With a meaty taste and few calories, portobello mushrooms make a healthy, flavorful alternative to hamburger patties and other traditional sandwich fillings. That said, a lean portobello sandwich can quickly turn into a calorie bomb if you pile on fatty toppings like cheese, mayonnaise or pesto sauce. Keep your lunch light by using reduced-fat cheese and topping your mushroom with fresh vegetables like lettuce, tomato and onion instead of the heavier alternatives.

One cup of grilled portobello mushroom slices contains just 35 calories. A hamburger bun has about 117 calories, while two slices of whole-wheat bread contain about 161 calories, so a plain sandwich may contain 152 or 196 calories, respectively. The calorie count will climb, of course, as you add toppings. A particularly rich recipe from "Fine Cooking" that includes fatty ingredients like mozzarella, Parmesan cheese and mayonnaise contains about 540 calories per sandwich.