Here are the rules of portion sizes…

• If they serve it, we will eat it.

• If they serve it on a bigger plate, we will eat more of it.

• If they serve a bigger portion, we will chew less, take bigger mouthfuls and eat more of it.

• If they serve a soft drink in a bigger cup, we will drink more of it.

• If packaged foods come in larger sizes, we’ll start to believe that those supersized amounts are perfectly normal—and, yes, eat more of them.

Are you noticing the pattern here? The truth is, portion sizes are a huge contributor to overeating. Recently, researchers at the University of Cambridge in London did a meta-analysis of 72 studies—most of them done in the US, as it turns out—and found overwhelming evidence that portions drive up our calories a lot. In a new article in The BMJ, the same researchers arrived at an even more startling conclusion. If the US eliminated oversized portions in restaurants and food packaging—a big if, admittedly—adults in this country would take in between 22% and 29% fewer calories.

That kind of calorie cut would certainly take a big chunk out of our obesity problem—and the diabetes epidemic it’s bringing on.

Unfortunately, educating the public on ways to reduce portion sizes doesn’t seem to have dawned on the federal government, which, for the first time in 20 years, is proposing changes to the nutrition facts label that appears on all packaged foods. One of the biggest changes: Increasing the “serving size” that appears on all foods that carry a nutrition label.

It boggles the mind.

WHAT IS A SERVING SIZE, ANYWAY?

Contrary to what you might think, the serving size that’s listed on the side of the package isn’t the recommended amount. Instead, it’s the amount that most people actually eat. And in the 20 years since the labels were last updated, people’s portion sizes have gotten bigger, so the new label will reflect that change.

Take ice cream. The current serving size is one-half cup, but under the new rule, a serving size will be one cup.

For sodas, both a 12-ounce bottle and a 20-ounce bottle will be listed as one serving. ­ A 20-ounce can of Coke has 65 grams of sugar (about 15 teaspoons)! Yet the new dietary guidelines advise us to limit added sugars to less than 10% of calories. So if you’re taking in 1,800 or 2,000 or even 2,400 calories daily, at just under four calories per gram, that single “serving” of Coke will give you more sugar than you should consume in an entire day.

The rationale: Nutrition experts say the new serving sizes may lead people to realize how many calories are in the foods they’re eating and eat less of them.

What’s more likely to happen: The exact opposite.

“People view serving sizes as recommendations, so this may give people license to eat more,” says Lisa R. Young, PhD, RD, CDN, adjunct professor of nutrition at New York University and author of The Portion Teller Plan. Ideally, she says, the new label would point out that the serving size represents the amount that’s typically consumed—and not the recommended portion. But that’s not part of the proposed new label.

HOW TO PROTECT YOURSELF: PORTION SAVVY

The new food labels aren’t finalized yet, so it’s always possible that they’ll be revised to actually make it easier for us to eat less. In the meantime, the new analysis of the harm of large portions should encourage all of us to redouble our efforts against the relentless supersizing that we see in supermarkets and restaurants. Some of Dr. Young’s recommendations for eating less may sound familiar, but they’re really important—and effective…

• Purchase single-serving portions. “This can really help you practice portion control while snacking,” says Dr. Young. If you do buy a large bag of, say, chips or pretzels, divide the contents into one-ounce single portions (approximately one fist full) and place them in small baggies. “This way, when you crave a snack, you just grab a small baggie and resist the temptation of overeating.”

• Order the smallest drink size. It’s especially easy to consume more liquid calories than you intend to because they don’t fill you up the same way that the calories in food do. Practice good portion control by choosing the smallest size you can find. And keep in mind that when it comes to packaging, the word “small” can be relative. In movie theaters, for example, the “small” size of soda can be a 32-ounce cup! When the US first introduced Coke, one of those iconic bottles held just 6.5 ounces.

• Use smaller plates. “Eat off of your grandmother’s dishes,” Dr. Young recommends. “They are sure to be smaller than your current plates.” Or use your salad plates instead of your dinner plates. Use smaller glasses and utensils, too.

• Don’t serve food family-style. It’s better to plate out your portions in the kitchen.

• Split portions when eating out. Share an entrée with your dinner companion, and order an extra salad or vegetable side dish. If your dining partner doesn’t want to share, split it with yourself—by immediately asking the server to wrap up half to take home. Get it off your plate as soon as possible so you’re not tempted to overeat.

• Stay away from all-you-can-eat restaurants. It’s just too tempting to try to “get your money’s worth!”

• For more tips, see Bottom Line’s  Lessons Learned from Watching Overweight People at a Buffet.