Types of Fasting
What is prolonged fasting? The term isn’t especially precise, but it is generally taken to mean a long (5+ days) period with extremely low calorie intake. There are several different variants (this list is non-exclusive):
- Water Fast: As the name might suggest, a water fast consists of ingesting nothing, except for water.
- Standard Fast: The most common form of fasting involves eating essentially no calories, but slightly expands on the water fast to allow the intake of non-caloric beverages (for example, black coffee and unsweetened tea), along with electrolytes, often in the form of broth.
- Fat Fast: The fast fast is more expansive than a standard fast, and allows for the intake of <500kcal fat per day, but no other macronutrients. This fat intake is generally understood to be spaced out in time, so as not to intake more than 100kcal or so at a time (about a tablespoon).
- Protein-Sparing Modified Fast (PSMF): This is similar to the fat fast in that food intake of <500kcal is allowed, but in this case, the permitted macronutrient is protein rather than fat. Again, this is to be spread out throughout the day, with less than 20g ingested at a time. PSMF is useful if you expect to perform heavy exercise and are worried about conserving muscle.
What to Expect
It’s my understanding that different people’s experiences while fasting have a pretty broad range. Some really enjoy it, while it is absolute torture for some. Given that, anything I say about what you can expect while fasting should be taken with a heaping spoonful of YMMV (this does not break the fast). However, there are some generalities. Most people seem to agree that the first day will be the most difficult, especially the period around 16 – 30 hours after the last pre-fast meal. You’ll likely feel hungry, irritable, and lethargic. The hunger should go away fairly quickly, surprisingly, but the feelings of low energy and irritability will likely stay for some time longer. At some point it is likely that you will start to experience periods of hypovolemia (low blood volume) and hyponatremia (low sodium), which manifests as dizziness, lightheadedness, headaches, increased heart rate, or your skin turning purple. Eventually your body will hit a turning point of sorts where it produces the energy you need via fatty acid metabolism, rather than through the glucose pathway. Once this happens, things get much better! Unfortunately, you may have to slog through a few days of feeling shitty to get to that point, but that is life–you have to work for good things, sometimes. At this point, your energy levels should be pretty good, although generally not quite where they would be on a normal day. Your hormone balance will have shifted noticeably by this point, and you can expect your mood to be quite good. You may find periods of time when you are very happy, expansive, or friendly. You may feel that you have lots of motivation. By now, you’re not ever really hungry, although food may become very fascinating to you. Don’t worry, spending your (unused) lunch hour making a comparative analysis of kimchi recipes is entirely normal. The trickiest part about fasting is actually doing it. Some general guidelines to make things easier:
Things to Make Your Fast Easier
- DO NOT FAST WITH LOW BODYFAT. IANAD(octor), but for a regular, healthy person, fasting for a week or so should be fine if care is taken. This is NOT the case when you have low bodyfat. If you have sufficient fat stores, your body will adapt to make up for the loss of caloric intake, and it won’t do anything terribly drastic. If, however, you don’t have sufficient fat storage, your body will do all sorts of damaging things to make sure it has calories to function–things that can have lasting, negative, consequences. Don’t do it. If you really want to fast and you have low bodyfat, gain weight first.
- Get into ketosis before you start the fast. Even better, become keto-adapted. There’s a wealth of knowledge out on the internet about how to do both of those, but I like to cut out carbs almost entirely (I leave a small amount of fibrous vegetables in my diet) for the week or so immediately preceding the fast. I wrote above that the worst part (in terms of subjective experience) of the fast is the period in which your body is transitioning from the glucose metabolism to fatty acid metabolism. If you are already getting your energy from fatty acid metabolism when you start fasting, the initial period of discomfort is much shorter, and much, much less painful. In terms of improving your quality of life while fasting, this is probably the best thing you can do.
- The second best thing you can do to make fasting easier is to have already fasted. Fasting gets MUCH easier the more you do it. My experience fasting nowadays is a world away from when I first tried it. Not only does your body adapt physically in ways that make fasting easier, having a successful fast under your belt turns fasting into a known experience. It’s much easier to do something that you have already done. If you have no experience with fasting, don’t jump into the deep end and try a week long fast. Start slow, and do a few single days fasts. Then you might try fasting for a long weekend for three or four times before you attempt a prolonged fast.
- Avoid artificial sweeteners. While artificial sweeteners are non-caloric, not only can they trigger an insulin response in some people, they have been shown to change your gut bacteria composition (and therefore your hormone balance, and your response to fasting). Also, in general abstaining from anything is easier the less you are reminded of it. Just as it is harder for a person who has recently quit smoking to abstain when someone nearby is puffing on a cigarette, it is harder for you to abstain from eating when you put something in your mouth that your body understands as a delicious sweet-tasting treat.
