Previous study has suggested that phase coherency (λ) between RSA and respiration serve as a normalized index of the degree of mental stress. In the present study, we investigated whether λ is altered by food ingestion or gum chewing. After 5 min of rest, subjects were asked to eat the control meal (CM) or to chew a tasteless chewing gum (CG) at their own pace. From the oscillatory signals of RRI and respiration, instantaneous phases were calculated and then λ was computed. To assess autonomic activation, the normalized power for low- (nLF) and high-frequency (nHF) components of RRI fluctuations were also computed. Unexpectedly, transient increases in HR and BP and decrease in λ were observed from rest to CM or CG, suggesting a shift of sympathovagal balance toward a sympathetic activation. The λ correlated to nHF and was inversely related with nLF/nHF, suggesting the association of autonomic activity.