For those of you who feel clear-headed after fasting for a day or two: you might have a delayed food allergy (google "IgG food intolerance"). The most common such allergies are to gluten (not just coeliac sufferers are sensitive to gluten) and dairy products. I believe these delayed-onset allergies are fairly common in western societies, and are due to a "leaky gut", or permeable intestinal lining, that results in an immune system response when those foods get absorbed. Because IgG allergies can manifest themselves up to three days after you eat the offending food, some people may never figure out the cause of their fatigue or foggy-headedness. IgG allergies might be worth looking into if you have those symptoms.