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There's never been a better time to be a health-conscious eater. Why? Because these days, so many delicious foods that used to be off-limits are now thought to be really good for you.

Still, “good for you” and “okay to eat in unlimited quantities” aren’t the same thing.

“Folks are always looking for that magical food that will make them healthy, slim, and beautiful. There are tons of foods on the planet that will do that, but if you eat anything in excess, it sort of backfires. Mother nature is kind of tricky that way,” says Frances Largeman-Roth, R.D., author of Eating in Color. Here, five surprising examples.

1. Red Wine

Healthy Foods That Turn Bad When You Eat Too Much

A glass of vino doesn’t just help you unwind. Red wine contains resveratrol, an antioxidant compound that’s thought to offer protection against heart disease, cancer, type 2 diabetes, and neurodegenerative diseases, like Alzheimer's. And according to one study, the stuff even appears to play a role in blocking the development of fat cells, which could help you maintain a healthy weight. Piceatannol, natural polyphenolic stilbene, inhibits adipogenesis via modulation of mitotic clonal expansion and insulin receptor-dependent insulin signaling in early phase of differentiation. Kwon JY, Seo SG, Heo YS. The Journal of biological chemistry, 2012, Jan.;287(14):1083-351X.

But a serving of red wine is a five-ounce glass—not the half a bottle you split with your friend when you go out to dinner. At 125 calories a serving, “drinking more than that could lead to weight gain, which would cancel out the heart-health benefits,” Largeman-Roth says. Plus, findings show that excessive alcohol consumption can raise your blood pressure and increase your risk for some cancers, such as liver, breast, and colon cancer.

What's Healthy: Reap the benefits without the bad stuff by sticking to no more than one five-ounce glass per day if you're a woman; or no more than two five-ounce glasses per day if you're a man.

2. Coconut Oil

Healthy Foods That Turn Bad When You Eat Too Much

There's a reason people love the tropical fat: Some research shows that compounds in coconut oil may play a role in reducing harmful belly fat—and one recent pilot study suggests that it may play a role in protecting neurons in the brain from Alzheimer's. Coconut oil attenuates the effects of amyloid-β on cortical neurons in vitro. Nafar F, Mearow KM. Journal of Alzheimer's disease : JAD, 2014, Oct.;39(2):1875-8908. Effects of dietary coconut oil on the biochemical and anthropometric profiles of women presenting abdominal obesity. Assunção ML, Ferreira HS, dos Santos AF. Lipids, 2009, May.;44(7):1558-9307. According to one nutritionist, it can even help with digestion. “It basically goes in and eradicates harmful bacteria in the digestive tract,” says Rania Batayneh, nutritionist and author of The One One One Diet.

Still, it can be easy to go overboard. Most experts recommend that 20 to 35 percent of your calories come from fat. But if you’re drizzling coconut oil on everything, it's easy to exceed that limit. “It’s a healthy food, but there needs to be room in your diet for protein and carbohydrates,” Batayneh says.

What's Healthy: When it comes to coconut oil (or any other cooking oil) stick to no more than a tablespoon per day.

3. Avocado

Healthy Foods That Turn Bad When You Eat Too Much

Avocados are loaded with heart-healthy monounsaturated fat and fiber—a combo that can help you stay fuller longer and stave off junky cravings. In fact, one study found that people who added the green fruit to their meals were less interested in snacking for up to five hours. (The researchers note, however, that this may be due to the additional calories in avocados. In other words, eating something richer made participants feel more full.)

But those mouthwatering recipes you see on Pinterest for avocado halves stuffed with eggs are probably overdoing it. A 50-calorie serving of avocado is actually just one-fifth of the green fruit—so eating the entire thing will cost you 240 calories. Whether or not eating that much is a good idea depends on how active you are. “If you’re fairly sedentary, the additional calories would be too much, unless you cut them from somewhere else in your diet,” Largeman-Roth says.

The good news? Since the fat in avocados is so good for you, you won’t run into heart-health risks from eating too much, adds Largeman-Roth.

What's Healthy: Unless you’re really active, use it like you would mayo or Parmesan cheese: in small amounts to add flavor, not in giant gobs.

4. Dark Chocolate

Healthy Foods That Turn Bad When You Eat Too Much

As long as it contains as least 70 percent cacao, dark chocolate is pretty much the healthiest way to satisfy your sweet tooth, Batayneh says. Thanks to a hefty dose of antioxidants called flavonoids, dark chocolate can help boost your heart health by lowering blood pressure and bad cholesterol and increasing blood flow to the heart. Flavonoids in food and their health benefits. Yao LH, Jiang YM, Shi J. Plant foods for human nutrition (Dordrecht, Netherlands), 2005, Mar.;59(3):0921-9668. Dark chocolate even contains compounds that boost your brain’s production of feel-good hormones like serotonin—which might explain why that square of Godiva always seems to leave you feeling so satisfied.

Thing is, a one-ounce serving is around 160 calories. Devour an entire 3.5-ounce bar, and you’re up to 600 calories that you could be spending on other good-for-you fare. “It’s not necessarily bad, but what it does is take up more of your calories that could be invested in other foods and nutrients,” Batayneh says. Not to mention it loads you up on sugar. A full-size bar delivers around 27 grams of the sweet stuff, which is as much as what the American Heart Association recommends women should consume in a day and two-thirds of what men should consume.

What's Healthy: Keep in mind how many calories you're consuming, and stick to smaller serving sizes—not the whole candy bar.

5. Lean, Grass-Fed Beef

Healthy Foods That Turn Bad When You Eat Too Much

Lean cuts of beef like sirloin, tenderloin, or flank steak are top sources of protein as well as iron—a nutrient that plenty of people (especially women) don’t always get enough of. And if you’re springing for grass-fed beef, you’ll get extra benefits like heart-healthy omega 3s and more antioxidants like vitamin E too.

Still, just because it’s grass-fed and lean—a 3.5-ounce serving only has about 9 grams of fat and about 4 grams of saturated fat—doesn’t mean it’s a good idea to load up every day. Research shows that diets high in red meat are linked to an increased risk of colon cancer.

What's Healthy: “Eating lean red meat two or three times a week is fine. But you need to keep the serving size in mind, which is 3.5 ounces,” Largeman-Roth says.

The Takeaway

“You can have it all—but you can’t have it all at once,” Batayneh says. No matter what sorts of health benefits a food might boast, eating endless amounts will usually do more harm than good. So enjoy your favorites in moderation.

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