Let’s say that dry eye is caused by lack of omega-3. If you eat enough omega-3, you’ll never get it. Here is a recently-discovered association with tuna consumption:
Tuna consumption [1 serving was 113 g (4 oz)] was inversely associated with DES [Dry Eye Syndrome] (OR: 0.81; 95% CI: 0.66, 0.99 for 2–4 servings/wk; OR: 0.32; 95% CI: 0.13, 0.79 for 5–6 servings/wk versus =< 1/wk P for trend = 0.005).
If tuna were a good source of omega-3, eating 5-6 servings per week would completely prevent dry eye. But it doesn’t. Which supports what I have come to believe for other reasons: oily fish, in the quantities most people eat it, is a mediocre source of omega-3. Even if you eat tuna almost once/day, you don’t get enough. To get enough omega-3, look elsewhere.