Gentlemen, if you want to keep your sperm swimming — and who doesn’t? — head on over to the nearest fish market and load up on tuna and salmon.

The fattier the fish, the better — because the same fatty acids that make these fish such healthy choices for everything from your heart to your eyes to your brain are also positively critical to your fertility.

The omega-3 fatty acid docosahexaenoic acid (DHA) is one of the essential building blocks of sperm — and a new study on mice shows how it’s also a key part of the acrosome, which is what enables the sperm to penetrate the egg.

You might say it’s the most important part of all. The mice would agree: When they were denied DHA, they produced fewer sperm — and the ones they did create were misshapen, rendering them infertile.

But once DHA was put back into their diets, they began to produce again like, well, mice. (Side note: There has to be a pest-control angle in here somewhere).

This is, of course, just one study on mice. But human studies have also shown how high levels of these essential fatty acids can boost your fertility.

One study from just a couple of years back found that fertile men tended to have higher blood levels of omega-3 fatty acids, while infertile men had higher levels of omega-6 fatty acids.

And these days, with diets heavy in grain fed animals from factory farms, most of us get less of the desirable omega-3s and far more of the undesirable omega-6 fatty acids.

Call it one more reason to switch to fresh all-natural grass-fed meats.

Naturally, omega-3 fatty acids aren’t the only answers for sperm health. A lot goes into male fertility — and studies over the years have shown that high levels of vitamin D can boost the speed and forward motion of sperm, an essential trait called motility.

Other studies have also shown that junk food, soda and the BPA used to line canned goods (including soda cans) can slash sperm levels and turn the ones that are left into the microscopic equivalent of couch potatoes: slow, lazy and uninterested in the quest for the egg.

That would explain the recent rise in male infertility.

“Brain shrinkage” sounds scary enough. Seriously, who wants a shrinking brain?

But in reality, all our brains shrink a little over the years — and in most cases it’s nothing to worry about.

Some brains, however, shrink faster than others — and since this rapid loss of gray matter is often a warning sign of dementia and Alzheimer’s disease, you want to limit your own shrinkage as much as possible.

And the best way to do that is with the vitamins you should be getting anyway.

I’ve told you how B vitamins can help slow the shrinkage linked to dementia and stop or even reverse cognitive decline. Now, blood tests on 104 seniors with an average age of 87 finds that those with the highest levels of vitamins B, C, D and E have the biggest brains.

Not coincidentally, seniors with high levels of these key nutrients also did the best on tests that measured certain cognitive functions — specifically, the ability to think, plan and solve problems. They also did better on tests measuring visuospatial skills and global cognitive function.

In plain talk, that means these vitamins can help keep your brain sharp enough to chase dementia away.

Along with those nutrients, make sure you get your share of omega-3 fatty acids — because the same study found that seniors with the highest levels of these essential fats had better cognitive function and less damage to the white matter of the brain.

And don’t forget while omega-3s can protect the brain, there’s another type of fat than can rot it from the inside: the dangerous trans fats used in so-called “healthy” products like margarine.

Seniors with the highest levels of those trans fats, which you’ll find in everything from coffee creamers to snack cakes, had smaller brains and did poorly on cognitive tests.

Keep in mind that food makers are allowed to round “low” levels of trans fats down to zero — so don’t trust the ingredients panel. Any product that has partially hydrogenated vegetable oils will have trans fats — so avoid them, even if it says “trans fat free” on the label.

It’s not the first study to show that nutrients can slow or stop dementia. As I mentioned earlier, B vitamins have proven time and again to prevent shrinkage, reduce inflammation and boost brainpower.

And while a good diet will include most of the vitamins you need to keep your brain sharp the exception to the rule is those Bs. You’ll need more of those than what you’ll find in food, so be sure to add a quality B complex to your regimen today.

“Fish is brain food” is the kind of age-old folk wisdom that’s been proven time and again by cutting-edge science — and the latest research confirms that the best way to keep your brain swimming in gray matter is with a steady diet of fish.

I mean that literally: Seniors who eat fish at least once a week have more of that critical gray matter, giving them a lower risk of dementia and Alzheimer’s disease.

The biggest boost is in the frontal and temporal lobes — specifically the areas most closely linked to memory and learning, or exactly where you want your extra gray matter to be.

And if you think the benefits inside the brain are impressive, you should see how that translates out in the real world: Just 3.2 percent of the fish-eaters developed cognitive decline over five years, versus 30.8 percent of those who ate little to no fish, according to data presented at a Radiological Society of North America meeting.

If there’s one area where the researchers found no benefit, it was in fried fish — and I have to wonder if it’s because of the frying… or because of the oils people tend to fry things in.

Most people don’t fry their foods — fish or otherwise — in healthy oils. They fry them in the unhealthiest polyunsaturated oils of all, including blended vegetable oils, soybean oil and safflower oil.

Try a healthier oil — like peanut oil — and all your fried foods will get a health boost (and taste better, too).

But let’s get back to seafood, cooked however you like — because a diet rich in fatty fish will do so much more than protect your brain. Fatty fish can help prevent heart disease, protect your vision, beat depression and even improve your gums.

Yet despite all those benefits, some simply won’t eat fish to save their lives. Maybe it’s the smell… the taste… the texture… or all three.

Whatever the reason, you don’t have to actually eat any fish at all to get the benefits — because almost all of those benefits come from the omega-3 fatty acids found in fish oil, which you can find almost anywhere as a supplement.

Shop around before you commit to one — some brands will leave you with the “fish burps,” which is a little counterproductive if you’re taking pills to avoid the taste of fish in the first place.

Buy smaller sizes or sample packs first — it might take a little trial-and-burping, but eventually, you’ll find one that works for you.

When it comes to heart disease, it looks like the eyes have it.

Researchers say they can spot who’s more likely to suffer the life-threatening condition by simply checking for yellow spots on the eyelids. People who have them face a 50-percent increase in the risk of a heart attack.

The study of nearly 13,000 people in Denmark also found that the yellow spots — actually little pockets of cholesterol called xanthelasmata — can up the odds of heart disease by 39 percent and an early death by 14 percent.

That may not sound like a lot, but over the years it can add up — and over the course of a decade, the researchers say a person with xanthelasmata has a one in five chance of developing heart disease.

You can see where this is going, right? Since the spots are made of cholesterol, the researchers wrote in BMJ that patients who have them should be given cholesterol treatment — a not-so-subtle code for meds like the statins that have become so overused.

But it’s just not that simple, because there’s no clear link between those yellow cholesterol pockets and blood levels of the fats. In fact, half the people who develop xanthelasmata have perfectly normal blood cholesterol levels — and even the new study found that the link to heart disease was there regardless of those blood cholesterol levels.

So instead of blindly flinging statins around, docs should use the yellow patches as a sign they need to dig deeper and get a more complete picture of your heart disease risk factors. As far as those risk factors go, both yellow eyelids and even those cholesterol levels are actually pretty low on the list.

The one that beats them both is homocysteine, the inflammation marker that can signal everything from heart problems to dementia risk — and you don’t need a drug to help lower it.

Something you probably have in your supplement cabinet at this very moment will do that for you: fish oil.

As I’ve told you before, the omega-3 fatty acids in fish oil can slash homocysteine levels — and, as a bonus, they can even lower your levels of deadly triglycerides and boost HDL cholesterol, aka “the good cholesterol.”

It’s like killing two birds with one fish.