Hydration for the Heat: Part 1

Published by ashthomo nutrition
on Aug 11, 2020


Perfecting your nutrition for an Ironman is a series of mathematical equations and questions that goes a little something like this; how long am I going to be out on course for, am I going to use liquid or food for nutrition, how am I going to fit my fuel and hydration on the bike/ take it with me on the run, what brand of hydration or food is better for me to use, how do I fuel to prevent gut issues and don’t even get me started on preventing cramping. All of these questions thrown at you in order to tailor your food intake is exhausting, then Kona throws another huge contributing factor in, its stinking hot!

This article will help you debunk and understand the biggest concerns when competing in the heat. 

Thy physiology of what happens when we add heat!

An increase in ambient temperature increases the bodies reliance on fluid and energy needs. Physiological response to heat alters our circulatory system, endocrine system and the ability to thermoregulate body temperature. These systems effect our ability to maintain blood pressure, mental acuity, maintain muscular function and regulate blood plasma volume. 

When we exercise the circulatory system constricts vascular supply to organs such as the kidneys, stomach and gut and prioritises blood flow to the active tissues e.g. skeletal muscles. During extreme heat, such as Kona, our body cannot meet the demands of proving sufficient blood flow to the skin and muscles to provide an adequate cooling effect. Maintaining blood pressure will take precedence over blood supply to the skin and muscles which will increase the risk of hyperthermia and metabolic disturbances (Increased heart rate, vasoconstriction of blood flow to the skin and muscles, Increased lactate level, decreased oxygen delivery, increase muscle temperature), ultimately decreasing performance capacity and time till fatigue (  

In very hot conditions the most effective dissipation of heat is through evaporation (98%) e.g. sweating, as other mechanisms are compromised. Any disturbances in the sweating response e.g. high humidity or dehydration can result in a 7% decrease in athletic performance (5). This highlights the critical importance of fluid availability in hot environments, while also taking in nutrition that is easily digested by the gut since blood flow to internal organs is compromised. 

Why most sports drinks are not assisting hydration and fluid availability

Major sports hydration companies have got caught up in their products being more about palatability and energy availability rather than assisting you with what they say they do, hydrate!

For fluid to pass from the gut into the bloodstream it needs to be less concentrated than our blood, I.e lower osmolarity (less than 270-300mOsm) (6). You also want it to have substances to help fluid transport into the cells e.g sodium, sucrose and glucose. Most sports drinks have 5-8% carbohydrate often containing maltodextrin and fructose and an osmolarity of 300mOsm (7). This combination requires us to draw water into the gut to dilute the substance before it enters our blood, causing GI stress and a drop in blood plasma volume which could lead to dehydration.

if you want to stay hydrated have a liquid with only 2-4% carbohydrate made of sucrose and glucose/ dextrose, around 200mg of potassium, 300mg of sodium and some calcium carbonate (8). 

Ash is the current Dietitian for CF racing, The Endurance Republik and The Lone Wolf Squad.


1.      Burton AC. The clinical importance of the physiology of temperature regulation. Can Med Assoc J. 1956;75(9):715–20.

2.     Kenny GP, Flouris AD. The human thermoregulatory system and its response to thermal stress. In: Wang F, Gao C, editors. Protective clothing: managing thermal stress. Cambridge: Woodhead Publishing Limited; 2014.

3.     Hardy JD. Physiology of temperature regulation. Physiol Rev. 1961;41:521–606.

4.     Benzinger TH. Heat regulation: homeostasis of central temperature in man. Physiol Rev. 1969;49(4):671–759.

5.     Sawka, MN. Cheuvront, SN. Kenefick, RW. 2011, High skin temperature and hypohydration impair aerobic performance, Available at:


6.     Faubel S, Topf J. The fluid, electrolyte, and acid-base companion, San Diego, CA: Available at: http://www.pbfluids.com/2009/10/fluidelectrolyte-and-acidbase.html1999

7.      Fluid osmolarity of sports drinks Ishijima T., Suzuki K., Hashimoto H., Higuchi M. (2011)  Chocolate, Fast Foods and Sweeteners: Consumption and Health, , pp. 213229. 

8.     David R. Lamb, Fluid Replacement and Heat Stress.  Institute of Medicine (US) Committee on Military Nutrition Research; Marriott BM, editor. Washington (DC): National Academies Press (US); 1994.


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Published by ashthomo nutrition
on Aug 11, 2020

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