Cycling: Heat and Nausea

Hey Mike, When I do centuries and it soars up to around 90 degrees, I start to feel ill. Sometimes it’s nausea, and sometimes there’s that odd feeling like I’ve inhaled a little water. Knowing this happens, the last time I rode a century in such heat I sipped ice water during our lunch break. First half of the ride felt easy, but it was so hot that I experienced that nausea the last 60 miles (of 115). Is it possible that I drank more carbohdrate solution than my gut could handle because it was so hot and I was drinking more than usual? Thoughts? Thanks, Jer

Hey Jerry, it sounds to me like your gut is having difficulty absorbing the fluids you are ingesting. At rest, our stomachs typically work efficiently with little distress, processing a variety of foods and liquids. Most of us lose this luxury when exercising.  Working muscles require a significant supply of blood to shuttle in nutrients and remove waste. As circulatory resources are diverted to muscles, less is available for other bodily functions without immediate need, such as digestion. Heat compounds this effect.¹ 


What is the largest organ of the body?

The skin. The skin is largely responsible for keeping you alive when it is 90 degrees out by effectively dissipating heat via sweating. As such, when circulatory demands are prioritized to working muscles and cooling, very little is left for gastric function. Therefore, when exercising in the heat it is important to supply the stomach with only the nutrients that are most important for survival and performance. Water tops the list, followed by an energy supply and in cases of longer duration exercise, electrolytes.² 

Optimize absorption and reduce the risk of gastric distress:

Pre-hydrate. If it’s going to be hot, hit the fluids early. Have a cold bottle of water or sports drink nearby for at least an hour pre-ride.  I would suggest at least 18 ounces of liquid before even getting onto your bike.

Dial back the osmolarity of your drink. The carbohydrate concentration, electrolyte content and any other additives all increase the osmolarity of a sports drink. Osmolarity is the number of particles in a solution, and the greater the osmolar concentration, the slower the gut will empty, which leads to intestinal distress.³  Most sports drinks are formulated with a 7 to 8% carbohydrate solution, which for most individuals is optimal for balancing energy demands and digestibility during exercise. However, if you are having gut issues, you should consider reducing this concentration to around 4%. A 4% solution will still offer a sufficient supply of calories, especially if your consumption increases in the heat.  What else does your sports drink contain? Protein, fat, bok choy? Although, all these things may be good for you, they will all cause an additional delay in gastric emptying so you may want to consider consuming them at another time when your gut is actually working. 


Mike’s Mix Sticker

Electrolytes. Electrolytes have been heavily touted by the sports market industry and many athletes believe that more is better (still waiting on some research to substantiate this). Electrolytes increase the osmolarity of a sports drink. If you are experiencing gastric distress then it is important to take a conservative approach to electrolyte replacement. A large bike water bottle holds 24 ounces. Start with 300 mg of Na (Sodium) as a base line.

Related Post: Gatorade, Galactose and Hydration

Use a sports drink with more than one carbohydrate source. Different carbohydrates have different molecular structures and possess different pathways for leaving the gut. Most athletes find a combination or variety of carbohydrate sources are better tolerated by the gut. Look for a sports drink that contains at least a couple of different carbohydrates: dextrose, maltodextrin, sucrose and fructose are all viable options. It should be noted that beverages that solely contain fructose, or have a high concentration of this sugar, are more likely to cause GI distress. However, a little fructose is good (research has demonstrated a competitive advantage for ultra-distance athletes when consuming  fructose containing drinks as it is preferentially processed by the liver and offers another mechanism for energy supply). So, look for a sports drink with fructose as the last ingredient or one that contains sucrose which dissociates into glucose and fructose.(4)

Related Post: Customize the amount of carbohydrate in your sports drink

Train your gut. Find a sports drink you like and stick with it. Much like you train your muscles and heart with persistent physical workouts, train your gut with frequent drinking during your workouts. Develop confidence and familiarity with the beverage of your choice and bring your own drinks to events as what is being provided is most likely at least somewhat different than what you have trained with. 

Shameless solicitation:

You know who makes a good simple and tasty sports drink powder? Mike’s Mix!

Having gut issues? Try mixing your Mike’s Mix Sports Drink at a 3.5% carbohydrate solution (one scoop per quart of water). This provides 250 mg of sodium. If your gut handles this without issue, you can try to increase the concentration.

Interested in putting more work into your hydration? 

Mike’s Mix sells the finest sports grade Tapioca Maltodextrin and Tapioca Dextrose. Although I don’t sell sea salt, it is cheap, available at any grocer and is nearly identical in electrolyte composition to what is lost through sweat. The same can be said for sucrose (table sugar), which will provide a source of fructose (sucrose is composed of one molecule of glucose and one of fructose).

Please leave comments with questions, concerns or even ridicule. Let’s me know folks are reading these posts.  


1 comment

  • Good read Mike. I hadn’t thought that much into the osmolarity part before but usually filled 1 of my bike bottles with fairly good strength sports drink and the other with ice water so I could swap back and forth depending on what I was feeling during different parts of the ride and how hard I bonked going up the last hill.


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