During cold exposure, an increase in sympathetic nerve activity evokes vasoconstriction (VC) of cutaneous vessels to minimize heat loss. In older adults, this reflex VC response is impaired thereby increasing their susceptibility to excess heat loss and hypothermia. Since L-tyrosine, the amino acid substrate necessary for catecholamine production, has been shown to augment reflex VC in aged skin, we hypothesize that oral ingestion of L-tyrosine will attenuate the decline in core temperature (Tc
) during whole-body cooling
in older adults.
In a randomized, double-blind design, nine young (25 ± 3 years) and nine older (72 ± 8 years) participants ingested either 150 mg/kg of L-tyrosine or placebo prior to commencing 90 minutes of whole-body cooling
to decrease skin temperature to ~29.5o
C. Esophageal temperature and forearm laser Doppler flux (LDF) were measured continuously throughout the protocol to provide an index of Tc
and skin blood flow
, respectively. The change in esophageal temperature (ΔTES
) was the difference in temperature at the end of cooling subtracted from baseline. Cutaneous vascular conductance (CVC) was calculated as CVC = LDF/mean arterial pressure and expressed as a percent change from baseline (%ΔCVCBASELINE
Oral tyrosine ingestion augmented the cutaneous VC response to cooling in older adults (Placebo = 14.4 ± 2.0, Tyrosine = 32.7 ± 1.7 %ΔCVCBASELINE
; p<0.05). Additionally, tyrosine improved Tc
maintenance throughout cooling in older adults (Placebo = -0.29 ± 0.07, Tyrosine = -0.07 ± 0.07 ΔTES
; p<0.05). Both the cutaneous VC and Tc
during cooling were similar between young and older adults supplemented with tyrosine (p>0.05).
These results indicate that L-tyrosine supplementation improves Tc
maintenance in response to acute cold exposure in an older population.