Some folks confuse renin with rennin (aka rennet) which is involved in the curdling of milk in the stomach. Adults have a lot less rennin than young kids. This curdling of the milk slows the passage of the casein from the stomach into the small intestines, and without it you can get poorer digestion of the milk and associated gastric problems associated with milk consumption. However, if you are already drinking lots of milk and don't have the shits or other noticeable problems then you needn't worry about it. Any sub-Q fluid retention from milk drinking goes away within days of discontinuing the milk, and this is a good idea if you need to look absolutely dry and ripped for a show or photo shoot. Otherwise it's just a little extra fluid under the skin which does not in any way affect your ability to lose fat or retain muscle.