<qa>  What is the optimal quantity to recommend?It is important to establish whether drinking extra water is indeed helpful for weight control and, if so, what is the optimal quantity to recommend, as this could have an important impact of public health and public health policies if it is indeed effective. Tap water is so readily available in this country that even if the improvement in weight maintenance or reduction is small, this will be a cost-effective intervention. If it is not effective, then it is important that people know this so that they can make an informed choice about buying bottles of water for their health. 