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I recently watched a BBC Horizon documentary that seems to argue that fasting is good for your general health and your mental acuity.

What scientific evidence is there for and against it?

The documentary talks about some experiments on rodents as part of the supporting evidence.

Personally, I feel like if I were to starve myself, then even if my mental acuity would increase, it would be directed primarily towards thoughts about food.

(Please add more relevant tags, if there are any)

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+1 for nice question. – Md. Mahbubur R. Aaman Apr 17 '13 at 13:43
up vote 10 down vote accepted

First, we should distinguish between "fasting" and "calorie restriction" because they are two quite different fields of study. Fasting is abstaining from food, while calorie restriction simply means consuming less energy-producing food such as carbohydrate and fat. There is some evidence on positive mental functioning on caloric restriction, particularly on the memory of elderly [1, 2]. However, a recent randomized controlled trial on 48 adults did not find any difference between the cognitive functions between the caloric restricted group and the comparison group [3]. The general conclusion is that larger and longer human studies are needed.

In animal models, much more have been done on restrictive calories and longevity and so far in the tested animals, there have been some good results. However, the one most commonly tested subject is just fruit fries [4]. We still have a long way to go until we can see some human data.

Now, fasting is a completely different story. In terms of research, scientists have not ventured into studying fasting as a healthy behavior but rather from the angle of school performance and breakfast consumption. Two studies that involve school children reveal a general benefit of NO fasting. One concluded that both overnight and morning fasting regimes can “included slower stimulus discrimination, increased errors, and slower memory recall.” [5] And another one describes a “gain some, lose some” effect that fasting hurts the accuracy during problem solving, but enhances immediate recall in short-term memory [6]. I haven't been able to identify similar studies done on adults; the results could differ in another age range.

I would suggest that informed caloric restriction is perhaps a better approach: fasting is associated with too much risk, and its benefits have been largely anecdotal. One particular risk is that prolonged fasting can increase the concentration of a chemical called ketones in our body. Ketones are formed when our body has to resort to rely on fat as the primary energy producer. Too much ketones in our body can cause loss of consciousness. However, this is an extreme outcome in famine, prolonged ketogenic diets [7], and poorly managed diabetes. Usual therapeutic type of fasting is unlikely to cause a healthy person to fall into coma. The reason of mentioning this ketone chemical is that the perceived “clear mind” in a lot of fasting participants could be an indicator of mild ketone intoxication. If you have not fasted before and would like to try it, consult some professionals (dietician, physician, and fasting treatment specialist) before doing so. Do not attempt fasting by yourself without noticing anyone, especially if you have metabolic diseases such as diabetes.

  1. Gillette-Guyonnet S, Vellas B: Caloric restriction and brain function. Current opinion in clinical nutrition and metabolic care 2008, 11(6):686-692.
  2. Witte AV, Fobker M, Gellner R, Knecht S, Floel A: Caloric restriction improves memory in elderly humans. Proceedings of the National Academy of Sciences of the United States of America 2009, 106(4):1255-1260.
  3. Martin CK, Anton SD, Han H, York-Crowe E, Redman LM, Ravussin E, Williamson DA: Examination of cognitive function during six months of calorie restriction: results of a randomized controlled trial. Rejuvenation research 2007, 10(2):179-190.
  4. Pijl H: Longevity. The allostatic load of dietary restriction. Physiology & behavior 2012, 106(1):51-57.
  5. Pollitt E, Cueto S, Jacoby ER: Fasting and cognition in well- and undernourished schoolchildren: a review of three experimental studies. The American journal of clinical nutrition 1998, 67(4):779S-784S.
  6. Pollitt E, Leibel RL, Greenfield D: Brief fasting, stress, and cognition in children. The American journal of clinical nutrition 1981, 34(8):1526-1533.
  7. Hartman AL, Vining EPG: Clinical aspects of ketogenic diet. Epilepsia 2007, 48(1):31-42.
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Just wondering: How come you know so much about this? Are you a nutritionist? – Oleg2718281828 Aug 14 '12 at 16:56
@Oleg2718281828 Yes, trained in nutrition and epidemiology. Nice to meet you. – Penguin_Knight Aug 14 '12 at 20:15
@Penguin_Knight while you are correct that too much ketones can cause loss of conciousness (through ketoacidosis), there is no way you can reach ketoacidosis through fasting. It is the end result of diabetes and thus of severe metabolic derangement. Ketones are fine, Inuit and Masai use them all the time. – w00t Aug 14 '12 at 20:33
@Penguin_Knight also, school children are not fully formed adults (especially regarding the brain and its relative energy consumption) and therefore you can't really extrapolate anything about fasting for adults. Other than that, nice answer. – w00t Aug 14 '12 at 20:41
@w00t Not exactly cited as ketoacidosis but as "coma" on page 37 table 3. I believe your >48 hr recommendation hits the nail at the head: it depends on how we define "fasting." If it's the therapeutic sort in terms of a couple days, I agree that one shouldn't fall into a coma; if it's the famine-like fasting, then physiologically ketoacidosis will happen. I revised the text to further distinguish it. Thanks! – Penguin_Knight Aug 14 '12 at 23:26

As a practical suggestion regarding all this talk about calorie restriction and ketosis, I suggest a drinking hot, low sodium chicken or beef broth nearly all day long. As a person who abstains on occasion from solid foods for religious reasons, I find that a steady intake of broth will keep the hunger pains away. And, oddly enough, I do find I'm more focused while I'm doing it.

That being said, I wouldn't do it longer than three days. And once you're off it, ween yourself back off it rather than gorging immediately. If you want more insight into this, do some internet research on cleanses -- I don't trust that they actually achieve any health benefits in and of themselves, but the people know how to readjust their diet after a fast/calorie restriction period.

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Not sure about scientific evidence, but I'm fasting and it's turned out quite efficient. It's not quite starving, and after a few days you get used to it.

Have a large meal at the start of the day. It should provide you with enough energy to last until the afternoon. Quite the contrary, you'll find yourself not even thinking about food and be able to maintain a routine throughout the day. I'd still recommend taking 15-25 mins off mid-day, maybe have a nap, do some stretching, or look at music videos to rest your mind. A heavy meal mid-day tends to make you a little sleepy afterwards; if you're fasting you don't have to worry about that.

Don't overindulge when breaking your fast. A regular dinner will do. I know a lot of people who eat buffets; they're usually starving the next day.. maybe because they get used to a high calorie intake. While I make do with a couple of burgers and I feel fine and energetic afterwards.

Of course, your mileage may vary. Some people do focus completely on eating. I find the most counter-productive thing about fasting is not having coffee, and waking up+cooking early for breakfast which messes up the sleep cycle.

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Ok, so your brain runs on glucose, except when it runs on ketones. In fact, your brain runs slightly better on ketones.

Ketones are made in a number of circumstances, but mostly just by fasting.

Furthermore, by fasting you are also definitely not eating a lot of carbohydrates. This means that your blood glucose level won't get a chance to wildly fluctuate and then crash, which is a possible effect of eating too much carbohydrate.

Add up a possible benefit and remove a possible issue and you get some chance of your brain working better with some fasting.

Finally, fasting doesn't have to mean not eating, in case you want to give this a try. See also intermittent fasting. Personally I often skip breakfast, having only a coffee with lots of cream. This means I do about a 14-16 hour fast and I'm therefore in mild ketosis during that time. (cream is mostly fat and very little protein/carbs - keeps you in a fasted state)

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