The nineteenth month, following immediately on the hospitality of the intercalary days, is the month of the fast. During nineteen days the fast is observed by abstaining from both food and drink from sunrise to sunset. As the month of the fast ends at the March equinox, the fast always falls in the same season, namely, spring in the Northern, and autumn in the Southern, Hemisphere; never in the extreme heart of summer nor in the extreme cold of winter, when hardship would be likely to result. At that season, moreover, the interval between sunrise and sunset is approximately the same all over the habitable portion of the globe, namely, from about 6 A.M. to 6 P.M. The fast is not binding on children and invalids, on travelers, or on those who are too old or too weak (including women who are with child or have babes at the breast).
There is much evidence to show that a periodical fast such as is enjoined by the Bahá'í teachings is beneficial as a measure of physical hygiene, but just as the reality of the Bahá'í fast does not lie in the consumption of physical food, but in the commemoration of God, which is our spiritual food, so the reality of the Bahá'í fast does not consist in abstention from physical food, although that may help in the purification of the body, but in the abstention from the desires and lusts of the flesh, and in severance from all save God. `Abdu'l-Bahá says: --
Fasting is a symbol. Fasting signifies abstinence from lust. Physical fasting is a symbol of that abstinence, and is a reminder; that is, just as a person abstains from physical appetites, he is to abstain from self-appetites and self-desires. But mere abstention from food has no effect on the spirit. It is only a symbol, a reminder. Otherwise it is of no importance. Fasting for this purpose does not mean entire abstinence from food. The golden rule as to food is, do not take too much or too little. Moderation is necessary. There is a sect in India who practice extreme abstinence, and gradually reduce their food until they exist on almost nothing. But their intelligence suffers. A man is not fit to do service for God with brain or body if he is weakened by lack of food. He cannot see clearly. (quoted by Miss E. S. Stevens in Fortnightly Review, June 1911).
-- Esslemont, Baha'u'llah and the New Era, p. 183-184.