As you start losing weight, initial weight loss is mostly due to loss of water and so you need to drink more water to avoid the risk of dehydration. This will save your kidneys from doing extra work from processing tap water. Staying on one of the above diets for too long causes malnourishment and this can lead to serious health issues. Exercise is a sensible way to lose weight, but you do not want to overdo it, especially in the beginning.