Reed W. Hoyt

This paper describes the use of a data-driven autoregressive integrated moving average model to predict body core temperature in humans during physical activity. We also propose a bootstrap technique to provide a measure of reliability of such predictions in the form of prediction intervals. We investigate the model's predictive capabilities and associated reliability using two distinct datasets, both obtained in the field under different environmental conditions. One dataset is used to develop the model, and the other one, containing an example of heat illness, is used to test the model. We demonstrate that accurate and reliable predictions of an extreme core temperature value of 39.5 degC, can be made 20 minutes ahead of time, even when the predictive model is developed on a different individual having core temperatures within healthy physiological limits. This result suggests that data-driven models can be made portable across different core temperature levels and across different individuals. Also, we show that the bootstrap prediction intervals cover the actual core temperature, and that they exhibit intuitively expected behavior as a function of the prediction horizon and the core temperature variability.

The increase in obesity in developed societies drives interest in the interplay of energy intake, metabolic energy expenditure, and body energy stores. A better understanding of energy management in physically active and undernourished humans should help guide strategies to manage obesity safely and effectively. This review focuses on field studies of men and women engaged in prolonged strenuous activities, ranging from ranger training to extreme expeditions. Although scientifically unconventional and limited, field studies of exercise and food deprivation have yielded interesting findings: 4-5% body fat is the normal lower limit to fat reserves in physically active underfed young adult men, and in response to exercise and underfeeding, women used more fat mass and less fat-free mass to meet metabolic fuel requirements. Field studies have shown that fat energy reserves in young adult men can be estimated as percentage body fat minus 5%, and initial body fat mass has a significant positive influence on fat oxidation rates per kilogram of fat-free mass during rapid weight loss associated with underfeeding and exercise. Data logging pedometers, activity monitors, global positioning systems, and wireless body and personal-area networks promise to make it easier to study and care for free-living humans.

Background:A challenging 7-d ranger field exercise (FEX) by cadets in the Norwegian Military Academy provided a venue in which to study the effects of negative energy balance. Objective:We quantified total energy expenditure (TEE), food intake, and changes in body composition in male and female cadets. Design:TEE (measured by doubly labeled water), food intake, activity patterns (measured by accelerometry), and body composition (measured by dual-energy X-ray absorptiometry) were measured in 16 cadets (10 men and 6 women aged 21–27 y). Results:The physically active (≈23 h/d) and semistarved (0.2–2.2 MJ/d) cadets lost weight (x̄ ± SD: men, −7.7 ± 1.1 kg; women, −5.9 ± 1.1 kg; P < 0.05). Absolute TEE differed by sex (men, 26.6 ± 2.0 MJ/d; women, 21.9 ± 2.0 MJ/d; P < 0.05) but body weight–specific TEE did not (men, 343 ± 26 kJ · kg⁻¹ · d⁻¹; women, 354 ± 18 kJ · kg⁻¹ · d⁻¹; NS). Fat-free mass (FFM) loss differed significantly by sex (men, −4.0 ± 1.2 kg; women, −2.5 ± 1.1 kg; P < 0.05), but percentage FFM loss did not (men, −6.3 ± 1.9%; women, −5.6 ± 2.4%). In contrast, absolute FM loss did not differ significantly by sex (men, −3.45 ± 0.72 kg; women, −3.42 ± 0.22 kg), but fat oxidation (men, 5.2 ± 1.0 mg · min⁻¹ · kg FFM⁻¹; women, 7.3 ± 0.5 mg · min⁻¹ · kg FFM⁻¹) and the relative contribution of FM to TEE (men, 74 ± 14%; women, 89 ± 6%) were significantly greater in women than in men (P < 0.05). Conclusion:Female cadets maintained a significantly more fat-predominant fuel metabolism than did male cadets in response to sustained exercise and semistarvation.

