You already know what junk food will do to your waistline -- but for men, a diet rich in poor choices can also hit below the belt.

To put it simply, junk food kills sperm the way RAID kills roaches.

Researchers examined sperm samples from 188 young men and found that those who ate the most fast food and other junk were far more likely to have serious problems "down there" than those who ate real food.

More specifically, their junk-fed sperm were more likely to drop dead before they'd ever have a chance to reach an egg -- and if you can't reach the egg, you sure can't fertilize it.

If that's not bad enough, a second new study finds that men who eat the most trans fats have dramatically lower sperm concentrations: 48 million sperm per ml in men who ate the most trans fats versus 79 million in those who ate the least.

What's more, these junk-filled men even had trans fat floating around in their semen -- almost as if the sperm were eating junk food, too. Give them a sofa, a TV, and a remote control and I bet they won't even try to reach the egg!

The obvious answer here is to avoid junk food if you hope to be a dad someday -- especially trans fats.

But believe it or not, that's not as easy as you'd think -- because if you rely on nutritional labels for information, you could find yourself loading up on trans fat without even realizing it.

There's a nasty little loophole in the labeling laws that allows food manufacturers to list trans fats as "zero" if the item contains .5 grams or less per serving.

Not only that, but the company can even put a "ZERO TRANS FATS!" blurb right onto the front of the package -- when in reality, just a few servings a day of these "zero trans fats" foods can push you right past the recommended limit of 2 grams.

Look at the ingredients carefully -- if something contains partially hydrogenated vegetable oils, it's going to have at least some trans fat... even if the label says otherwise.