Fasting when you have health problems is probably not advisable. You can however chose plain or dull food ie. Food contrary to your desire. Like if you want a cold beer in hot weather have cold water. When you eat meals have what is plain and sufficient for you needs, ie you can fast from sauces and gravies, from sugar on your ceŕeal/coffee tea etc. In this manner you still eat sufficient food but at the same time detach your self from the pleasure of food which ofcourse is the point of mortification. Also you should note that fasting should be something you do to detach yourself from your attachments so if you aren’t attached to food in a disorderly way then rather concentrate on attachments you need to mortify.