In ancient esoteric and mystery schools, a simple diet was generally followed as an aid to spiritual growth as well as physical health.  That diet was made up of mostly fresh fruits and vegetables, nuts and raw honey.  Such foods not only supply needed nutrients, but are also cleansing and reduce acidity.  Meat, especially red meat, was generally avoided.  While some cooked food was eaten, particularly whole grain bread, most food was eaten raw.      
    In today's ICC community , such a strict diet is difficult, but it is still recommended that it be followed when possible.  Sometimes, especially when traveling, we have to eat what is available, but we should still make the healthier choices where possible.  Choose whole grain bread instead of white bread.  Choose to include salads in your meals regularly, but avoid heavy dressing.   Choose fish or fowl rather than red meat.  
    Natural and organic foods are preferable to processed foods.  If you can grow some of your own food in a back yard garden, it is a good idea to do so and grow it without pesticides or artificial fertilizer.  Honey or raw sugar is preferable to white sugar or corn syrup.  Olive oil, sesame oil, or peanut oil is preferable to lard or tallow (beef fat).  Cold pressed oils are better than processed vegetable oil (extra virgin olive oil is almost always cold pressed even though it may not say so on the label).  And drink bottled spring water or tap water that has been filtered.  
    A healthy diet helps us spiritually because it doesn't use up our energy that we need for spiritual growth and it prevents illness that makes it difficult to work on our spiritual growth.