There's a good chance a can of tuna is sitting in your cupboard right now. Maybe two. It's the food you grab when you're tired, the lunch you make when there's nothing else.

Most of us think of that can as filler. Cheap protein. Something to stretch a salad.

But a large study out of Boston suggests that plain can, and the fish in the case at the store, may be doing a quiet job for a part of you that you rarely think about until it starts to slip. Your hearing.

To see why that matters, let me start with a bird...

It starts small.

You reach for a word you’ve used a thousand times, and it’s just… gone. You finish a page, turn it, and realize none of it stuck. You go to introduce your neighbor and freeze on her name.

Most of us blame stress, bad sleep, or simply getting older. And sometimes that really is all it is.

But researchers studying human brain tissue have uncovered something most people have never heard of: microscopic particles that settle in the brain and quietly build up, year after year.

They turned up in every sample tested. And people diagnosed with dementia had some of the highest levels of all.

You can’t feel them. You can’t see them. And most people have no idea they’re taking them in every single day.

The encouraging part? Once you know where they come from, cutting back is surprisingly simple.

==> See What Researchers Found Hiding in the Brain

In 1988, two teams of scientists in the U.S. printed their work side by side in the journal Science. Both had done something unkind to young chicks. They played them very loud sound for hours, loud enough to wipe out part of the inner ear.

Then they waited. Within days, new sound-sensing cells began to grow in the hurt spots. In about a week and a half, many were back. The chicks had healed their own hearing.

That finding shook the field. Birds can do it. Fish and frogs can do it. People can't.

In you and me, once those cells are gone, they're gone for good. No pill, no surgery and no food grows them back. So the only real plan is to look after the ones you still have.

Deep in each ear sits an organ shaped like a snail shell. It is lined with rows of very small cells. Each one wears a tuft of stiff little hairs on top.

Sound reaches you as waves in the air. Those waves make the fluid inside the snail shell ripple, and the ripples bend the tufts. When a tuft bends, its cell sends a message to the brain. That's how a voice turns into words you understand.

You're born with a set number of these cells. Loud sound, some drugs and the long wear of the years knock them out, a few at a time. The high notes tend to fade first. That's why soft sounds like "s" and "f" can blur before anything else does.

Nothing hurts while it happens. You just notice, one day, that you need people to face you when they talk.

Now back to Boston. Sharon Curhan, a doctor at Brigham and Women's Hospital, led a team that followed 65,215 women from 1991 to 2009. All of them were nurses. Every few years, they filled out long forms on what they ate. They were also asked whether they had trouble hearing.

Over those 18 years, 11,606 of the women said their hearing had slipped. The team lined those cases up against what had been on their plates.

Women who ate fish two to four times a week were about 20% less likely to report hearing loss than women who almost never ate it. That held after the team allowed for age and other habits.

Here's the part I find most useful. It didn't seem to matter much which fish it was. Tuna, darker fish such as salmon, lighter fish such as cod, and shellfish all leaned the same way.

Brigham and Women's Hospital, Boston (2014): Among 65,215 women followed for 18 years, those who ate fish two to four times a week were about 20% less likely to report hearing loss than those who rarely ate it. Each type of fish leaned the same way. Published in the American Journal of Clinical Nutrition.

Nobody knows for sure. Fish carry a special kind of fat that your body can't make much of on its own. In the study, women who took in more of that fat also reported less hearing loss.

One idea is that this fat helps calm the slow, low-grade swelling that wears on fragile cells over the years. Another is that it helps build the thin outer skin of each cell. Both are theories. No one has watched it happen inside a living human ear.

This was a study that watched people. It can't prove the fish did the work. Women who eat fish twice a week may also eat more greens, move more or see the doctor more often.

The women judged their own hearing, too. Nobody tested their ears in a sound booth. And since every one of them was a woman, the study says nothing direct about men.

Fish also won't bring back cells that are already lost. At best, it may help slow a loss. It can't undo one.

Canned light tuna, canned salmon and sardines all count as a fish meal. They keep for months, cost less than fresh fillets and need no cooking. Drain, flake and fold into a salad, or pile onto whole grain toast.

Most fish sold in the U.S. is low in mercury. Salmon, shrimp, cod, tilapia, sardines and canned light tuna sit on the government's "best choices" list. Swordfish, shark, king mackerel and bigeye tuna are the ones to skip.

Food is one half. Loud sound is the other. Fall means mowers and leaf blowers, so keep a pair of foam earplugs next to them. If the person beside you can hear your earbuds, they're up too high.

Those chicks in 1988 could grow new hearing cells in under two weeks. You can't. That makes the ones you were born with worth some care.

The Boston numbers don't promise anything. What they point to is a habit that takes a can opener and ten minutes, twice a week.

So open a can this week. Set a fish dinner for the weekend. Your ears have been working for you every day of your life. Feed them like it.