Jorge Granados

Performing loaded work in hot ambient environments is a common challenge for military soldiers. Blood delivery to the skeletal muscles in the attempt to maintain body temperature within certain boundaries place simultaneous demands on the volume of blood being pumped by the heart, also known as cardiac output (Q). Competition between these processes leads to significant cardiovascular strain and reduced blood flow to the intestinal tract. This results in increased risk of exertional heat illness (EHI).

Given that EHI is driven by a lack of Q, and given that Q is prioritized to deliver oxygen to active skeletal muscles, we theorized that if oxygen (O₂) delivery to the act of skeletal muscle could be increased independent of an increase in Q, then both EHI and EHI – associated symptoms could be reduced. As such, this research question has particular significance for US military personnel.

Beetroot juice (BR) contains inorganic nitrate (NO₃^-) and has been shown to improve cardiovascular endurance. These benefits are meditated by improvements in the efficiency of mitochondrial respiration in type two muscle fibers and translate into reduced oxygen (O₂) consumption during submaximal exercise. BR has also been shown to improve exercise performed in hypoxic conditions.

The study’s objective was to determine whether it's physiologically relevant dose of     a concentrated BR  supplement (8.4 mmol NO₃^-)  reduce  the  physiological strain associated with performing a 45 minute military battle march in a hot environment   and subjects were wearing a hypoxic simulation mask and full gear to simulate   military conditions.

Six males (25.9±3.4 yrs; 11.8±3.2% body fat; VO_2peak 51.6±3.9ml/kg/min) completed the randomized double-blind, placebo-controlled, crossover study. BR (8.4 mmol NO₃^-/140 ml) or placebo (NO₃^- depleted BR) was ingested for six days, separated by a one-week washout. On the sixth supplementation day, they performed a treadmill military march (45 minutes, 3.1 mph, 1% grade) in a hot environment (40°C, 30% RH), while wearing a hypoxia simulation mask (altitude 2,743 m) in full military gear. Prior to exercise, subjects underwent passive heating in a hot tub (40°C) to increase core temperature (T_C) to 38.5°C.

During each trial, the subjects T_C, skin temperature (T_SK), heart rate (HR), oxygen consumption (VO₂), carbon dioxide production (VCO₂), ventilation (V_E), tidal volume (V_T), and respiratory rate (RR) were monitored every 10 seconds. Ratings of perceived exertion (RPE), thermal comfort (THC), overall comfort (OAC), and oxygen saturation were measured every five minutes.

Subjects who performed the military march under the BR condition significantly increased their core temperature, skin temperature, heart rate, resting of perceived exertion, and thermal comfort values over PL. All other measured variables were not significantly altered.

Contrary to expectations, the preliminary data suggests that BR supplementation does not reduce cardiovascular strength associated with a 45-minute military march in hot/hypoxic environments, but may increase the risk of suffering EHI by increasing T_C T_SK and HR.

Suggested Further Readings

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2.    Gonzales-Alonso J, Crandall CG, Johnson JM. The cardiovascular challenge of exercising in the heat. J Physiol. 2008; 586(1):45-53.

3.    Dokladny K, Moseley PL, Ma TY. Physiologically relevant increase in temperature causes an increase in intestinal epithelial tight junction permeability. Am J Physiol Gastrointest Liver Physiol. 2006; 290(2):G204-12.

4.    Kuennen M, Gillum T, Dokladny K, Bedrick E, Schneider S, Moseley P. Thermotolerance and heat acclimation may share a common mechanism in humans. Am J Physiol Regul Integr Comp Physiol 2011; 301:524-33.

5.    Lim CL, Mckinnon LT. The roles of exercise-induced immune system disturbances in the pathology of heat stroke: the dual pathway model of heat stroke. Sports Med. 2006; 36(1):39-64.

6.    Bedno SA, Li Y, Han W, Cowan DN, Scott CT, Cavicchia MA, Niebuhr DW. Exertional heat illness among overweight U.S. Army recruits in basic training. Aviat space Environ Med. 2010;81(2):107-11.

7.    Vanhatalo A, Bailey SJ< Blackwell JR, DiMenna FJ, Pavey TG, Wilkerson DP, Benjamin N, Winyard PG, Jones AM. Acute and chronic effects of dietary nitrate supplementation on blood pressure and physiological response to moderate-intensity and incremental exercise. Am J Physiol Regul Integr Comp Physiol. 2010; 299:1121-31.

8.    Ferguson SK, Hirai DM, Copp SW, Holdsworth CT, Allen JD, Jones AM, Musch TI, Poole DC. Impact on dietary nitrate supplementation via beetroot juice on exercising muscular vascular control in rats. J Physiol 2012; 591:547-557.

9.    Wylie LJ, Kelly J, Bailey SJ, Blackwell JR, Skiba PF, Winyard PG, Jeukendrup AE, Vantahalo A, Jones AM. Beetroot juice and exercise: pharmacodynamics and dose-response relationships. J Appl Physiol. 2013; 372: 1152-73.