According to Bahá’u’lláh Himself, the Bahá’í fast is adapted from the fast ordained in the Bayan. The Bab’s fast, mentioned in both the Arabic and Persian Bayans, occupied the last month of the Babi calendar, the month of ‘Ala‘, roughly 2-20 March. Believers were to fast from the age of eleven (numerically equivalent to huva, ‘He‘) until forty-two (bala, ‘Yea‘). Children could fast until noon for the first eleven days. Those over forty-two were exempted from fasting. Those fasting had to abstain from food, drink and sexual relations from sunrise to sunset—preferably from slightly before sunrise until slightly after sunset. No exemptions are mentioned. The real meaning of the fast, the Bab said, was abstention from the love of other than the Manifestation of God. The continuation of the fast was contingent on the acceptance of Him Whom God shall make manifest. Although Bahá’u’lláh accepted the fast of the Bab, He altered the details of its regulations in many important respects. The Bahá’í fast is binding on all believers from the age of maturity, which for Bahá’ís is fifteen, until seventy. There is no provision made for children fasting. The following individuals are exempted from fasting: Travellers, providing their journey is to last at least nine hours or two hours on foot. If they break their journey for more than nineteen days, they are only exempt for the first three days after their arrival. If they return home, they must begin fasting on arrival. The sick. Women who are pregnant or nursing. Women who menstruating, who must instead repeat the phrase ‘Glorified be God, the Lord of Splendour and Beauty’ ninety-five times between one noon and the next. Those engaged in heavy labour, who are advised to be discrete and restrained in availing themselves of this exemption. These groups are also exempted from fasting in Islam. Bahá’u’lláh does not require missed days of fasting to be made up later, nor does He mention abstention from sexual relations.
(John Walbridge, Fasting, published in Sacred Acts, Sacred Space, Sacred Time: Bahá’í Studies volume 1 http://bahai-library.com/walbridge_encyclopedia_fasting )
Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires.
(Bahá’u’lláh, The Kitáb-i-Aqdas, p. 177)
Keeping the Fast is enjoined upon all Bahá’ís, regardless of nationality; it has a very salutary effect both physically and spiritually, and the friends should realize Bahá’u’lláh never would have instituted it if it were detrimental to the health.
(Shoghi Effendi, Lights of Guidance, p. 233)
O God! as I am fasting from the appetites of the body and not occupied with eating and drinking, even so purify and make holy my heart and my life from aught else save Thy Love, and protect and preserve my soul from self-passions and animal traits. Thus may the spirit associate with the Fragrances of Holiness and fast from everything else save Thy mention.
(‘Abdu’l-Bahá, Star of the West, Vol. IV, No.18, p. 305)