I’ve long been a skeptic when it comes to disclosing information about how doctors practice medicine, how hospitals treat patients and what both doctors and hospitals charge for their services. While I am all for transparency, it’s still an open question how patients or consumers of medical care can actually use that stuff to find “the one that’s right for you” or “the best” as marketers like to say.

I’m dropping my skepticism, though, about disclosing calories in food. The Affordable Care Act calls for calorie labeling on restaurant menus, a provision that many policy wonks and much of the media have overlooked. While insurance coverage has grabbed most of the headlines, insurance policies may not have the biggest effect on health in the long run. Calorie labeling, however, might … if it helps Americans choose healthier diets.

A year from now, the law will require chain restaurants with 20 or more national outlets to reveal how many calories are in their mega-cheeseburgers, overstuffed burritos and bacon-glazed donuts. Customers of chains, where more and more Americans consume their meals, will be able to see how many calories each selection contributes to their daily intake. A proposed FDA rule that will guide the labeling provisions also calls for disclosure by supermarkets as well as convenience stores, where Americans increasingly turn for their meals.

The hope is that eventually customers at McDonald’s, for example, will select a healthier option, like a premium McWrap with bacon and grilled chicken with 460 calories, than a Big Mac with 550 or a Bacon Club House burger with 750. The Club House burger supplies about one-third of someone’s calories for the day (based on a 2,000-calorie diet). And that’s without fries and a Coke.

Add a medium portion of fries, which McDonald’s prominently advertises in my neighborhood restaurant, and a small Coke, and the calorie total climbs to 1,270. That doesn’t leave a lot of room for breakfast or dinner or even a glass of wine at night. When you add the numbers, you can see how easy it is to overeat.

“I’ve been stunned by how many calories are in popular restaurant foods and how difficult it is to tell the difference between items,” says Margo Wootan, director of nutrition policy at the Center for Science in the Public Interest, a food advocacy group. “There’s a real difference between a regular hamburger at 200 to 300 calories, a bigger hamburger that has 400 to 500 calories, and a triple burger with 700.”

Sometimes, Wootan told me, a tuna salad sandwich may have 50 percent more calories than one made with roast beef. We’ve been conditioned to think of tuna as a health food of sorts and that beef is laden with fat. But few of us think about the giant scoop of salad in the sandwich (probably more than we need) or the mayo slathered on the bread. Only labels will reveal the option with the fewest calories.

Restaurants sell consumers these extra calories using a value for money proposition. A larger amount might cost less even as it delivers more food — and more calories. As portion sizes get larger, it’s all the more important to read the labels and understand what you are getting. More and cheaper is not always better. The bigger the portion size, the more you eat.

New York City has required calorie labeling since 2008, and early studies showed little or no change in eating behavior. Wootan says some of those studies were too small to say whether labels made a difference. Larger studies have been more conclusive. One New York study found that 17 percent of fast food customers purchased 100 fewer calories after the labeling law took effect.

A Stanford University study assessing labels on Starbucks products found they had no effect on beverage consumption but contributed to a 14 percent decrease in the purchase of other Starbucks offerings. Customers were not going to give up their 470-calorie white chocolate mochas but thought twice before buying a 480-calorie glazed donut.

Some restaurants reformulated their products in response to the New York law. One chain reduced the number of calories in its signature salad from 600 to 380 when it cut the amount of cheese in half. Starbucks made similar changes. A manager of one New York City Starbucks told me that the calorie law had prompted the chain to start using two percent instead of whole milk and take an apple fritter with some 600 calories off the menu.

Wootan says a good rule of thumb for people to use in judging their calorie intake is about 600 calories per meal plus two snacks. If people follow that advice with the help of calorie labeling and menu reformulations, one can imagine less obesity, less illness and a healthier population. That would not be a bad legacy for the Affordable Care Act.

Trudy Lieberman is a journalist and an adjunct associate professor of public health, Hunter College, New York, NY. She blogs on the Prepared Patient blog.