There are tons of diets out there telling you to ditch certain foods. No fruit, no whole grains, no sweet potatoes, no pizza, no ice cream — what kind of life is that? Aside from making you unhappy, that kind of lifestyle isn't sustainable. Maybe you can ditch those delicious foods for a week or two, but soon the cravings will come on so strong that you'll say, "Forget this!" and have a huge binge. Of course that won't help you lose weight.
One of the most important things to take into consideration when either trying to lose or put on weight is output versus input. It is important to know the amount of energy your body is using every day, so that your intake fits the needs of one's personal weight goal. Someone wanting to lose weight would want a smaller energy intake than what they put out. There is increasing research-based evidence that low-fat vegetarian diets consistently lead to healthy weight loss and management, a decrease in diabetic symptoms as well as improved cardiac health.