Abstract
This physiology study assesses whole-body heat loss in physically active middle-aged and older men with vs without type 2 diabetes after aerobic cycling to evaluate whether type 2 diabetes impairs heat loss and by what mechanism.
Performing exercise in the heat can increase the risk of health complications, especially among middle-aged and older adults who have impaired whole-body heat loss (WBHL) relative to young adults.1 That risk may be higher among patients with type 2 diabetes due to abnormalities in cutaneous vasodilation and sweating, which facilitate WBHL.2 However, repeated brief exercise for 7 days or more during heat exposure (heat acclimation) may mitigate this risk by enhancing WBHL.3 We therefore assessed whether type 2 diabetes impairs heat loss in physically active middle-aged and older adults during exercise heat stress and whether heat acclimation could offset any impairment.
Methods
Following written informed consent, physically active men who performed 150 or more minutes per week of moderate exercise; were aged 50 through 70 years; lived in the Ottawa, Ontario, Canada, region; and had or did not have type 2 diabetes volunteered for the study, which was approved by the University of Ottawa Ethics Board and ran from April 2014 to May 2018. Only patients with well-controlled type 2 diabetes who had been diagnosed for 5 years or longer, had hemoglobin A1c concentrations that ranged from 5.5% to 9.0%, and had no diabetes-related complications were included. Participants completed three 30-minute bouts of cycling at increasing metabolic heat productions (150, 200, or 250 W/m2) in dry heat (40°C, 15% relative humidity) within a direct air calorimeter (a device to precisely measure WBHL).4 Participants were invited to repeat this protocol after a 7-day heat acclimation involving exercise (50% of V̇o2peak [peak aerobic power]) for 90 minutes per day in similar conditions.3
Rates (W/m2) of WBHL (dry + evaporative heat loss) and body heat storage (metabolic heat production) as well as heart rate (expressed as percent heart rate reserve) were measured continuously and expressed as peak responses (mean of the final 5 minutes of exercise at the highest metabolic heat production [250 W/m2]). The change in body heat storage (kJ) was used to derive the change in mean body temperature.4 Data were compared between groups using unpaired t tests and within each group using paired t tests (α = .05 [2-tailed]; Prism 8, GraphPad).
Results
Of the 34 participants, 17 had type 2 diabetes and 17 did not. Of the 34 participants, 8 with and 10 without type 2 diabetes completed heat acclimation. No statistically significant between-group differences in physical characteristics were observed (Table 1). Overall, peak WBHL was statistically significantly lower in patients with type 2 diabetes (187 W/m2) vs those without (215 W/m2) (difference, −28 W/m2; 95% CI, −46 to −10 W/m2; P = .003), due to reduced evaporative heat loss (Table 2). Peak body heat storage, mean body temperature change, and heart rate reserve were statistically significantly greater in patients with type 2 diabetes than in those without (Table 2).
Table 1. Physical Characteristics of All Participants and Subgroups Who Completed Heat Acclimation.
| Variablea | All Participants | Subgroups Completing Heat Acclimation | ||||
|---|---|---|---|---|---|---|
| Type 2 Diabetes (n=17) | No Diabetes (n=17) | P Valueb | Type 2 Diabetes (n=8) | No Diabetes (n=10) | P Valueb | |
| Age, y | 59 (6) | 61 (5) | .42 | 58 (5) | 61 (6) | .27 |
| Height, m | 1.75 (0.05) | 1.75 (0.05) | .71 | 1.76 (0.03) | 1.74 (0.04) | .24 |
| Mass, kg | 84 (13) | 83 (11) | .68 | 85 (13) | 77 (5) | .10 |
| Body surface area, m2c | 2.0 (0.1) | 2.0 (0.1) | .82 | 2.0 (0.1) | 1.9 (0.1) | .50 |
| BMI | 27.6 (4.0) | 26.9 (3.5) | .56 | 27.6 (4.2) | 25.6 (2.0) | .22 |
| Body fat, %d | 26.9 (4.9) | 26.1 (7.3) | .72 | 25.8 (4.7) | 23.1 (4.0) | .21 |
| V̇o2peak, mL/kg/min | 33 (7) | 36 (5) | .12 | 32 (7) | 37 (5) | .11 |
| Physical activity, min/wke | 345 (194) | 383 (260) | .63 | 343 (172) | 324 (124) | .78 |
| HbA1c, % | 6.9 (1.0) | 7.0 (1.0) | ||||
| Duration of diabetes, y | 10 (7) | 10 (8) | ||||
Abbreviations: BMI, body mass index, calculated as weight in kilograms divided by height in meters squared; HbA1c, hemoglobin A1c; V̇o2peak, peak aerobic power as determined during an incremental cycle test to volitional fatigue.
Data are presented as mean (SD).
Between-group comparison (unpaired 2-tailed t test).
Body surface area was calculated as 0.20247 × height in meters0.725 × mass in kilograms0.425.
Body fat was estimated using the hydrostatic weighing technique.
Physical activity was quantified as the average weekly duration spent performing structured physical activity of moderate to vigorous intensity using a questionnaire.
