Tomatoes can help you lose weight. If you are on a sensible diet and exercise plan, build lots of tomatoes into your everyday eating. They make a great snack and can be used to “bulk up” salads, casseroles, sandwiches and other meals. Because tomatoes contain lots of water and fiber, they are what Weight Watchers calls a “filling food,” one of those foods that fills you up fast without adding a lot of calories or fat.