A Catecholamine Precursor Does Not Influence Exercise Performance in Warm Conditions

The effect of prescription and over-the-counter medications on core temperature in adults during heat stress: a systematic review and meta-analysis

Background

Heat stress impacts are an escalating global health concern. Public health bodies such as the World Health Organization (WHO) warn that certain medications impair thermoregulation, with limited supporting evidence. Our aim was to investigate whether medications listed by the WHO increase core temperature responses during heat stress.

Methods

For this systematic review and meta-analysis, MEDLINE, PubMed, Scopus, CINAHL, Web of Science, and EMBASE were searched up to Jan.30, 2024. Randomised studies exposing humans to exertional and/or passive heat stress that investigated a drug identified by WHO compared to no drug/placebo were eligible. The primary outcome was core temperature (e.g., rectal, oesophageal, aural, tympanic). We assessed risk of bias (Cochrane's Risk of Bias 2) and certainty of evidence (GRADE). The study was pre-registered on PROSPERO (CRD42020170684).

Findings

Thirty-five studies were included enrolling 353 individuals (16 women; 4.5%). Twenty-seven unique medications were tested. The average age of participants across studies was <30 years, and only one study included a clinical population. Under heat stress, there was moderate quality evidence that drugs with high anticholinergic properties increased core temperature at air temperatures ≥30°C (+0.42°C; 95% CI 0.04, 0.79°C; p = 0.03) alongside reduced sweating, although evidence is limited to the drug atropine. Similarly, non-selective beta-blockers (+0.11°C; 95% CI 0.02, 0.19°C; p = 0.02), adrenaline (+0.41°C; 95% CI 0.21, 0.61°C) and anti-Parkinson's agents (+0.13°C; 95% CI 0.07, 0.19°C; p = 0.02) elevated core temperature. Antidepressants, diuretics, or drugs with weak anticholinergic effects did not alter core temperature responses.

Interpretation

Current evidence supports strong anticholinergics, non-selective beta-blockers, adrenaline, and anti-Parkinson's agents impairing thermoregulation during heat stress. No evidence indicated thermoregulation is impacted by other WHO-listed medications. Evidence is predominantly limited to healthy young men, with short heat stress exposures. Studies over longer durations, in women, older adults and those with chronic diseases are required to better inform the pharmaceutical management of patients during hot weather.

Funding

This study was supported by a National Health and Medical Research Council (NHMRC) Investigator Grant (2021/GNT2009507; Holder: O. Jay).

No Influence of Low-, Medium-, or High-Dose Tyrosine on Exercise in a Warm Environment

PURPOSE

Tyrosine administration may counter exercise fatigue in a warm environment, but the typical dose is inconclusive, with little known about higher doses. We explored how three tyrosine doses influenced the circulating ratio of tyrosine/amino acids competing for brain uptake and hypothesized that a medium and high dose would enhance exercise performance in a warm environment.

METHODS

Eight recreationally trained, non-heat-acclimated male individuals (mean ± SD age, 23 ± 4 yr; stature, 181 ± 7 cm; body mass, 76.1 ± 5.9 kg; peak oxygen uptake, 4.1 ± 0.5 L·min) performed a peak oxygen uptake test, two familiarization trials, then four experimental trials in a randomized order separated by 7 d. Before exercise, subjects drank 2 × 300 mL sugar-free drinks delivering 0 (PLA), 150 (LOW), 300 (MED), or 400 (HIGH) mg·kg body mass tyrosine in a double-blind fashion. Subjects performed a 60-min constant intensity cycling then a simulated time trial in 30°C and 60% relative humidity.

RESULTS

Time trial performance (P = 0.579) was not influenced by tyrosine ingestion. The plasma ratio of tyrosine/∑(free-tryptophan, leucine, isoleucine, valine, phenylalanine, methionine), a key determinant of brain tyrosine influx, increased relative to PLA (P < 0.001). The increase was similar (P > 0.05) in MED (7.7-fold) and HIGH (8.2-fold), and greater than that in LOW (5.3-fold; P < 0.05). No differences existed between trials in core and skin temperature, heart rate, RPE, or thermal sensation (P > 0.05).

CONCLUSION

Exercise performance in a warm environment was not influenced by tyrosine availability in recreationally trained male individuals. The results provide novel data informing future studies, on the tyrosine dose maximizing the circulating ratio of tyrosine/amino acids competing for brain uptake.