For meditative purposes many choose to not eat prior to [or have a light something] because a full stomach or other gut issue [gas, bloating] can be distracting and make it hard to clear your mind and relax.
Regarding 'Wiccan diets' Lark is correct that whatever feels right for you is the diet. Like with the faith, you must do some soul searching and decide for yourself what is the best diet. [as in regular eating plan] some feel you do harm killing the animal and become vegetarian, others feel honey and eggs exploit and cause harm to the animal and become vegans. You need to ask yourself what a 'Wiccan diet' is for you. I limit junk food to occasions treats [say the movies or a party] and while I'm not a vegetarian/vegan there are certain meats I avoid because of various personal reasons.
Most Wiccans I know are mindful of what they eat and try to stick to a balanced diet with more natural foods [fruits, vegetables, nuts, certain meats] over man made [fast food, frozen foods, highly processed foods]