1. It messes with your metabolism
So you've been blessed with a speedy metabolism and tend to eat whatever you want. Yeah, that won't last forever: That's the lesson that 12 college-aged men, all relatively healthy, learned when they were put on a high-fat diet for five days. According to the research, published in the journal Obesity, as the guys gorged on things like mac and cheese, sausage biscuits, and microwaveable meals, their muscles began to less effectively turn glucose into energy. While the five-day trial didn't change their whole-body insulin sensitivity, it did show major potential for screwing up their metabolisms if they had stayed on the diet.
2. It narrows your arteries almost immediately
Habitually eating fast food leads to high cholesterol–you know that. What might be more of a surprise is that even a single meal high in saturated fat (and let's be real, this is most drive-thru meals) causes immediate damage to your arteries. While cholesterol builds over time to narrow artery walls, scientists who fed perfectly healthy men a meal high in saturated fat–read: a sausage, egg, and cheese sandwich with three sides of hash browns–saw subjects' artery walls narrow by 24 percent only two hours after eating, according to a Canadian study. Narrowed arteries mean high blood pressure, and potentially heart problems down the road if your fave fast-food joint knows you by name.
3. It could throw off your concentration
If your focus is waning after lunch, it might not just be because you need another latte. Scientists at the University of Cambridge noticed that after only nine days on a high-fat diet, rats made more mistakes running through a maze than rats fed a normal diet. The same researchers are now studying similar effects in humans.
4. It saps your energy and makes you cranky
Oh, the all-too-familiar sugar crash. Empty calories are a surefire way to feel tired, shaky, and ravenous within a couple hours of justifying an office birthday cupcake as your lunch. A sugar surge means a shit-ton of insulin gets pumped into your body, telling your tissues to soak up all that glucose. The tissues soak it up so quickly that the rapid change in blood sugar leaves you feeling tired and irritable. Watch out, world.
5. It might make you look less attractive
Sure, junk food makes your stomach feel bloated. But a new study published in Immunity suggests that high-fat food leaves your skin bloated, too. When researchers fed mice a high-fat diet comparable to the grub that humans would get at the drive-thru, they saw changes in the animals' skin proteins, which led to inflammation. Ever wake up with puffy eyes the morning after an alcohol-fueled junk food binge? Maybe this explains why.
6. It alters your brain chemistry
There might be more truth to declaring yourself a "chocoholic" than you may think. Studies have shown that foods high in sugar encourage the release of opioids and dopamine–the same brain chemicals released when you take addictive drugs. What's more, foods high in both sugar and fat also hit you with a greasy, oily mouthfeel that some scientists say ups the addiction ante.
7. It changes your gut bacteria
No, we're not talking about your waistline (but regularly eating junk food will change that, too). We mean your actual gut–as in the organs that make up your digestive system. Scientists recently discovered that switching rats to a high-fat diet immediately begins to change the population of bacteria in their guts, causing some bacteria to multiply and damage the gut-to-brain communication chain that tells you to stop eating.