Table 2. Peak Responses in Patients With and Without Type 2 Diabetes During Exercise Before and After Heat Acclimationa.
| Variables, Mean (SD) | All Participants | Subgroups Completing Heat Acclimationb | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Type 2 Diabetes | Type 2 Diabetes vs No Diabetes, Difference (95% CI) | P Valuec | Type 2 Diabetes | Type 2 Diabetes vs No Diabetes, Difference (95% CI) | P Valuec | |||||
| Yes (n = 17) | No (n = 17) | Yes (n = 8) | P Valued | No (n = 10) | P Valued | |||||
| Whole-body heat loss, W/m2e | ||||||||||
| Before heat acclimation | 187 (28) | 215 (23) | −28 (−46 to −10) | .003 | 197 (20) | 225 (13) | −28 (−44 to −12) | .002 | ||
| After heat acclimation | 225 (23) | 236 (12) | −11 (−29 to 6) | .20 | ||||||
| Difference (95% CI) | 28 (15 to 41) |
.001 | 11 (1 to 22) |
.04 | 17 (2 to 32) | .03 | ||||
| Dry heat loss, W/m2e | ||||||||||
| Before acclimation | −49 (18) | −53 (19) | 4 (−9 to 17) | .52 | −45 (12) | −51 (23) | 6 (−13 to 25) | .49 | ||
| After heat acclimation | −54 (15) | −57 (13) | 3 (−11 to 17) | .63 | ||||||
| Difference (95% CI) | −9 (−20 to 1) |
.08 | −6 (−18 to 6) |
.27 | −3 (−18 to 12) | .66 | ||||
| Evaporative heat loss, W/m2e | ||||||||||
| Before heat acclimation | 236 (35) | 268 (31) | −32 (−56 to −9) | .008 | 242 (29) | 276 (26) | −34 (−62 to −7) | .02 | ||
| After heat acclimation | 279 (27) | 293 (19) | −14 (−38 to 9) | .21 | ||||||
| Difference (95% CI) | 37 (26 to 48) |
<.001 | 17 (6 to 28) |
.007 | 20 (5 to 35) | .01 | ||||
| Body heat storage, W/m2e | ||||||||||
| Before heat acclimation | 68 (32) | 40 (23) | 28 (9 to 47) | .005 | 56 (21) | 33 (17) | 23 (4 to 42) | .02 | ||
| After heat acclimation | 24 (23) | 16 (12) | 8 (−9 to 26) | .33 | ||||||
| Difference (95% CI) | −32 (−49 to −15) |
.003 | −17 (−28 to −6) |
.008 | −15 (−33 to 3) | .09 | ||||
| Change in body temperature, °Ce | ||||||||||
| Before heat acclimation | 2.2 (0.8) | 1.6 (0.5) | 0.6 (−1.1 to −0.1) | .01 | 1.8 (0.4) | 1.4 (0.3) | 0.4 (−0.0 to 0.7) | .07 | ||
| After heat acclimation | 1.3 (0.4) | 1.0 (0.4) | 0.2 (−0.2 to 0.6) | .32 | ||||||
| Difference (95% CI) | −0.5 (−0.8 to −0.2) |
.003 | −0.4 (−0.6 to −0.1) |
.008 | 0.1 (−0.2 to 0.5) | .40 | ||||
| Heart rate reserve, %e | ||||||||||
| Before heat acclimation | 84 (13) | 71 (18) | 13 (2 to 25) | .02 | 81 (15) | 71 (18) | 10 (−6 to 27) | .21 | ||
| After heat acclimation | 66 (12) | 62 (17) | 4 (−11 to 19) | .58 | ||||||
| Difference (95% CI) | −15 (−29 to −2) |
.03 | −9 (−16 to −2) |
.02 | −6 (−20 to 7) | .32 | ||||
Data represent peak responses (mean over the final 5 minutes of exercise at the highest metabolic heat production [250 W/m2]) or cumulative changes over the entire exercise protocol (change in mean body temperature) performed in dry heat (40°C, 15% relative humidity) within a direct air calorimeter (a device to precisely measure whole-body heat loss).
Subgroups refer to participants who volunteered to complete the exercise protocol again after a 7-day heat acclimation.
Between-group comparison of those with and without type 2 diabetes (unpaired 2-tailed t test).
Within-group comparison before vs after heat acclimation (paired 2-tailed t test).
Higher whole-body heat loss and evaporative heat loss with lower dry heat loss (ie, heat gain from the environment) are advantageous for thermoregulation, while lower body heat storage, mean body temperature change, and heart rate reserve indicate lower thermoregulatory and cardiovascular strain.
In participants completing heat acclimation, peak WBHL was statistically significantly lower in patients with type 2 diabetes (197 W/m2) vs those without (225 W/m2) (difference, −28 W/m2; −44 to −11 W/m2; P = .002; Table 2) before heat acclimation. Following heat acclimation, however, there was a statistically significant increase in peak WBHL in both subgroups (Table 2), with the magnitude of that improvement being statistically significantly greater in patients with type 2 diabetes (28 W/m2) vs those without (11 W/m2), for a difference of 17 W/m2 (95% CI, 2 to 32 W/m2; P = .03). Subsequently, no statistically significant between-group differences in other outcomes were observed following heat acclimation (Table 2).
Discussion
Relative to healthy counterparts, physically active middle-aged and older men with well-controlled type 2 diabetes had attenuated heat-loss capacity during exercise in the heat, due primarily to impaired sweat evaporation, which exacerbated thermal (body temperature) and cardiovascular (heart rate) strain. These preliminary findings indicate that exercise heat stress may pose a health concern in patients with type 2 diabetes, especially because physical activity is recommended for diabetes management.5 However, participants with type 2 diabetes demonstrated a greater improvement in heat-loss capacity than did healthy controls after heat acclimation. A randomized clinical trial of repeated brief, supervised exercise is warranted to determine whether heat acclimation during heat exposure offsets diabetes-related thermoregulatory impairments and health complications. Study limitations include the small sample size, the inclusion of only men and patients with well-controlled diabetes, and the specific exercise and environmental conditions.
Section Editor: Jody W. Zylke, MD, Deputy Editor.
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