I modelled my recent 4-day fast on Dr Michael Mosley's Horizon programme [0] in which he eats nothing but a miso soup (under 50 calories) per day and drinks nothing but water. Miso soup has the added benefit of sodium and other electrolytes without any caloric impact.

I personally supplemented my water intake with peppermint tea as that has an incredibly low nutritional footprint and does not contain any extra substances such as caffeine. It's surprising how 'clinical' drinking cold water can feel after a couple of days.

Others here are saying absolutely water only. It would be really interesting for me to find out the effects of these before I fast again, as I am likely to do before the end of the year.

I also found the below article really helpful in giving me an overview.[1]

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[0] - http://www.bbc.co.uk/programmes/b01lxyzc

[1] - http://www.yogicwayoflife.com/three-day-water-fast-and-its-b...

Can you tell us more about the change of taste of water (or even other things post fasting) ?

I am not a doctor.

On days I don't eat, I supplement various vitamins and electrolytes to minimize risks of refeeding syndrome[0] and heart problems[1,2]. I don't know how bad the risks are in practice for these for short fasts (I've only gone up to 3 days), but I am sufficiently paranoid that I supplement anyway. Supplementing electrolytes also means I don't get headaches, restless leg syndrome, or (as much) weak feeling in muscles, all of which I used to get when I tried unsupplemented water fasting.

I also try to break the fast with skim milk or yoghurt so that I'm not phosphate depleted (refeeding syndrome again).

For what it's worth, my supplement regime amounts to: a generic A-Z multivitamin, 800mg potassium chloride, 300mg magnesium citrate (it seems to be important that it's citrate for bioavailability reasons), 800mg calcium, and a B-vitamin complex (as I remember it, vitamin B1, or thiamine, is important when refeeding, for reasons I can no longer find).

[0] https://en.wikipedia.org/wiki/Refeeding_syndrome

[1] http://ajcn.nutrition.org/content/34/4/453.short

[2] http://www.ncbi.nlm.nih.gov/pubmed/6859052

When I was fasting (about 2.5 weeks), I ingested nothing but water. I don't think you need to be worried about running out of essential nutrients when you are only fasting for 3 days. Personal experience, of course. No empirical evidence.

I would definitely worry about not eating for 15 days.

Ingesting anything will ruin the fast. Even brushing your teeth becomes tricky, as toothpaste can ruin everything. I ended up using a salt water solution (and an electric tooth brush) to do the job, but in hindsight, I could've used something like ToothSoap. It's extremely important to rinse the mouth thoroughly, so you don't swallow anything.

It is possible to run low on vitamins (depending on how long you go). If it's just 3-7 days, then it doesn't matter as much, but some people go a full 28 days (you can theoretically go about 40 days without causing problems, but as it will take another 40 days to break the fast, it proves to be too much for someone's first time (with 3-7 days working better)).

The step in a water fast that does most people in is the breaking of the fast. The key to a successful water fast is properly breaking the fast. It can be even more difficult, as while any hunger usually goes away within the first day or two, when the fast is first broken (with your first taste of 50% watermelon + 50% water, for example), it will kick the digestive system back into action, and you may easily become overwhelmed by an intense hunger. If you don't have the willpower to ignore this hunger, then you may end up eating something prematurely or eating too much.

The solution that works for most is to either buffer up on vitamins and minerals in the weeks leading up to the fast (which also ensures they don't have any hidden deficiencies), or to start with a juice fast (vegetables blended in a Vitamix, for example).

Do you have any sources for "ingesting anything will ruin the fast"?

and whats wrong with eating too much - I understand that you say breaking the fast is important ... but why?

I'm not clear on the details of why, but I know if the fast isn't properly broken, then the body can react very poorly. I suppose it needs time to get back in the habit of processing food. The main thing to remember about a fast is that it will take as long as you fasted to break it (unless it's 24 hours or less). So if you water fast for 3 days, you'll need another 3 days to ramp back up to normal eating. If you juice fast, then it's 1/3rd of the time (ie, 1 day for a 3-day juice fast).

I've read about people ending up with allergic reactions, swollen joints, pain, and many other issues from improperly breaking the fast. Also, me not remembering or being clear on the details doesn't mean they aren't clearly laid out on sites/articles about this topic. That is, you can search online for a bit, and it should clear things up for you.

I'm not a doctor, but see https://en.wikipedia.org/wiki/Refeeding_syndrome

I don't know any of the science behind it, but it's my understanding that during long periods of not eating, metabolism gradually slows to a crawl to allow maximum duration from the amount of 'energy' available in the body.

I'd wager it gradually ramps back up again once food is reintroduced, and overeating during this period could probably lead to some pretty hefty weight gain.

People died of eating too much when the prisoner camps where liberated at the end of WWII.

IANAD.

My understanding is that you are avoiding calories. More specifically, you are avoiding the bodily processes (especially insulin response, etc.) that accompany digestion.

So water, coffee and tea fit into that. Coffee or tea with milk, however, does not.

I am not sure about the vitamin supplements. Even if they are zero calorie, they have to be digested, etc., in a way that goes beyond coffee and tea ...

I would just wait until lunch/dinner, but I only IF for 16-20 hours at a time.

I used to have coffee and tea while water fasting but not anymore. While coffee helps with the initial headaches and may provide more energy in the following days, the fact that caffeine is a psychoactive drug started to worry me when doing the fasts. Surely, I don't think it causes any damage and by all means, have some if it helps you. But I've noticed it affects my primary goal with fasting: putting my system in a more natural state than before. It took me a while to remove it from my fasting practice.

I did water, a zero-calorie powerade, and a few pinches of salt. Not having any calories is the goal.

exactly what I thought when I read the article.