It is forbidden for a person to fast until midday on Shabbat. Even one who involves himself in prayer and Torah study on Shabbat morning must make a point of eating and preferably beginning the Shabbat meal before midday. If a person knows that he will be in the synagogue until midday and thus be unable to begin his Shabbat meal before that point, he must drink something such as water or tea before services, so that he will not have "fasted" until midday.
A person who is accustomed to observing a fast every day, and will experience discomfort by eating on Shabbat due to the sudden change in routine, is allowed to fast on Shabbat. It is indeed reported that Rabbi Yehuda Hachasid (Germany, 1150-1217) fasted on Shabbat, as for him fasting amounted to "enjoyment." Nevertheless, even such a person should ensure to partake of at least a Ke'zayit of bread for each of the three required meals on Shabbat.
If eating even a Ke'zayit of bread causes one discomfort or threatens his health, then he is exempt from the obligation to eat three meals on Shabbat. However, a person may not refrain from eating the three meals for purely dietetic reasons; the exemption applies only to a person for whom eating will cause actual harm.
Summary: One should preferably begin his Shabbat meal before midday; if he knows that he will be unable to eat before this point, he should drink something before the morning prayers. One who finds it more comfortable to refrain from eating is allowed to refrain from eating on Shabbat, but should eat at least a Ke'zayit of bread for each of the three meals. If even this amount causes him discomfort, he may refrain from eating entirely.
