Praise be to Allaah.
There is a sharâi concession which allows breaking the fast in Ramadan for one who is sick and unable to fast, or one who fears that he will become sick or is his sickness will get worse. Allah says (interpretation of the meaning): âwhoever is ill or on a journey, the same number of days which one did not observe Sawm (fasts) must be made up from other daysâ al-Baqarah 2:185.
Ibn Qudaamah (may Allah have mercy on him) said in al-Mughni (4/403):
The healthy person who fears that he will become sick by fasting is like a sick person who fears that his sickness will get worse with regard to the concession allowing breaking the fast, because the sick person is only allowed not to fast for fear of what fasting may lead to of the sickness getting worse or lasting longer. So the fear of becoming sick comes under the same category. End quote.
Al-Zaylaâi said: The healthy person who fears that he will become sick by fasting comes under the same category as the sick person. End quote. Tabyeen al-Haqaaâiq, 1/333
Based on that:
If fasting will definitely make you sick or will cause you physical harm because of being severely underweight, then in this case it is allowed for you not to fast, but you have to make it up when you are able to.
To decide concerning this matter, it is essential to consult trustworthy doctors.
As for merely getting tired or exhausted, or feeling weak because of fasting, that is not an excuse for not fasting, because fasting -- especially on hot and long days -- is not free of some hardship.
The scholars have stated that those who do hard (physical) work should start their day fasting, then if they encounter severe thirst or hunger from which it is feared that harm will result, it is permissible for them to break the fast, but they must make up the days on which they broke the fast. It should be noted that study and work are not excuses for not fasting. Fasting is one of the pillars of Islam and one of the greatest obligatory duties of this religion. So it is not permissible for the Muslim to be so heedless about it that he gives precedence over it to study or work that he has no urgent need to do and which he can do without.
For more information please see the answer to question no. 43772 and 65803.
And Allah knows best.