This is a complicated question and I am not sure there is a definitive answer. The fasting experts we generally use for reference here do allow for very small amounts of lemon juice in water, mostly for the taste. I am not sure the alkaline ash produced by such a small addition of the juice would be enough to affect the natural acidity produced by ketosis. The caloric issue is not in question either. The main problem would be the carbohydrates and minerals contained in the juice. While neither one is bad, the question is how would they affect the fast? Ideally, no carbohydrates or any other food source should be ingested during a pure fast. The body need to depend on ketones for its primary fuel source. Carbohydrates would be the first fuel of choice to produce glucose. This might result in muscle wasting since the body is still trying to metabolize glucose and goes after any source including glycogens stored in the muscle tissue. Of course protein needs to be avoided during fasting to prevent stress on the kidneys and too much acidity. (Fuhrman has some excellent points in the dangers of being in ketosis while still eating high protein foods in "Fasting And Eating For Health")
Another issue that really needs to be addressed is taking vitamin and mineral supplements during a water only fast. While, once again, a small amount of lemon juice would probably not cause adverse effects, vitamins and minerals should not be taken during a healing fast. When fasting the body not only begins to break down adipose tissues for fuel, it also begins to leach out minerals. The first places called upon to release mineral deposits are extraneous tissues like abscesses, fungal growths, and deposits in joints. This is one reason that many people get relief from arthritic conditions during a fast. The body also breaks down fatty deposits in blood vessels, along nerve paths, and on the outside of organs. This helps reduce inflammation in the body. I don't understand the whole process of reducing inflammation, but it is supposed to be one of the great benefits of water only fasting. If supplements are taken, then the body will not catabolize accumulated wastes and unnecessary tissues for their mineral content and the body will not experience the same healing potential.
The regulars on this board cannot, in good conscience, recommend unsupervised water only fasting for more than ten days. Experienced fasters may be able to go twenty one days without supervision since they have experienced considerable detoxification and know what to expect from their own body. Anyone who wants to go longer is putting themselves at risk. While we can measure blood pressure, blood Sugar
levels and to some extent ketosis and blood ph at home, people on extended fasts need careful monitoring of their blood and electrolyte counts to be sure they are still fasting in good health. The best way to begin and experience water only fasting is to start out one day at a time, after a while, try three days. That way you can safely detox and learn how your body reacts to fasting. As we've stated over and over, water only fasting is not a "quick fix", nor is it a good permanent weight loss plan. Water only fasting allows the body to heal itself through cleansing, detoxification, and catabolism of extraneous tissues. This all takes time and patience. It must be done correctly and safely or fasting could prove to be detrimental to health instead of healing.