Comparison of a Novel Condensed Heat Acclimation Programme With a Traditional Longer-term Heat Acclimation Programme

Repeated exposure to heat in a laboratory setting (acclimation) elicits a range of adaptations, which reduce heat illness risk and increase work capacity in the heat. Traditional approaches to heat acclimation require daily heat exposures of 1 to 2 hours over ~7 to 10 consecutive days. Heat acclimation approaches which reduce the number of days to achieve acclimation may have utility. The primary purpose of the proposed research is to determine whether it is possible to achieve a similar degree of heat acclimation to that seen with a traditional longer-term heat acclimation approach by increasing the frequency of heat exposure, utilising multiple daily heat exposures over a smaller number of days. Secondary aims of the research are to examine whether heat acclimation provides cross-adaptation to a hypoxic stressor and whether heat acclimation improves aerobic fitness.

Study Overview

• Status: Completed
• Conditions: Heat Stress
• Intervention / Treatment: Other: Heat acclimation

Detailed Description

Individuals are often required to operate in adverse conditions and may be exposed to prolonged periods of high ambient heat and humidity. High environmental temperatures impair work capacity and increase heat-illness risk. Repeated exposure to heat in a laboratory setting (acclimation) elicits a range of adaptations, which reduce heat illness risk and increase work capacity in the heat. However, traditional approaches to heat acclimation are time consuming, typically requiring daily heat exposures of 1 to 2 hours over ~7 to 10 consecutive days. This can be logistically difficult to implement and impractical, particularly in situations where an individual must be rapidly deployed. Therefore, heat acclimation approaches which reduce the number of days to achieve acclimation may have utility. In addition, recent research studies have suggested that adaptation to heat may improve tolerance to hypoxia (cross-adaptation) and improve aerobic fitness; these effects may also be beneficial in a military context. Therefore, the primary purpose of the proposed research is to determine whether it is possible to achieve a similar degree of heat acclimation to that seen with a traditional longer-term heat acclimation approach by increasing the frequency of heat exposure, utilising multiple daily heat exposures over a smaller number of days. Secondary aims of the research are to examine whether heat acclimation provides cross-adaptation to a hypoxic stressor and whether heat acclimation improves aerobic fitness. Heat acclimation will be evaluated using a range of whole-body (cardiovascular, thermoregulatory, psychophysiological) and biochemical markers.

• Study Type: Interventional
• Enrollment (Actual): 37
• Phase: Not Applicable

Contacts and Locations

Study Locations

• United Kingdom
  • Hampshire
    • Portsmouth, Hampshire, United Kingdom, PO1 2ER
      • University of Portsmouth

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: 18 years to 45 years (Adult)
• Accepts Healthy Volunteers: Yes

Description

Inclusion Criteria:

• Male
• Physically active and healthy as determined by pre-participation health screening questionnaire.
• Free of COVID-19 symptoms (loss or change in sense of smell or taste, fever and new, continuous cough) as determined by pre-screening questionnaire.
• Resting diastolic blood pressure between 60 and 90 mmHg; resting systolic blood pressure between 90 and 150 mmHg A resting ECG is required if: individuals are over 30 years of age; they are unfamiliar with exercise of a maximal nature; they have a family history of adverse cardiac events. The ECG is to be reviewed by the Independent Medical Officer.

Exclusion Criteria:

• Current smokers
• Recent (< 3 months) participation in a formal heat acclimation regimen, or regular frequent heat exposures that may render the participant partially heat acclimated (other than regular exercise)
• Recent (< 3 months) high altitude (>1 500m ) sojourn that may render the participant partially altitude acclimated
• Diagnosed cardiovascular, metabolic or respiratory conditions (excluding asthma)
• Prior history of heat illness
• Prior history of collapse or intolerance with exposure to altitude
• Recent blood donation (within 3 months of commencing study)
• Baseline serum sodium level <135 mmol.L-1
• Any other extant medical condition which may be exacerbated by participation
• Participants with inadequate understanding of English.
• Any volunteers who are currently participating in any other research studies which may influence their responses or the results obtained

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Prevention
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: None (Open Label)

