A 48-hour zero-calory diet significantly alters physiological and psychological functions

Reduced nutrient and energy uptake plays a major role in a number of clinical conditions (anorexia, cachexia, obesity); however, the direct and indirect effects of short and long-term dieting have rarely been investigated.

Methods: We investigated 16 healthy female volunteers (21.4±2.1 years, 21.7±1.6 BMI) for the effects of a zero-calory diet for 48 hours. The following functions were investigated after 24 and 48 hours of fasting and compared to control conditions following a regular meal:

a) Energy expenditure during rest and after 10min standardized ergometer cycling;

b) cardiovascular autonomic functions (HRV) during rest and following physiological (tilt table, valsalva) and psychological stress (Stroop, mental arithmetic);

c) smell and taste functions with standardized tests (Sniffin Sticks, Taste Strips);

d) neurocognitive processing of visual food cues with an eye-tracking device (Sensomotoric Instruments SMI-Highspeed);

e) psychological functions (mood, hunger, depression, somatization) and stress responses (saliva cortisol profile across the day).

Results: a) Resting energy expenditure increased and resting cardiovascular functions (HRV) decreased significantly after 24 hours and continued to do so after 48h of fasting.

b) Responses to physiological and psychological autonomic stress tasks remained mainly unaltered throughout the experiment, but cardiac functions (HRV) were altered during resting.

c) Olfactory sensitivity did not change, but taste sensitivity overall increased with lower sensitivity for sweet and higher sensitivity for bitter, sour, and salty tastes.

d) The diurnal cortisol profile (8.00h to 17.00h) dose-dependently shifted towards higher levels after 24 and after 48 hours fasting.

e) Food cues induced shorter reaction times compared to non-food signals after 24 hours, but not after 48h. Subjects' mood and psychological well-being was moderately affected by fasting in a dose-dependent fashion.

Conclusion: Short-term fasting on a zero-calory diet induces many acute physiological and psychological changes within the first 24 hours that in part are reversed in the subsequent 24 hours. This indicates counter-regulatory mechanisms that may be responsible for the „revolving door“ effects of diets in the treatment of obesity.