Heart rate variability (HRV) is a common measure of the autonomic modulation of the heart. To ensure accuracy, various external factors known to affect HRV such as medication use and concurrent illnesses are frequently controlled during measurement. Despite extensive recognition of the importance of such confounds, the influence of ostensibly innocuous daily activities such as drinking and eating have received remarkably little attention in comparison. Water consumption provokes a powerful pressor stimulus that is buffered by changes in cardiovagal outflow yet it is sporadically controlled during experimentation. This paper examines the dose-dependent magnitude of the effect of water on HRV, the loss of HRV during a common attentional task after water consumption, and the typical scenario of water and mixed meal ingestion. Water affects HRV in a dose-dependent manner, and exaggerates attentionally- mediated HRV reduction. This effect is antagonised by simultaneous mixed meal consumption, which strongly increases cardiac sympathetic activity. Water consumption during or previous to HRV measurement should be carefully controlled, if possible. Considering the diverse nature of experimental protocols and populations, we suggest a hierarchy of methods to control for water consumption in HRV research.