Ruling 1552

If a fasting person who is aware of the fact that he is fasting intentionally eats or drinks something, his fast becomes invalid, irrespective of whether the thing he ate or drank was something normal – such as bread and water – or not – such as earth and the sap of a tree – and irrespective of whether it was a little or a lot. In fact, even if one takes a toothbrush out of his mouth and then puts it back into his mouth and swallows the moisture, his fast becomes invalid unless the moisture on the toothbrush was so little that it could be said to have disappeared in his saliva.