Total energy expenditure (TEE) assessment using pedometers provide an easy and less expensive method than doubly labeled water (DLW). This study assessed TEE by a new pedometry method (TEEpedo) compared to the doubly labeled water method (TEEdlw). Shipboard sailors (7 men, age: 23.0+/-3.9 yrs; ht: 180.2+/-6.5 cm; wt: 83.8+/-11.8 kg, and 10 women, age: 24.7+/-4.4 yrs; ht: 165.2+/-8.0 cm; wt: 63.5+/-14.0 kg) (Mean +/-SD) were studied for 8 days. The energy cost of activity was estimated using (a) total body weight, (b) foot-ground contact times [Tc] during running, walking, and non-exercise activity [NEAT], and (c) the known proportion of time spent in each activity category. Resting metabolic rate (RMR) was estimated from lean body mass. TEEPEDO was calculated as: TEEpedo (MJ) = (1440 x [%Run Time x ((0.0761 x [Total Body Weight/TcRun]) - 7.598) +%Walk Time x ((0.056 x [Total Body Weight/TcWalk]) - 2.938) + (%NEAT Time x 0.1 x [RMR/Minute])] + RMR)/239. This method, explained 79% of the variance of TEEpedo with a 95% confidence interval of +/-0.81 MJ/day, relative to TEEdlw (12.55+/-3.3MJ/day). Mean TEEpedo (12.65+/-3.1 MJ/day) did not differ from mean TEEdlw (p=0.95). At TEEs >14 MJ/day, the TEEpedo method underestimated actual TEE, possibly due to unaccounted for upper body exercise. At more moderate TEEs of 9 to 14 MJ/day, the Tc pedometry method provided accurate estimates of TEE.

Energy balance and thermal status was assessed in 14 male U.S. Marine Corps volunteers (age = 25 +/- 3 y MEAN +/- SD; ht = 178 +/- 5 Cm; wt = 81.0 +/- 4.1 kg; %body fat = 15 +/- 3; load = 42.6 +/- 3.2 kg) during a 10-day field exercise (FEX) at Quantico, VA. Question: Does intense physical activity, limited sleep, and restricted rations, combined with cold/damp weather, result in excessively negative energy balance and hypothermia? Methods: Total daily energy expenditure and water turnover were measured by doubly labeled water; daily food intake was assessed by ration wrapper collection; daily activity logs were maintained; weather data were collected with an automated weather station. Each volunteer wore wearable sensors that collected body core temperature, heart rate, activity patterns, and geolocation. Results: energy expenditure = 5378 +/- 678 kcal/d (22.51 +/- 2.84 MJ/d); energy intake 1333 +/- 196 kcal/d (5.58 +/- 0.82 MJ/d); water turnover = 3.23 +/- 0.38 L/d. Carbohydrate intake was inadequate at 168 +/- 30 g/man/d. Rapid decreases in core temperature (approx. 0.8 0C/h) were sometimes evident during inactivity/sleep. Conclusions: (1) one MRE ration per day provided inadequate carbohydrate - an additional 225 g/d of carbohydrate was needed to meet the minimum 400 g/d requirement, and (2) additional sleeping gear is needed to avoid rapid decreases in core temperature and sleep disruption.

In January and February, 1998, fifty Marine Recruits (thirty men, twenty women) were studied during the Crucible, a 54 hour training exercise which culminates Marine Corps Recruit Basic Training. The purpose of this study was to measure body core temperature and energy expenditure during the Crucible to: 1) assess the risk of hypothermia and 2) determine if ration restrictions should be eased. Core temperature was measured with a lightweight (3 oz) Body Core Temperature Monitor (BCTM) and an FDA-approved ingestible temperature sensor. Energy expenditure was measured using the doubly labeled water (D2180) technique. Energy intake was estimated by collecting Meal Ready to Eat (MRE) ration wrappers saved by each volunteer during the exercise. Weather data were obtained from portable stations. Results: 1) mean maximum core temperature was 38.51 deg C (range, 38.15-39.14 deg C); 2) mean minimum core temperature was 35.97 deg C (range, 35.28-36.64 deg C); 3) mean energy expenditure (kcal) over 54-h was 13,790 plus or minus 410 (6128 kcal/day) and 10,635 plus or minus 315 (4727 kcal/day) for the men and women, respectively; 4) mean energy intake for 54-h was 3,200 and 2,600 kcals for the men and women, respectively; 5) mean air temperature was 10.8 deg C (range, 3.6-18.8 deg C) in January and 13.8 deg C (range, 6.3-21.4 deg C) in February. In conclusion, mean core temperature did not fall to hypothermic levels (<35.5 deg C) and the energy deficit was approximately 10,100 kcal in the men and 7,700 kcal in the women.