Sweatlabs: Can Heat Adaptation Lower Carb Usage in the Heat?


• In hot conditions, carbohydrate oxidization was 15–19% lower for runners who were heat adapted compared to those who were not heat adapted. Running intensity was 75%–85% of VO2max (reductions were greatest at the lower intensity).
• When running in the heat, the heat adapted runners saw an increase in velocity (4%) at Ventilatory Threshold 1 (this aerobic threshold is where the body transitions from mostly fat utilization to
  increased carbohydrate dependency).
• After 4 weeks of heat adaptation, runners also saw increased plasma volume (4%), increased hemoglobin (2%), and increased testosterone (18%). They also experienced a decrease in core temperature (0.4°C), an increase in sweat rate (21%), an increase in lactate clearance rate
  (19%), and an increase in running economy (6%) when running in the heat.

• 18 well trained (VO2max ~62 mL/min/kg)middle and long-distance runners were split into a control and a heat adaptation group. The control group did normal training and the heat adaptation group did a four-week protocol of 5 heat sessions per week (the intensity of work during each heat session was adjusted to match the RPE of the control group’s session).
• Incremental treadmill tests and running economy tests in the heat were conducted before and after the protocols. Temperatures were 30°C/91°F with high humidity.
• Standard lab methodologies measured physiological metrics. CORE sensors were used to measure core temperature during the heat sessions and treadmill tests.

When running in the heat, the heat adapted group used 36 grams/hour fewer carbohydrates than the control group. With amateur athletes consuming 60–90 g/hr during endurance events, heat adaptation means a massive decrease in the reliance on carbs during hot races.

Hot races are a three-pronged fueling nightmare for the non-heat adapted athlete:

• The body needs more carbs because of the heat.
• Fewer of the ingested carbs are converted to blood glucose because of a hot core temp.
• The digestive system has a lower capacity for digesting fuel because blood is diverted away from the gut to the skin for cooling (this is GI distress!).

The heat adapted athlete better copes with all three of these issues:

• An increase in carbs is not needed.
• The core temp stays cooler, so the efficiency of the digestive system is preserved.
• Less blood is diverted away from the gut to the skin, so more fuel can be tolerated.

Study conditions were very hot and running intensities were equivalent to marathon pace for trained athletes. Cooler conditions and lower intensities would very likely see smaller decreases in carb utilization. 

The underlying mechanism for the decreased carb usage appears to be greater aerobic capacity, as evidenced by the enhancement of the ventilatory thresholds. This allows more efficient delivery of oxygen to the muscles and less need for muscle glycogen. Which all means it is likely that heat training reduces carb usage also in cool conditions and at lower intensities.

The mechanisms that cause increased carbohydrate utilization in hot conditions appear to be well understood. Likewise, it seems apparent why heat adapted athletes do not need additional carbs in hot conditions. 

But a question remains about carb usage in heat adapted athletes during temperate conditions and in lower-intensity settings.  While it appears likely that they would use less than non-heat adapted athletes, more study is needed. 

The studies we would like to see:

• What is the lowest environmental temperature where heat adapted athletes use fewer carbohydrates than non-heat adapted athletes?
• What is the lowest intensity threshold (% of VO2max) where heat adapted athletes use fewer carbohydrates than non-heat adapted athletes? In other words, does this also apply to ultra-endurance runs, or an age-grouper’s Ironman?
• Can a model be developed to predict carb usage based on heat adaptation level, intensity of exercise, and environmental conditions?