- Manage your electrolytes (particularly the big ones–sodium, potassium, and magnesium). It can be a little tricky to get the dosing right, especially since your body is using and excreting minerals in different amounts than it otherwise would, but if you do you can ameliorate some of the negative effects of fasting. I can’t give any general guidelines, but I think over time, if you listen to your body you can pick up on the warning signs of low levels of any of these (look for: dry eyes, involuntary muscle twitches, cramps, and elevated heart rate). Most people use bouillon, but you can use table salt and any of the various forms of potassium and magnesium you can get online.
- Get lots of low intensity cardio (aka walking), but do not do any high intensity cardio or heavy resistance training. Just treat your fast as a deload week. Low intensity cardio is great because it will use up your glycogen stores (in the early part of your fast) or stimulate fatty acid metabolism (in the later part of the fast), but causes no damage to your muscles. Normally, muscle damage is a good thing in the context of working out, as your body treats it as a hormetic stressor and builds the damaged muscle back better than before. However, during fasting, you don’t have any dietary protein intake, and so the body has to take protein from elsewhere in the body.
- Schedule your fast for a time when you don’t have a lot of important or stressful things happening in your life. You are not going to be at 100% physically, and while you may at times be pretty on point mentally some of the time, it’s best not to count on being that way. Don’t plan on fasting while you are moving out of your house, or wrapping up a big project at work.
- Although I’ve never had trouble with this, it seems to be very common to have stomach trouble for the first meal after fasting. People suggest different foods you should eat when you break your fast, but I expect this is pretty individual. Generally, try for something pretty inoffensive to the stomach–high in fiber, not too much fat, and no spicy flavors. Also, plan your meal beforehand. If you start trying to figure out what you are going to eat a few hours before you break your fast, you are going to end up with an entire grocery store on your plate.
- Expect and plan for the post-fast rebound. Fasting is a pretty poor way to lose weight long term, simply because the body will trigger some pretty intense cravings once you start eating again. It’s OK to eat significantly more calories than normal while breaking your fast (although be aware that your stomach will have shrunk), but you also want to keep a lid on things. It’s not a bad idea to plan out your meals for three days following the fast.
- Just relax, and go with the flow. Accept that there will be times when you feel poorly, or lethargic, and times when you feel calm and pleasant. Be aware that changes will happen not only during the fasting period itself, but for a few days afterwards as well. The mental states you can get into when fasting can be pretty far outside of your normal range of experiences. That can be jarring if you let it, but it can also be a really exciting window into another way to view life, if you let it. It’s all a matter of perspective.
Why Should You Fast?
Why fast? There are any number of reasons, but I can only really speak for myself. I think the things that would motivate people to undergo a prolonged fast are pretty specific to that person. That being said, in my case:
- Health benefits. Unfortunately, the literature on prolonged fasting in humans is pretty sparse, but there is a fair amount of evidence for the effects of prolonged fasting in animal models. Additionally, there is evidence about related practices (time restricted eating, ketogenesis, calorie restriction, etc.) that we can infer things from. Some of the benefits include:
- Fasting is a cancer prophylactic
- Greatly increased rates of cell apoptosis, autophagy, mitochondrial biogenesis, and mitophagy
- Greatly increased stem cell production
- Improved metabolic functioning and flexibility (including insulin sensitivity, blood glucose levels, and number of mitochondria)
- Improvements in the immune system
- Improvements in a whole host of biomarkers related to cardiovascular health and inflammation reduction (including LDL(p), C-Reactive protein, TNF-alpha, and IL-6)
- Reduces stress
- I find that there are generally long periods of time in which I am very calm, and relaxed. I feel very equanimous; events that would normally cause me a lot of stress seem to just pass me by, and I find that I am able to make difficult decisions clearly and rationally. I don’t know the exact mechanism for this, but it is often pretty pronounced and seems to happen fairly regularly on long fasts.
- There’s something to be said for willful deprivation of anything you take for granted. If you have three square meals of good food every day, you become blind to the value of food. It’s just how the mind works–because attention is a scarce resource, the mind prefers to allocate attention to new stimuli and ignores what it has seen before. By foregoing eating for some time, your mind doesn’t take food for granted in the same way. The Stoics practiced this principle by periodically eating gruel, wearing only a loincloth, or sleeping outdoors, in order to better appreciate food, clothes, and shelter. Also, there is always value in intentionally doing something difficult. It’s good for you!
