Beat the Heat at the Tour Down Under: Why You’re Burning Energy Faster

Beat the Heat at the Tour Down Under: Why You’re Burning Energy Faster Than You Think

If you’re heading to Adelaide to chase the action this January, riding between stages, climbing the same hills, or rolling out to watch the race, you’re stepping straight into one of the hottest cycling environments of the year. That heat doesn’t just make you sweat; it rapidly drains your glycogen, slows carb absorption, and sets you up for a spectacular bonk if you’re not prepared. Understanding how the heat destroys your energy levels is the key to staying strong on those scorching South Australian roads.

How Does Heat Make You Burn Through Glycogen Faster?
Why Does Heat Reduce How Well You Absorb & Use Carbs?
How Does Relying On Stored Carbs Increase Bonking Risk?
How Does Dehydration Speed Up Fatigue & Carb Use?
How Does Low Fluid Intake Affect Cortisol & Stress Levels?
Summary: How Adelaide’s Heat Impacts Cycling Performance (and Increases Bonking Risk)

How Does Heat Make You Burn Through Glycogen Faster?

Exercising in hot conditions increases whole-body carbohydrate use and speeds up muscle glycogen breakdown compared with cooler environments, partly because more blood is diverted to the skin to cool you down, leaving less oxygen for working muscles and shifting metabolism toward glycogen over fat¹⁻⁵.
More blood is diverted to the skin to help you cool down, leaving less oxygen available for your working muscles. This shifts your metabolism toward using glycogen instead of fat³.
Even when hydration is maintained, the rise in core and muscle temperature alone increases the rate at which your body burns through glycogen⁶,⁷.

Why Does Heat Reduce How Well You Absorb & Use Carbs?

Heat stress slows the rate at which your gut can absorb and oxidise ingested carbohydrates, reducing the amount of fuel coming from what you eat¹,².
More blood is redirected to the skin for cooling, leaving less blood flow to the gut, this limits carbohydrate absorption and increases gut discomfort or slowing³.
Reduced blood flow and higher gut temperature can impair gastric emptying, meaning carbs move more slowly from the stomach to the intestine³,⁴.
Heat raises stress hormones like adrenaline, which increases reliance on stored glycogen and decreases the body’s use of ingested carbohydrate⁵,⁸.

How Does Relying On Stored Carbs Increase Bonking Risk?

You burn through muscle glycogen faster in the heat, so your “fuel tank” empties sooner than it would in cooler conditions¹,².
Because your body can’t use ingested carbs as efficiently, you can’t top up your energy stores quickly enough to match the higher burn rate²,⁵.
Once glycogen drops below a critical level, your brain and muscles struggle to get enough energy, leading to the classic “bonk” of sudden fatigue, dizziness, or loss of power¹,²,⁴.
Heat stress also raises fatigue-related hormones and increases cardiovascular strain, making the effects of low glycogen hit harder and earlier in your session or race⁵,⁸.

How Does Dehydration Speed Up Fatigue & Carb Use?

Dehydration reduces blood volume, which lowers muscle blood flow and oxygen delivery, forcing your body to rely more on glycogen for energy¹,³.
Lower blood flow also limits heat dissipation, causing higher core temperatures and accelerating fatigue¹,³,⁴.
Dehydration increases cardiovascular strain, making the heart work harder and muscles tire faster, even at the same exercise intensity¹,³,⁴.
Reduced plasma volume impairs the delivery of nutrients, including glucose, to muscles, further increasing glycogen use¹,³,⁴.

How Does Low Fluid Intake Affect Cortisol & Stress Levels?

Habitually low fluid intake increases stress-hormone reactivity (cortisol and catecholamines) during exercise⁵,⁸.
Elevated stress hormones accelerate glycogen breakdown, contributing to earlier fatigue and higher carb use⁵,⁸.
Chronic suboptimal hydration may influence long-term hormonal balance and overall stress responses, potentially affecting performance and recovery⁵,⁸.

Summary: How Adelaide’s Heat Impacts Cycling Performance (and Increases Bonking Risk)

Adelaide’s summer heat dramatically speeds up glycogen burn, reduces carb absorption, and spikes stress hormones, making bonking far more likely for anyone riding between stages or following the race. Because the body relies on 20-25% more stored glycogen in the heat, riders should increase carb intake the day before, prioritise high-carb recovery meals after every ride, and fuel more consistently across the day to ensure they start each session fully topped up. In hot conditions, replacing what you burn becomes even more important than in cooler weather, making strategic pre- and post-ride nutrition essential for staying strong on the bike.

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

References

Mougin L, Macrae HZ, Taylor L, et al. The effect of heat stress and dehydration on carbohydrate use during endurance exercise: A systematic review and meta-analysis. Sports Med. 2025;55:2825–2847. doi:10.1007/s40279-025-02294-3
Jentjens RL, Wagenmakers AJ, Jeukendrup AE. Heat stress increases muscle glycogen oxidation but reduces oxidation of ingested carbohydrates during prolonged exercise. J Appl Physiol. 2002;92:1562–1572. doi:10.1152/japplphysiol.00482.2001
Périard JD, Racinais S, Sawka MN. Adaptations and mechanisms of human heat acclimation. Scand J Med Sci Sports. 2015;25(S1):20–38.
Nybo L. Hyperthermia and fatigue. J Appl Physiol. 2008;104:871–878.
Tatterson AJ, Hahn AG, Martini DT, Febbraio MA. Effects of heat stress on physiological responses and exercise performance. J Appl Physiol. 2000;89:1150–1156.
Hargreaves M, Angus D, Howlett K, Conus NM, Febbraio M. Effect of heat stress on muscle metabolism during exercise. Acta Physiol Scand. 1996;158:365–371.
González-Alonso J, Teller C, Andersen SL, Jensen FB, Hyldig T, Nielsen B. Influence of body temperature on the development of fatigue during prolonged exercise in the heat. J Appl Physiol. 2005;98:1032–1039.
Armstrong LE, Ganio MS, Casa DJ, Lee EC, McDermott BP, Klau JF, et al. Mild dehydration increases cortisol and catecholamine responses to exercise. J Appl Physiol. 2012;112:119–126.

Disclaimer:

The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.