Hi Sadhana, I heard that coffee and tea can have a drying effect on the bones and joints due to the tannins, acids and caffeine in them. People with arthritis are generally recommended to refrain from drinking and eating acidic foods and drinks containing caffeine or tannins because of the adverse affects. It is said to dry out the joints and bones more. Especially if there is not enough water taken in to counter the effects. I suppose on a general level tannins are used in tanneries to dry out the animal pelts. So rather than be dehydrating in the way that we need more fluids to rehydrate, it is more detrimental as above. It also takes the body 6 hours to detoxify one cup of coffee from the body so it is easy to build up a dependency and withdrawal symptoms from it.