Thin people fasting the fast of Daawood
9-8-2013 | IslamWeb
Question:
Salaam 'Alaykum wa Rahmatullah wa Barakatuh, is it recommended for people who are thin & slim to observe the fast of Dawood (aleyhi salaam), meaning to fast every second day, if they want to? May Allah guide us to his mercy. Ameen.
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The fasting of Prophet Daawood (David) 
is the most beloved fasting to Allaah The Almighty as stated by the Prophet, sallallaahu ‘alayhi wa sallam. So, whoever is able to observe it and it does not prevent him from fulfilling what is obligatory upon him, then let him fast in that way for it is truly a great deed.
However, if fasting prevents one from what is obligatory upon him, then he should not observe it. The same applies if fasting prevents one from offering the acts of worship that are better and more rewardable; in this case one should give priority to the better acts of worship.
Imaam Ibn Rajab 
said: “The best form of fasting is the one that does not weaken the body so that it becomes unable to do what is much better than it (voluntary fasting) such as fulfilling the rights of Allaah The Almighty, or the necessary rights of His servants. So, if it weakens one to do any of such things that is better than it, then not observing it will be better: The first case is when excessive fasting leads to weakness that in turn makes it difficult for one to offer prayers, remembrance of Allaah The Almighty and to seek knowledge; as it is said that forbidding voluntary fast on Friday and the Day of 'Arafah is because it leads to weakness that makes it difficult for one to do remembrance of Allaah or supplicating to Him on these two days. Ibn Mas‘ood used to fast a little and say: 'Fasting prevents me from reciting Quran while reciting Quran is more beloved to me.' Also, reciting Quran is better than fasting as stated by Sufyaan (Ath-Thawry).' The second case is when fasting weakens one and thereby reduces his ability to provide for his family or to satisfy his wife. This latter meaning is implied in the saying of the Prophet, sallallaahu ‘alayhi wa sallam: “Surely, your wife has a right over you.” [End quote]
Allaah Knows best.