

I weighed myself yesterday morning before and after my regular Saturday ride before work and found that I had lost 2 kg (mostly water) despite eating and drinking more during the ride. These 3 hour rides, when warmer, make me feel like I'm making my body work double time. I've noticed that when the temperature is higher, I need to eat more (during the ride) or else I cannot keep up with my friends.

On Saturday rides that are cooler, I sometimes don't even eat the banana that I bring with me and can still ride well without tiring.

Turns out that training in Singapore's heat and humidity does not just make one more comfortable in hot conditions, it actually changes how our muscles use energy (Xu et al, 2025).

The authors in the study quoted above show that a month of structured heat acclimation training can help change your metabolism to race stronger when it's hot and probably in cooler temperatures too.

The researchers split 18 trained middle and long distance runners into 2 groups. Both groups completed treadmill tests and metabolic assessments at temperatures of 30-32 °C (or 86-89 °F).

The first group trained in normal temperatures (20-25 °Celsius or 68-77 °F). The other group did 20 sessions of heat acclimation over 4 weeks. They ran in tem[eratures between 30-36 °C that pushed their core temperatures to 39-40 °C (or 102-104 °F).

No prizes for guessing that the heat acclimated group made more notable adaptations compared to the control group. Their core temperature during the subsequent treadmill test was lower by 0.4 °C, a clear sign of improved thermoregulation. They were also more efficient aerobically, as oxygen uptake improved by 4 and 3.7 percent at the *first and second ventilatory thresholds.

Plasma volume improved by 4 percent, haemoglobin by 2 percent and erythropoietin by 13 percent after heat acclimation, showing better oxygen support. All signs of heat adaptation.

At submaximal intensities (75 and 85 percent VO2 max, carbohydrate oxidation dropped by 15-19 percent in the heat acclimated group. The runners used less blood glucose and muscle glycogen, using more of the body fat instead. Exactly like I wrote above when I do not need to eat as much during cooler rides. In the picture above, carbohydrate (CHO) use and energy expenditure (EE) before and after control (black bars) and heat training (red).

Upon finishing the treadmill test, the heat acclimated runners cleared lactate more efficiently demonstrating improved recovery.

However, VO2 max did not change, suggesting that having a higher VO2 max (compared to someone else) may not mean you will race faster. The efficiency gains were more beneficial.

Take home message? Those of us who live in sunny and super humid Singapore can train almost all year round in such conditions. A 4 week block of 5 days a week may give you the metabolic edge of using less carbohydrates at higher intensities. This spares your glycogen stores and delays fatigue. The thermoregulatory and blood adaptations will help you stay cooler and deliver oxygen more effectively, especially if you are going to be racing in a friendlier climate. 

You will need to monitor the above parameters and allow for adequate recovery to get the benefits. This is when your blood, sweat (lots of it) and tears will pay off if you do it correctly.

Reference

Xu Y, Ye C, Ma S et al (2025). Four-Week Heat Acclimation Lowers Carbohydrate Oxidation Of Trained Runners During Submaximal Exercise In The Heat. Frontal Physiol. DOI: 10.3389/fphys.2025.1581594/full

*First ventilatory threshold (VT1) is when your breathing becomes noticeably deeper and lactate starts to accumulate in the blood. This indicates a shift from using mostly fat to carbohydrate during exercise.

VT2 or second ventilatory threshold occurs later, this is often when breathing is rapid, more labored leading to a sharp increase in carbon dioxide production. This is when a significant shift to anaerobic metabolism.

Still not too hot