As much as we enjoy anytime we can get on our bikes, some days are a bit tougher than others. Not every ride is at the optimal 68 degrees Fahrenheit. Let’s face it we’ve all walked out of a nice air conditioned room into the searing sun and gritted our teeth thinking “it’s going to be a hot one.” Usually we also do some quick “recalculating” about how far and fast we intend to ride or ride at all.
It takes a bit of “guts” to go ahead clip in and start pedaling when the temperature reaches close to or above triple digits. The heat doesn’t just take some “guts” to ride in, it quickly, literally challenges your guts – intestines.
The higher the temperature gets above 68 the more of a strain it puts on several of your body’s organs, metabolic and physiological systems. Even at rest in a hot environment your heart rate increases and changes in vasomotor tone take place.
When you’re out on the bike in hot weather your body is subjected to heat from three sources: radiant heat from the sun, convection from the road and metabolic heat generated by your working muscles. The effects of this place tremendous strains on your heart to pump the blood necessary to maintain vital organs and the working muscles.
The most critical survival factor when cycling in high temperatures is heat dissipation. As much as 60% of your cardiac output is directed to the peripheral circulation to shed all the heat building up internally. This leaves a remaining 40% that has to support your brain, working muscles and digestives system – stomach, small and large intestines.
Studies have shown that intestinal blood flow can be reduced by 80% of normal. Several significant consequences flow from this dramatic reduction in perfusion.  . This is understandable if you consider it from a survival perspective. When your body is struggling to survive and maintain consciousness, the last thing it’s thinking about is digesting a nice meal.
The most obvious or most common result, experience is that even though we try to drink and fuel according to the best recommendations our intestines are simply not getting sufficient blood flow to support passive or active transport systems. On top of that what does get absorbed is carried away with only 20% of normal blood flow. 
Fluid fills up and food partially digested sits in the gut longer than normal and can contribute to the bloating, cramping or gaseous experience most cyclists have had at one time or another. So despite experimenting with nutritional and hydration products that best suit us, digestion and absorption will most likely be impaired regardless.
The problem with intestinal hypoxia and ischemia can get even more significant. Recently studies have been done demonstrating a heat dependent rate  and level of gut permeability – “leakage” that contributes to increased levels of fatigue, heat-shock-proteins [LPS] and pro-inflammatory cytokines [IL-6].  IL-6 is believed also to be a thermoregulatory sensor in the gut and communicates to the brain. Recently, IL-6 has been implicated in signaling the CNS and influencing perceptions of fatigue and performance during exercise.  It is now theorized that increased levels of LPS and IL-6 signal the Central Nervous System (CNS) to modulate exertional activity in the heat. 
A 2012 study demonstrated that while acclimation reduces the increases in LPS, HSP, and IL-6 levels are still increased while cycling in high temperatures. . Likewise conditioned athletes tend to demonstrate lower GI permeability exercising in the heat than unconditioned subjects. .
In our field study conducted at the University of California Davis 12 of the 13 participants self-reported lower GI distress while cycling in the heat [84-92 d F]. Although the semi-quantitative results were not quite statistically significant under the study conditions it was a notable observation.
Given the results and implications of recent studies as well as the underlying physiological principles we believe that sustainable cooling while cycling in the heat can have an impact – improve splanchnic blood flow indirectly by lowering the circulatory demands of the peripheral circulation and thus improving the percentage of cardiac output directed to the gastrointestinal tract.
Once again, the importance of thermoregulatory systems and assistance as a central – pivotal role in performance, endurance, fatigue, recovery and safety is underscored.
You may or may not have learned something new in the material presented here. But most likely you have not experienced the solution developed to allow for these improvements.
There is now a practical useable system that allows cyclists to keep their skin 15-20+ degrees cooler on the bike and during rides of any length. It’s definitely worth a try.
Spruzza is really…”A Cooler Way to Ride.”
[1 ] http://www.tandfonline.com/doi/full/10.1080/23328940.2016.1179380?src=recsys Heat Stress on GI system and permeability.
 MATTHEW BARBERO GI DISTRESS HEAT CYCLING https://etd.auburn.edu/bitstream/handle/10415/3601/Barberio_Dissertation_FINAL.pdf?sequence=2
 STOMACH PROBLEMS WHILE RIDING http://www.bicycling.com/training/injury-prevention/tips-make-your-belly-happy-every-ride