The effect of time of day on orthostatic tolerance and the cardiovascular effects of a high carbohydrate meal in healthy young subjects

Ingestion of a high carbohydrate meal leads to a fall in blood pressure but does not change orthostatic tolerance in healthy elderly subjects. Much smaller effects are observed in young subjects, but studies have only been performed in the morning. The purpose of this study was to compare orthostatic responses and the effect of a high carbohydrate meal in the morning and in the afternoon in young subjects. Fourteen healthy, young subjects (four female, mean BMI = 22.0 kg/m2, age range 21-27 years) were studied on two occasions, in the morning (0830 h) and in the afternoon (16.30 h) on separate days. Blood pressure, heart rate and cardiac output were measured noninvasively before and after the ingestion of a standard meal with 76-77% of the energy being provided by carbohydrate. Time of day had no effect on blood pressure and heart rate responses to tilting after food. Cardiac output fell significantly during tilting in the fasted state. In the morning there was a progressive fall in cardiac output with a decrease of 0.83 l/min at 45 degrees (95% confidence interval of the change -1.37 to -0.27 to l/min) and a fall of a further 0.15 l/min at 75 degrees. By contrast in the afternoon cardiac output fell 0.84 l/min at 45 degrees (95% confidence interval of the change -1.31 to -0.35 l/min) and then rose by 0.61 l/min on tilting to 75 degrees (95% confidence interval of the change +0.22 to +0.98 l/min. Interaction effect p = 0.02 ANOVA). Supine cardiac output increased after food ingestion at both times of day (p less than 0.01 ANOVA).