Nutrition & Hydration Considerations When Training In The Heat
By Sarah Wiseman, RD, LDN
What physiological factors are affected when training in the heat?
Carbohydrates are broken down at a faster rate in the heat than in cooler temperatures, which will impact your metabolism and muscle function.
Blood volume is increased (thicker blood), which then increases the strain on your heart.
Motor control center of the central nervous system can become sluggish.
Essentially, when our body temperature rises our physiology stops working optimally. If we let our temperature get too high, our body will start to shut down. Optimal training in the heat depends on you keeping your body temperature down.
Dehydration + training in the heat
If you are dehydrated by 2-7% of your body weight (3-10lbs of fluid loss for a 150lb person), you will see a significant decline in performance, particularly if the outdoor temperature is over 86℉.
An increase in body temperature by 1°C OR being 4% dehydrated causes
a reduction in your heart’s stroke volume (amount of blood pumped out of the heart per beat) and an increase in your heart rate. Essentially, your body will automatically have to work harder if either of these scenarios exist, even if you’re running your regular training/racing pace. This will ultimately lead to a poorer performance if not managed properly.
An increase in BOTH body temperature by 1°C AND being 4% dehydration causes:
A decrease in your heart’s stroke volume by over 2x the impact of either the rise in body temperature or dehydration alone
A decrease in the amount of blood your heart can put out per minute
A decrease blood pressure, which makes it harder for your blood to get to your muscles
An increase in the resistance on your vascular system (your blood is thicker and has a harder time moving through your veins and arteries)
All this science-y stuff is all to say that the result of mismanaging your body temperature and hydration will force you to have a harder time running ( or exercising in general) because:
Your heart has to work harder to get blood to your muscles
Your blood has a harder time getting to your heart and muscles because it is thicker