Types of Intermittent Fasting
Intermittent fasting, as the name implies, consists of alternating ‘eating time’ with ‘fasting time.’ Everyone does this naturally as part of their living cycle, even if ‘fasting time’ is only equivalent to the time you sleep. Most people wouldn’t consider that intermittent fasting, however. So let’s limit ourselves to fasts that last at least 12 hours.
Now, as it happens, 12 hours roughly corresponds to the hours of darkness in tropical regions, where the difference between the longest day of the year and the shortest is much less than in more temperate regions. So for our early ancestors, and our chimpanzee cousins, fasting 12 hours a day was probably the norm.
I don’t know if any studies have been done on such short-term fasting, but if so I haven’t seen them. For the health and longevity benefits associated with intermittent fasting, most people assume we need at least 18 hour fasts, and most people on intermittent fasting regimes fast longer than that. But in the end, there are no hard and fast rules, because research into this is in its early stages, and very few of the possible permutations have been explored.
Alternate Day Fasting
The term alternate day fasting (ADF) was coined by scientists to describe feeding patterns (indeed, they often use the interchangeable term ‘alternate day feeding’) that they subject experimental animals to in their research. As a type of intermittent fast, this term is usually misleading and confusing, since most people who follow what they call alternate day fasts are really just fasting part of every day — but the fasting parts of two days come together to make one long fast of about 24 hours. So they talk of ‘eating days’ and ‘fasting days’ but mean 24 hour periods by those terms, not calendar days.
It gets even more confusing because some people really do have eating days and fasting days on an ADF regime, but they don’t mean by these terms what common sense might dictate. These people have what I call asymmetric ADF regimes — they eat more on some days, and less on the alternate days. Again, most of them are fasting about 24 hours at a time, but they do so in a way that leaves them with more food (eating days) on some days and less (fasting days) on alternate days. So the fasting day is really a day when they eat less food, and they still fast part of the eating day.
Other people on ADF schedules try to balance the amount of calorie intake each day. A typical schedule of this type might follow the usual American meal pattern: these people eat dinner on day 1, then breakfast and lunch on day 2, after which they fast until dinner time the following day. If they ate the same as before the fasting was begun, they would be reducing their calories by 50%, but few people can long withstand such extreme calorie deprivation.
A third type of ADF diet does not try to restrict calories at all, but only limits the timing of those calories, so they are consumed outside the 24 hour fasting period in every 48 hours. This is the kind of fast we describe on the ‘our diet‘ page. We eat about the same amount of food as we always did, having one large meal and one small snack each day. For convenience, we only fast 23 hours, which allows us to have our main meal at the same time each day (between 2:00 PM and 3:00 PM). We refer to this by the generic term intermittent fasting, or use the numeric notation (hours fasting / hours eating) or ‘23/25 fast’ for a more descriptive name. But neither of these is really adequately descriptive, since we have found that timing of the fasting/eating hours is highly significant in determining how easily one can stick to the schedule. Calling it ADF is confusing beyond measure.
Partial Day Fasts
Another popular technique for implementing intermittent fasting is to simply eat during a short time period at the same time each day. Bert Herring popularized this with his ‘Fast-5′ diet plan, which calls for eating between 5:00 PM and 10:00 PM each day. This was intended as a weight-loss diet — the author recommends making the eating window longer when you reach your ideal weight. It is doubtful that any health or longevity benefit would come from such a lifestyle, if the fasting period were reduced to less than 18 hours each day (beyond the obvious weight-loss health benefits).
We have been experimenting with this type of diet, which we refer to as 19/5 using the numeric coding described above. Our experience, as you can see from earlier posts on this blog, suggests that timing is critical in making such a diet comfortable — we found eating late in the evening very difficult, while shifting the eating window to 2:00 PM to 7:00 PM has made it much easier. Still, this is a weight-loss diet, and not a lifestyle. I think the longer fasts of our 23/25 diet are probably healthier, and certainly easier to maintain over the long term.
Another partial day fast that is somewhat popular is simply eating one meal a day, and fasting the rest of the time — 23/1 schedule. I will have to reserve comment on that until I try it (which I’m not to anxious to do — it sounds very difficult), but I think for most people it would be too arduous to keep up such a schedule even for a few weeks to lose weight, let alone as a lifestyle. Maybe if you believe suffering is good for the soul — but I prefer to enjoy my life.
Long Term Fasts
There are a variety of long-term fasts undertaken for ‘cleansing’ or for rapid weight loss, but these are rarely repeated at such regular intervals to make them a form of intermittent fasting. One exception is a kind of alternate day fasting where they really do use calendar days — so they eat nothing all of one day, which combined with eight hours sleep the night before and eight hours sleep afterwards, make a total fasting period of about 30 to 32 hours out of each 48. Again, this may be a great weight-loss diet if you can keep with it, but I can not see it as a viable lifestyle option for most people.
Other non-intermittent fasts are outside of the subject area for this blog. Religious fasting, juice fasting — or any other fast that includes caloric nutriment consumption during the fast, such as cleansing fasts, as well as medical fasting prior to procedures, and all other occasional fasts are fundamentally different from, and unrelated to intermittent fasting. We will be discussing non-fasting weight-loss diets and other health-related matters here, but always in an effort to elucidate the effects and benefits of intermittent fasting.