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
Active Comparator: Traditional heat acclimation; The traditional heat acclimation programme will consist of daily 75-minute heat exposures for eight consecutive days	Other: Heat acclimation; Repeated heat exposures with light exercise sufficient to elevate deep body temperature to ~ 38.5C
Experimental: Condensed heat acclimation; The condensed heat acclimation programme will consist of two consecutive days with four, 75-minute heat exposures undertaken on each day	Other: Heat acclimation; Repeated heat exposures with light exercise sufficient to elevate deep body temperature to ~ 38.5C

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Reduction in the peak deep body temperature	Rectal temperature measurement during a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Reduction in the resting deep body temperature	Rectal temperature measurement during a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Reduction in the peak mean body temperature	Weighted rectal and mean skin temperature measurement during a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Reduction in the resting mean body temperature	Weighted rectal and mean skin temperature measurement during a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Increased plasma volume	Change in plasma volume calculated from resting haemoglobin and haematocrit prior to a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Increased sweating rate	Sweating rate calculated from change in nude body mass and accounting for fluid consumption during a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Reduced sweat sodium concentration	Sweating sodium concentration assessed from sweat collected in a custom sweat patch on the back during a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Reduced exercise heart rate	Heart rate measured using chest telemetry during a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Increased resting intracellular heat shock protein 70 content	Assessed in peripheral blood mononuclear cells obtained before a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Increased post heat stress intracellular heat shock protein 70 content	Assessed in peripheral blood mononuclear cells obtained after a heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Increased resting intracellular heat shock protein 90 alpha content	Assessed in peripheral blood mononuclear cells obtained before a heat stress tests	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Increased post heat stress intracellular heat shock protein 90 alpha content	Assessed in peripheral blood mononuclear cells obtained after a heat stress tests	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Maximal oxygen uptake	Highest rate of oxygen uptake during graded cycling exercise	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Lactate threshold	Power output at 4 mmol.L-1 blood lactate during graded cycling exercise	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Gross mechanical efficiency	Percentage ratio of external work achieved compared to the total energy expenditure during graded cycling exercise below the lactate threshold	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Peak power output	Highest power output achieved during exhaustive cycling exercise	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Peripheral oxygen saturation during exposure to hypoxia	Assessed using ear or finger tip pulse oximetry during a hypoxic tolerance test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Heart rate during exposure to hypoxia	Assessed using chest telemetry during a hypoxic tolerance test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Minute ventilation during exposure to hypoxia	Expire volume of air per minute during a hypoxic tolerance test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Plasma neutrophil gelatinase-associated lipocalin	Concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Urine neutrophil gelatinase-associated lipocalin	Concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Plasma kidney injury molecule-1	Concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Urine kidney injury molecule-1	Concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Tissue inhibitor of metalloproteinase 2	Urine concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Insulin-like growth factor-binding protein 7	Urine concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Intestinal fatty acid binding protein	Serum concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Soluble cluster of differentiation 14	Plasma concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Lipopolysaccharide binding protein	Plasma concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Interleukin-6	Plasma concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)
Cortisol	Plasma concentration change pre to post heat stress test	Pre heat acclimation intervention (up to 1 week) to post heat acclimation intervention (up to 1 week)

Collaborators and Investigators

• Sponsor: University of Portsmouth
• Collaborators: Teesside University; Coventry University

Study record dates

Study Major Dates

• Study Start (Actual): January 17, 2020
• Primary Completion (Actual): January 4, 2023
• Study Completion (Actual): January 4, 2023

Study Registration Dates

• First Submitted: October 21, 2022
• First Submitted That Met QC Criteria: October 26, 2022
• First Posted (Actual): October 31, 2022

Study Record Updates

• Last Update Posted (Estimated): December 6, 2023
• Last Update Submitted That Met QC Criteria: December 5, 2023
• Last Verified: December 1, 2023

More Information

Terms related to this study

• Additional Relevant MeSH Terms: Wounds and Injuries; Heat Stress Disorders
• Other Study ID Numbers: 1017/MoDREC/19

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO
• IPD Plan Description: Data will be published as measures of central tendency and dispersion (e.g. Mean and Standard deviation)

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No