Tuna is a very healthy choice, rich in protein, low in calories and high in omega-3 fatty acids. But when you order a tuna sandwich in a restaurant, you might be getting a very high calorie, fatty dish. Tuna salad filled with fattening mayo could contain as many as 720 calories and 43 grams of fat! Eat tuna at home, mixed with fat-free mayo and served on whole wheat bread. And when you eat a sandwich out, opt for lean turkey or roast beef instead.