Spiritual fasting assists the soul “to refresh and reinvigorate the spiritual forces latent” in it

Shoghi Effendi indicates that the fasting period, which involves complete abstention from food and drink from sunrise till sunset, is ‘...essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, therefore, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires.’ 
(The Universal House of Justice, ‘Notes’ section of the ‘Kitab-i-Aqdas’)