A 2021 scoping review on the role of nutrition in the development

and prevention of age-related hearing loss screened thousands of papers

and concluded that “Diets rich in saturated fats and cholesterol

have deleterious effects on hearing that could be prevented by lower consumption.”

The case of the Mabaan I noted before makes for a compelling story,

but on what exactly are they basing their conclusion?

It's true that in laboratory animals you can prove it.

Randomize rats to added saturated fat or chinchillas to added dietary cholesterol,

and scientists can show atherosclerosis-inducing diets

exacerbate inner ear damage and hearing loss,

even without noise exposure.

But that doesn't necessarily mean the same is true in people.

There is cogent epidemiological data.

For example, a study of thousands of twins was able to draw a significant link

between a high cholesterol diet and hearing impairment.

In the Blue Mountains Hearing Study, which enrolled thousands of older men

and women to study age-related hearing loss,

dietary cholesterol was the nutrient component

most associated with hearing loss.

Those eating about two eggs worth of cholesterol a day

had 34 percent greater odds of hearing loss

compared to those only getting about a single egg's worth.

Consistent with a vascular cause, those on statins,

and particularly those at higher doses, appeared to be at lower risk.

The researchers suggest that high cholesterol intake

may interfere with the circulation within the inner ear itself,

or atherosclerotic inflammatory changes could reduce blood flow

to the tiny arteries feeding the inner ear.

But how about looking inside the arteries to see if this is actually true?

The extent and severity of coronary artery disease in the heart,

as determined by angiogram, was found to closely correlate to hearing loss.

Since atherosclerosis is a systemic disease affecting the entire arterial tree,

this has relevance for the arteries feeding the inner ear.

The same was found for the amount of atherosclerotic plaque

found in the carotid arteries in the neck.

The greater the plaque, the poorer the hearing, and the more plaque they found,

the greater risk of further hearing impairment

measured over the subsequent five years.

Getting closer, but how about the arteries that directly supply the inner ear?

Early autopsy data suggested sclerotic changes in the blood vessels of older ears

contribute to inner ear degeneration, and these days,

direct imaging studies correlate the degree of hearing loss

with atherosclerotic narrowing of those inner ear arteries.

Now all we need is an interventional trial to wrap it all up in a bow.

Yes, high-cholesterol and high saturated fat diets

have been shown to kill off those critical inner ear cells

and cause hearing loss in lab animals,

but it's not like you can lock hundreds of people up for a few years

and force them to eat different amounts of saturated fat

and see what happens to their hearing.

Oh, but you can, and they did: The Finnish Mental Hospital Study.

In 1960, one of two mental hospitals near Helsinki had their menus changed

to decrease their intake of saturated animal fat.

Then, after a few years, the two hospitals switched their menus over.

It was one of the first interventional trials of its kind

suggesting that you could decrease heart disease deaths

by decreasing saturated fat intake.

Ah, but what happened to their hearing? It followed the exact same pattern.

As their heart disease got worse, so did their hearing.

And then when the hospitals switched, the reverse happened.

And not just by a little.

Those in their 50s in the lower saturated fat hospital

ended up with significantly better hearing

than the group in the control hospital that was ten years younger.

The researchers concluded: "our audiological studies lead us to conclude

that diet is an important factor in the prevention of hearing loss.”