Summary

Background: Fasting is required by the Jewish and Islamic religions, and may sometimes be necessary for non­religious reasons as well. Very little empiric data are available on the effect of 24 hours of food and water deprivation.

Objectives:  To compare the effects of the dietary composition of different pre-fast meals on subjective discom­fort and various other parameters of a 24 hour food and water fast.

Methods: Thirteen volunteers of both genders participated in a non-randomized crossover study. Each consumed three different equicaloric pre-fast meals in which the main source of calories was protein (49% of calories), carbohydrate (86%), or fat (69%). Weight, heart rate, blood pressure, blood and urine were tested before and after 24 hours of fasting, and the subjective evaluations of the discomfort during the three fasts were compared.

Results: After the protein-rich meal greater discomfort and more side effects were reported. Weight and blood pressure decreased at the end of the fasts that followed each of the three meals heart rate increased after the high fat and carbohydrate meals but not after the protein meal. The main laboratory findings were a 40% increase in blood urea nitrogen and higher urine osmolarity after the protein-rich meal than after the other meals.

Conclusion: A protein-poor pre-fast meal is likely to be followed by easier fasting.