Sandwich architecture (a phrase once used by Ted Allen on an episode of Food Network's "The Best Thing I Ever Ate") emphasizes the importance of balance in each layer of a sandwich. When it comes to tuna salad sandwiches, the type of bread you use is quite essential.
The top bread choice for tuna salad is multigrain, mainly because it's hearty enough to hold ample tuna salad in place. Additionally, multigrain bread is made with grains like oats, barley, and flax, meaning it’s nutrient-dense and high in fiber.
Multigrain bread is fairly mild with a touch of sweetness, which is enhanced by the nutty flavor of the incorporated grains, so it strikes a better balance than simple white bread. However, its light tasting profile won't overpower the flavor of the tuna, like a sourdough or rye would.