The water contained in food is considered from food third of the belly
12-1-2015 | IslamWeb
Question:
With regards to the hadeeth: (rough translation) 'The human does not fill any container that is worse than his stomach. It is sufficient for the son of Adam to eat what will support his back. If this is not possible, then a third for food, a third for drink, and third for his breath.". Is the water content in food considered? For example, if a person eats an apple weighing 150 grams, should he count the water content in the apple (which could be between 120 and 130 grams) as going towards drink, or shouuld he count all 150 grams as going towards food?
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.
Ibn Al-Qayyim 
said when commenting on this Hadeeth:
"This is most useful for both body and heart. For if the belly is filled with food, it has not enough space for drink, and when the drink is added to it, this even leaves little space for breath. Thus the belly is afflicted with distress and fatigue, and it bears this like one carrying a heavy burden. This state will also lead to heart trouble and the limbs become too lazy to perform the obligatory rituals, and conversely they move swiftly in submission to desires brought about by satiety (i.e. the state of being fed or gratified to or beyond capacity). So, filling one’s stomach (in excess) with food is harmful for both the heart and body. However, if it is only done occasionally, then there is no harm. Abu Hurayrah 
drank milk in the presence of the Prophet, sallallaahu ‘alayhi wa sallam, until he said: 'I swear by the One Who has sent you with the truth, I cannot find any more space for it!' And the Companions often ate in his (the Prophet’s) presence, until they were satiated (filled to capacity).
Excessive satiety, however, weakens the faculties and the body, even if it nourishes it. The body only becomes strong in accordance with the quality of the food it accepts not its quantity.
Since the human being is made from three elements: (1) earth, (2) water, and (3) air, the Prophet, sallallaahu ‘alayhi wa sallam, apportioned his diet into these three categories." [Excerpted from Zaad Al-Ma‘aad by Ibn Al-Qayyim]
Allaah Knows best.