Jim Carrey (Lloyd) and Jeff Daniels (Harry) were "Dumb and Dumber" (or maybe it was the other way around) in the 1994 movie that showcased how ignorance could be bliss — and mildly amusing.

While it was a hit, it left moviegoers wondering how two characters could be so half-witted.

Well, researchers at the Linus Pauling Institute have the answer: by eating a high-sugar, high saturated-fat diet.

We've long known that artery-clogging saturated and trans fats, and inflammation-producing added sugars and syrups, damage your cardiovascular and immune systems.

Now it's clear that the sugar- and saturated-fat-laden diets of most North Americans slam the brakes on cognitive flexibility, the ability to adapt to changing circumstances and think on your feet.

And there's more: The study found that a high-sugar diet also dims short- and long-term memory.

Why are added sugars and unhealthy fats so brain-dulling?

The research showed that they alter the way your gut bacteria communicate with your brain.

Healthy, happy gut bacteria release compounds that act as neurotransmitters, which stimulate sensory nerves and other biological functions.

But they can't run smoothly if there's a lot of sugar and saturated fat disrupting their fuel lines. In lab-based research, it took just four weeks for saturated fat- and sugar-eating mice to become dim-witted.

So stay sharp. Avoid life-shortening sugars and fats in processed foods as well as saturated fats in red and processed meats.

Because, as Lloyd says to Harry: "Life's a fragile thing, Harr. One minute you're chewin' on a burger, and the next minute you're dead meat."