If one was to engage in an extended water fasting of longer than 3 days (maybe a week or 2) what effects would there be if:
1. You supplemented with a multi-vitamin tablet each day?
2. You supplemented with BCAA (Branch Chain Amino Acids) 3 times a day?
The BCAA I am thinking will help avoid muscle loss and yet I know very little about Physiology and so would welcome your opinion on whether this will adversely affect the benefits of fasting for weight loss.
Thank you for the effort you have made in providing this website.
Overall, it's not recommended that you take suppplements during a fast, and especially not during a water fast, where a complete rest for the digestive processes is desired. Taking a supplement would then require the production of certain enzymes to digest it. And most multi-vitamins have various fillers, and sometimes even the vitamins in them are indigestible and/or not assimilable by the body, creating a burden on the body to dispense of these materials.
This would apply to the branched chain amino acids also. Taking them in pill form, you'd want to know if the branched chain amino acids are naturally-derived or laboratory-derived. Are they assimilable by the body? Are there fillers or other additives?
To my way of thinking, it is always better to get our necessary nutrients from food sources rather than from manufactured pills. Meat, eggs, and milk are all good sources for these amino acids. While I wouldn't recommend the meat during any fast, eggs or milk could be used (pasture-raised, antibiotic-free eggs and raw milk). It would no longer technically be a water fast, but a water and milk fast or a water and egg fast.
My understanding is that athletes like to take the amino acids both right before a training session and right after, for the added protein/muscle synthesis, or in your particular case, to avoid muscle loss from fasting. So you could do your modified water fast by staying on just water until right before training, when you would have, say, a glass of raw milk. And then AFTER the training session, you would have another glass of raw milk, then just water until the next day's training.
These are just my thoughts and I'm no expert on fasting for athletes. There are some websites devoted to such though, run by professional trainers. You can search for "weightlifting and fasting" or "body building and fasting" for those websites.
If weight loss is your goal, I'd say a little supplementation of egg or milk to the water fast wouldn't severely affect your results. But, then, I have to add, losing weight by fasting or by any other means can be tricky because of our emotional and mental patterns. One can never be dependent on losing weight by fasting. See Using Fasting for Weight Loss and Fasting to Lose Weight for more information on this.
Also, I wouldn't jump right into a longer fast of this nature. I would suggest you take it slow and with an attitude of experimentation. Just try it for a day and see how you feel, look for your particular body's reactions, keep a journal perhaps to note the changes you feel.
Good luck on your journey!