Since Glen has posted about his dietary auto-experiment, I'll likewise relate my role as lab rat. Once a week, I fast for an entire day. More precisely, since I stop taking in calories after dinner the day before my fast and don't start again until the morning after, I fast for around 36 hours every seven days.
I fast for a number of reasons. Firstly and most importantly, I figure it helps to develop my self-discipline. I regard will-power as akin to a muscle: The more you exercise it it, the stronger it gets. (That view, by the way, nicely dissipates the need for philosophical bloviating about the supposed paradox of akrasia, or weakness of will.) Granted, fasting primarily developes will-power over my appetite. Consistent with the athletic metaphor, however, I calculate that I get cross-over benefits in other areas of my life. Fasting thus arguably helps me get my work done on time, control my temper, avoid smoking meth, and so forth.
Secondly, fasting gives me a weekly opportunity to develop good psychological habits. Especially in the afternoons of my fast days, when my blood sugar starts to crash, I tend to feel cranky and depressed. Knowing the physiological causes of my mood, I have a sound basis to reason my way through it. Later, should I find myself feeling similarly, I can draw on the cognitive skills I've practiced while fasting.
Thirdly, to recur to Glen's theme, I hypothesize (not "theorize", Glen!) that fasting replicates conditions somewhat like those that obtained during the formative years of the human species. I doubt that our ancestors routinely ate three square meals a day. I thus reason that our digestive systems are adapted for occasional fasting, and that I may do myself good by recreating, in small measure, that environment. At the least, I don't seem to be doing myself noticeable harm. As far as I can tell from searching my long-running journal, I've been fasting once a week for at least fifteen years.
Fourthly, fasting reminds me how lucky I am to live in a world of plenty. Until very recently, starvation threatened almost all humans. It still threatens many. I appreciate my good fortune much more deeply when I feel the sharp pang of hunger.
Note that I do not include weight loss among the benefits of fasting. It may or may not have that effect. I eat rather more than usual on the day after my fast, so I am not sure whether on net I reduce my caloric intake. Granted, I'd say I'm quite trim for a 40 year-old, but perhaps my genes or my ardent exercise habits deserve the credit for that.
Why do I call this my "fast food diet"? Because the oxymoronic flavor of that phrase—"fast" food meaning not just speedily prepared food but also no food—nicely fits the rationale for my habit. It's a "diet" in the sense of a dietary practice, but not in the sense that I primarily aim to lose weight. Instead of "fast food diet," then, you might call it a "non-food non-diet."