A vegetarian diet doesn’t necessarily lead to weight loss -- especially if you eat out at restaurants often,” says Tallmadge. “A lot of times, the only vegetarian dishes on the menu are cheesy and fattening.” It can be hard to find restaurants serving soy burgers or beans and rice, and eating restaurant-size portions of pasta, rice, nuts and cheese could quickly add up to weight gain. According to Tallmadge, the desire to eat lighter meals that provide adequate protein is what makes many vegetarians change their minds and start eating fish.