As the previous poster replied, it's related to https://en.wikipedia.org/wiki/Ketosis
I don't know if you can really get rid of the taste, I'd be curious to know of course; but you kind of get used to it really.
I personally don't mind taking fruit juice, as it does give a small energy boost, especially in the morning.
Not everyone gets it. Fasting puts you in nutritional ketosis as does ketogenic / very-low-carb dieting. But the metallic taste is usually a temporary and harmless keto-adaptation symptom. The body's ketone production and consumption aren't fully balanced at first for quite a while, during which some people flush out most excess ketones in urine while others breathe out more of them. As you keto-adapt (takes from a week to a few months depending on individual), ketone bodies (a byproduct of fat oxidation) are fully utilized in place of glucose by more and more tissues/cells over time, until balance is achieved.
The metallic taste is excess ketones. If your body is not used to using ketones, some will be excreted in the urine and saliva.
If you really goose your ketogenic metabolism, your body gets better at using ketones and the taste will go away. You should also be sure to eat/drink extra salt during a fast or ketogenic diet due to water loss.
