Introduction
During cold exposure, an immediate and sustained increase in sympathetic nerve activity evokes vasoconstriction (VC) of cutaneous vessels to minimize heat loss and maintain body core temperature. In older adults, this reflex VC response is impaired, thereby increasing their susceptibility to excess heat loss and hypothermia. This may be explained in part by reduced bioavailability of the amino acid substrate, L-tyrosine, for catecholamine production. Thus, the thermoregulatory benefit of tyrosine supplementation in older adults is unknown.
Objective
We hypothesize that oral L-tyrosine ingestion will augment the cutaneous VC response and thereby attenuate the decline in core temperature resulting from prolonged whole-body cooling in older adults.
Methods
In a randomized, double-blind design, 8 older participants (aged 68±4 years) ingested either 150 mg/kg of L-tyrosine or placebo before commencing 90 minutes of whole-body cooling to decrease skin temperature to ∼30°C. Esophageal temperature (TES) and forearm laser Doppler flux was measured continuously throughout the protocol to provide an index of core temperature and skin blood flow, respectively. Cutaneous vascular conductance (CVC) was calculated as CVC = laser Doppler flux / mean arterial pressure and expressed as a percent change from baseline (%ΔCVC). The change in esophageal temperature (ΔTES) was the difference in temperature at the end of cooling subtracted from baseline. Data were analyzed using 2-way (drug by cooling time) analysis of variance with repeated measures with Bonferroni post hoc analysis.
Results
Oral tyrosine supplementation improved the reflex cutaneous VC response to cooling in older adults (placebo = 15.0±0.9, tyrosine = 29.6±0.7 %ΔCVC; P < .05). Additionally, tyrosine maintained body core temperature throughout cooling (placebo = –0.31±0.04, tyrosine = –0.09±0.03 ΔTES; P < .05).
Conclusions
These results indicate that L-tyrosine supplementation may improve thermoregulatory function in response to acute cold exposure in an older population.