Mayonnaisse Do: Light Mayo
I don't know about you, but I can't tell the difference in taste between regular mayonnaisse and light mayonaisse. But "real" mayonnaise contains twice the calories and fat than light mayo.
Mayonnaisse Don't: Real Mayo
A one tablespoon serving of real mayo contains 90 calories - 100% of which are from fat! Light mayo, on the other hand, contains 35 calories per one tablespoon serving, 30 calories from fat. Do the math: that's 60 fewer fat calories.
Especially if you're going to make a tuna sandwich with all the fixin's why use real mayonnaisse? You'll never notice the difference in taste by using light mayo. And you'll save quite a bit of fat and calories.