I've used Slimfast, but not as a strict diet like the instructions say. If you just drink Slimfast, you'll feel weak and tired quickly. I drink it  in the morning because it's something I can grab quick and go. Sometimes I have it for lunch also and as a bedtime snack. But you need at least one good nutritional meal a day. It does keep me from getting hungary for about 4 hours. Also, as you age, the only way to really loose weight is to exercise. Not eating will not cause you to lose weight. Your body compensates for that.