Tuna is relatively low is calories. Typically, a three ounce serving of the fish contains only around 120 calories. However, most of the calories found within tuna salad is again due to the high fat content. Per gram, protein--which is found in tuna and other meats-- contain around 4 calories per gram, while one gram of fat contains 9 calories! This means that gram per gram, fat is over twice as dense in calories as the same amount of protein! It is no surprise then, since tuna salad is typically loaded with tons of mayonnaise, that it can be relatively high in calories.