Caffeine acts as a diuretic and therefore makes you lose water which is potentially dangerous as dehydration can weaken muscles including your heart. Dehydration of muscles also affects your body's ability to utilise fat, hence decreasing your metabolic rate. The scales may say you have lost weight, but in reality no fat has been lost. If consuming a lot of coffee and tea it is advisable to counteract the diuretic effects of the caffeine by drinking an equal amount in water. Dehydration makes you hungry and can hinder weight loss.