Effects of a 48-h fast on heart rate variability and cortisol levels in healthy female subjects.

BACKGROUND/OBJECTIVES: The physiological changes that occur during fasting are not completely understood, regardless of the cause for fasting (for example, medical, lifestyle, religious, political or famine). The purpose of this study was to examine the effects of a 48-h fast on heart rate variability (HRV) and cortisol levels in healthy young female volunteers.

SUBJECTS/METHODS: A total of 16 young healthy female volunteers underwent 48 h of total fasting under 24-h medical surveillance. Psychological (subjective feeling of hunger) as well as physiological data (HRV, diurnal cortisol profiles) were measured upon admission (Day 1), and after 24 (Day 2) and 48 h (Day 3) of fasting.

RESULTS

There was a measured weight loss from Day 1 to Day 3 that resulted in significant body mass index (BMI) reduction across all subjects (P<0.001). The slope of the diurnal cortisol profile significantly shifted towards lower values from baseline to the end of experiment (P=0.002). HRV during resting showed a significant (P<.001) decrease in standard deviation of the normal-to-normal interval (SDNN) and root mean square of successive differences (RMSSDs) from Day 1 to Day 3 of the experiment, with a small increase after 24 h that did not reach statistical significance. A 48 h of fasting also induced a significant (P<.001) decrease of mean interbeat intervals (IBIs), SDNN, RMSSD and log high-frequency (HF) power during head-up tilt testing.

CONCLUSIONS

An acute (48 h) total fast induced parasympathetic withdrawal with simultaneous sympathetic activation. These changes appear to reflect stress. Further studies are needed to demonstrate the specificity of these changes to fasting.