-Denethor Goes Mad From Lead Poisoning
First, a brief history lesson. The tomato was introduced to Europe in the early 1500's when the Spanish conquistadors brought it back from Mesoamerica. Because tomatoes are so acidic, when they were placed upon European pewter plates or cooked in pewter pots, the acidity would cause a greater amount of the lead in the pewter to leach into the food. The resulting increase of lead poisoning lead to the tomato being blamed for causing sickness and death.
In The Return of the King, as Pippin sings for Denethor as he feasts, he is explicitly shown eating tomatoes from a pewter platter. He is also shown drinking from a pewter cup.
I believe this was very intentional by Peter Jackson to subtly imply that lead poisoning was contributing to Denethor's unbalanced mental state.
