Ontario FireRangers exhibit high annual injury rates and cite fatigue as a contributor, however several factors influence fatigue, including:​ energy expenditure, energy intake, stress levels and recovery time. The purpose of this research was to evaluate the accuracy of heart rate variability (HRV) based estimates of energy expenditure (kilocalories) and to then assess energy balance, physiological responses, and nutritional quality in Ontario FireRangers during different types of fire deployments. Firstbeat Bodyguard2 and Zephyr BioHarness3 monitors were used to collect HRV data, and individual audio-visual food logs were kept using an iPod Touch and analyzed in NutriBase Pro11 software. Sleep quantity was also measured using actisleep monitors, to assist with energy expenditure calculations. The findings of this research support the use of HRV monitoring for free-living, energy expenditure estimation. Furthermore, this research indicates that Ontario FireRangers exhibit high daily energy demands, under-consume kilocalories, deviate from ideal nutrient consumption profiles and have varying levels of stress and recovery time, depending on deployment type.