Heat Balance Considerations when Estimating the Rate of Body Heat Storage

Dear Editors

Altareki et al. recently published an article in the International Journal of Sports Medicine [1] suggesting that afferent feedback arising from changes in the rate of body heat storage (S) during the early stages of exercise may mediate performance decrements during a self-paced 4-km cycling time trial in a HOT (35°C) relative to a COOL (13°C) environment so that thermal homeostasis is preserved in both conditions. The primary evidence for this conclusion was a slower HOT time trial (by 7.3 s), coupled with large differences in thermometrically estimated values of S between HOT and COOL conditions after the first kilometer, and a tendency for this difference in S between conditions to subsequently decline during the final kilometer. However when viewing their data from an alternative viewpoint, a different conclusion becomes apparent.

By definition, rate of body heat storage (S) at any point is the difference between the rates of metabolic heat production (M-W) and net heat loss by combined evaporative and dry (radiation, conduction/convection) heat exchange. However, the authors [1] estimated S using a traditional thermometry approach, calculating time-dependent changes in body heat content which itself was estimated using the product of body mass (m _b ), an estimated average specific heat of 3.48 kJ·kg⁻¹·°C⁻¹ and mean body temperature (T _b ) derived from the 2-compartment thermometry model of “core” (T _es ) and “shell” (T _sk ) [2]. The relative contribution of each compartment was determined by fixed sum-to-one weighting coefficients. Since m _b remained constant throughout, changes in S were solely dependent on time-dependent changes in T _b , or more specifically T _es and/or T _sk . While this method for estimating S is understandable considering its widespread use in thermophysiological studies, it has been repeatedly shown to yield erroneous values particularly during non steady-state body temperature conditions [3] [4] [8] [10] and can thus not be considered a reliable tool for determining S as a central mechanistic component during exercise. Such inaccuracies are evident in the present study [1]. A primary example being the S of −200 Wm⁻² reported after 1-km; an estimation clearly influenced by early changes in T _sk and not possibly representative of whole-body heat storage. According to VO₂ data (RQ estimated to be 0.85 since the reported RER values (∼1.10) indicate non steady-state hyperventilation) and external work rates presented, such a value for S would require a rate of total heat dissipation of ∼700 Wm⁻² (M-W: ∼500 Wm⁻²). However, at 13°C with an air velocity of 5.6 ms⁻¹ rate of dry heat loss would only be ∼260 Wm⁻² [3] accompanied by minimal sweating since T _es was only elevated by ∼0.1°C. Indeed, the original article citing evidence of an anticipatory reduction in exercise performance arising from early changes in S [9] has been criticized for basing their conclusions on early heat storage estimations that are similarly flawed [6].

A more accurate assessment of the actual rate of body heat storage can be derived from other data reported by Altareki et al. [1]. The rate of metabolic energy expenditure (M) was similar throughout cycling between COOL (650 Wm⁻²) and HOT (680 Wm⁻²) conditions. Correspondingly, the rate of external work (W) was ∼30 to 10 Wm⁻² greater in the COOL condition throughout, giving an almost identical rate of metabolic energy expenditure (M-W) in both environmental conditions. According to changes in nude body mass, evaporation was not different. Therefore the only source of difference in the rate of body heat storage would be a function of dry heat loss. It follows that the difference between HOT and COOL dry heat loss was 240 Wm⁻² after 1-km and 205 Wm⁻² after 4-km [3], thus demonstrating that the difference in the actual rate of body heat storage between environmental conditions was very similar from the beginning to the end of cycling (supported by the progressively greater T _es in the HOT relative to COOL condition). Any changes in self-paced exercise intensity elicited by a greater rate of body heat storage during the first kilometer in the HOT condition were therefore hardly sufficient to alter heat balance and certainly did not preserve thermal homeostasis. An alternative conclusion is that the modest decrements in cycling performance observed in the heat by Altareki et al. [1] were due to an incrementally greater level of hyperthermia-induced central fatigue [7].

Ollie Jay

References

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