This post has been a long time coming, and I've posted bits and pieces of it in various threads, but the amount of misunderstanding, misconceptions, and misinformation out there finally compelled me to put metaphorical pen to metaphorical paper here.
First thing first, just to get it out of the way, intermittent fasting is not a miracle cure for all that ails you. I think that there is a lot there to recommend it to people, I practice my version of it, and have been at it for coming on three years now, and I fully intend to keep doing it. I do recognize, however, that it cannot be all things to all people. If anyone tries to tell you that one size fits all, then you are dealing with a blatant charlatan and you ought to govern yourself accordingly.
What is intermittent fasting?
There is no clear cut definition, unfortunately, which contributes to the misunderstandings. Wikipedia maintains that "Intermittent fasting (IF) is a pattern of eating that alternates between periods of fasting (usually meaning consumption of water only) and non-fasting." This definition is less than useless, as it practically suggests that eating the traditional three meals a day, and drinking water in between meals, which constitutes a pattern of eating alternating between fasting and non-fasting, is intermittent fasting.
For my part, I suggest that we adopt a pragmatic definition: "intermittent fasting is a pattern of eating that involves a greater interval between meals than generally culturally prevalent." So, if you habitually skip breakfast, you are fasting intermittently by our definition. Skip breakfast and lunch? Great, you fit the bill too.
One thing ought to be glaringly obvious in this definition due to its conspicuous absence: nobody said anything about calories. Should you choose to eat breakfast, lunch, and dinner all at 7:32 PM EST every day, then you are intermittently fasting by this definition. With that out of the way, we can dispell the first myth of IF - it is not starving. Starving yourself does fit within the definition, but it is a degenarate case. If you have some masochistic need to starve yourself and then call it IF, you certainly can, but the definition in no way requires it.
Why would you fast intermittently? The Intuitive Answer.
Going back to the intermittent fasting definition, at some point someone realized that to a large extent in modern society meals and meal times are culturally rather than biologically determined. It didn't always use to be that way, and we have the linguistic archeological record to prove it. The word breakfast (as said by Sean Connery, the best of the Bonds in my opinion) used to define a functional meal, the one that broke your fast, even if that were daily at 7:32 PM EST. In modern society, the word has been co-opted and transformed from a functional definition into a cultural one where breakfast is your morning meal, generally consumed before heading off to work. I expect that this arose as a consequence of industrialization given that factories thrive on predictability and conformity. It is not clear to me that people thrive on conformity to the factory whistle, a point most eloquently made by Connery's Bond who eats because he is hungry ( a biological imperative ), not because the time is appropriate for the meal ( a cultural imperative ). The question is then, what is an eating frequency that is compatible with a biological imperative?
Another intuition that helps us answer the frequency question is the realization that there is a continuum of possible feeding schedules, and most of it represents ridiculous extremes. Assuming we are aiming at a 3000 kCal diet, and sleep for 8 hours a day, we could eat 0.05 kCal for the 57600 seconds that we are awake. That is, we could literally be constantly eating, albeit a miniscule amount of food each time. This represents a practical upper bound on how frequently one could eat. For a practical lower bound, consider attempting to eat a week's worth of food at one sitting, or 21000 kCal at one meal. Assuming that our diet consisted of the most energy dense food we know, dietary fat, we would need to eat 2.33 kg, or 5.1 lbs, of fat at this meal. Never mind the practical aspects of actually digesting this amount of fat, just the thought of attempting to do this is enough to revolt me. So, it would seem that a reasonable continuum has once a week at one end, and once a second at the other, with the true value probably being much more narrowly banded. I think enough of us know someone, or perhaps we are that someone, who does not eat breakfast because they do not feel hungry in the morning, even though they do not call that IF. Given this it is not much of a stretch to make the intuitive leap from not eating breakfast, to just eating dinner, as being a reasonable bit of the meal frequency continuum.
Why would you fast intermittently? The Appeal to Authority Answer.
Quick, name the first organized religion that comes to mind, and I'll bet you that they have a tradition of fasting. Whether Christian, Buddhist, Hindu, Muslim, or Jew, people have been fasting for better or worse for thousands of years now. I really don't have much to say about a religious justification for fasting, but I will say that in Islam, we have a population that fasts every year for the month of Ramadan, eating and drinking nothing between sunrise and sunset. Studying Muslim fasting populations allows scientist great insight into the effects of fasting on the human body, which brings us nicely to:
Why would you fast intermittently? The Scientific / Metabolic Answer.
One interesting aspect of fasting during Ramadan is that people tend to compress their eating schedule as the same quantitiy of food is ingested, but this is done in what amounts to one large meal in the evening. So, an interesting question to ask would be whether a greater interval between meals had any metabolic effect. It turns out that if you fast during Ramadan, you can expect:
- A decrease in LDL cholesterol, and a concomittant increase in HDL (as found in this study, as well as this one).
- A decrease in serum triglycerides ( same source as above. )
- No change in your plasma cortisol concentration.
- A pronounced lowering of insulin during the daily fast portion "favor[ing] a predominant lipolytic state."
- No change in thyroid hormone levels (T3, T4).
- Lowered levels of homocysteine and C-reactive protein ( indicators of inflamation ).
- You may experience some changes in body composition, but the studies are contradictory, although you are unlikely to gain weight.
Like I said, fasting during Ramadan has been extensively studied, and I could go on, but I think you get the point. I have linked to all of the studies, but unfortunately, only some of them provide the full text. The remainder are just abstracts, but worth reading, all the same.
In the above list, pay close attention to points 3, 4, and 5 because they relate to the hormonal impact of Ramadan fasting, and basically allow us to conclude that metabolically speaking, as far as the body is concerned, it's all business as usual, if not better. I make this point because invariably, when discussing intermittent fasting in the form of only eating one large meal in the evening, someone will start frantically beating the "starvation response" drum. You can politely ask them how eating a day's worth of calories in one meal can possibly constitute starvation, and then point them in the direction of the studies cited in 3, 4 and 5.
For me, this is absolutely fascinating because all you have done is monkeyed about with the timing of your meals, and derived a pretty good benefit from it for your effort. It is almost as though you may have gotten something for nothing. Almost, but not quite. Even the most basic understanding of the digestive process must include the realization that not all the things you put in your mouth are good for you. Consequently, a large amount of digestive energy is expended in performing triage, separating the desirable aspects of our diet from the undesirable, assimilating the good bits, and disposing of the bad. It would seem plausible that the observed benefits of Ramadan fasting might be due to the fact that the body is not continuously engaging in this metabolically taxing process throughout the day, but rather doing it all in one concentrated batch.
In the interest of full disclosure, I do not follow Ramadan style fasting insofar as I find the proscription on drinking during the day too strict. I understand that this means that the conclusions reached in the studies I referenced therefore may not apply to me, as the observed effects may be due to the drinking behaviour in conjunction with the eating. I don't believe that this is the case, but my beliefs do not seem to affect objective reality, much to my consternation.
As I think I am nearing the 10000 character limit for a post, and as this seems a good place to pause, I'll let you digest and give my cramping fingers a break.
Stay tuned for part 2 ...