There are different ‘levels’ or types of the fast, depending on one’s abilities, health condition and choice:
● eat moderately while avoiding beans and grains;
● eat only fruits, nuts, and non-cooked, raw food;
● drink only water;
● full fast without water (nirjala);
● full fast without water (nirjala) and without sleeping.
You can also fast fully (with or without water) until noon, and then you can continue with one of the two last fasting levels (fruits or cooked food). This way of observing Ekadasi is also mentioned in the scriptures.
Kindly keep in mind, that it is not the fast and its level itself that are important on Ekadasi day. We fast in order to have more time for chanting, reading scriptures, and engaging in kirtan, instead of losing time on cooking or eating food. If one focuses only on torturing their body (especially if it harms them!), this kind of fasting will be in a mood of passion (raja-guna), and it is not the proper way to observe Ekadasi.
Never fast just for the fast itself, and never do it on a basis of pride! If one has a judgemental or an offensive mood towards others (example, ‘I fast and they do not fast, I am better!’), especially vaisnavas, it will never please Sri Krishna, and then there will be no point in fasting.
Our body is Sri Krishna’s temple, He personally resides there as Paramatma (Supersoul). If one has any health condition, or their body is not fit for observing severe fasting — let them fast according to their ability. Srila Prabhupada himself observed Ekadasi by honoring Ekadasi prasadam, however, he was always happy to hear when any disciple was healthy enough to fast fully with the proper mood, and would always say that, ‘On Ekadasi we can eat some fruits and take some milk’. The main rule is that the mind has to be focused on the Lord, on spirituality.