Wondering why the same tried-and-true tricks you used to lose weight back in your teens and twenties no longer work after 40? From a biological point of view, the answer is pretty simple: If we maintain the same diet and exercise habits we began in our twenties, most of us will inevitably put on some weight — and the process of losing weight becomes much more difficult in middle age. Why is this? Our body composition, dietary needs, and hormones all shift, beginning for some as early as the mid-thirties. In order to maintain a healthy weight, certain lifestyle habits must be developed to counteract those changes. Here are five things you need to know about losing weight after 40.