Asking for less food isn't something most people think about when ordering from a menu. A new study suggests that asking people if they want less food and, in turn, less calories, before they order is key.

A server offers you the option to downsize the fried rice side in your Chinese takeout order by half. She tells you that if you accept her offer, you'll save at least 200 calories.

Do you take it?

About one-third of diners (out of several hundred) who were given that choice at a Chinese takeout restaurant said yes, according to a study out today in the journal Health Affairs. The diners who chose the smaller noodle and rice dishes also ended up eating less overall — and avoided overeating — compared with those who ordered a full serving.

"People are willing to downsize, but you have to ask them do it," lead author Janet Schwartz, a psychologist and assistant professor of marketing at Tulane University, tells The Salt. "They're not going to do it on their own."

Indeed, hardly any patrons in Schwartz's study asked for a smaller portion right off the bat. Considering portion size isn't really programmed into the way we order at restaurants, according to Schwartz. And her paper argues that calorie labeling alone isn't having much of an impact either. In fact, most people use a well-rehearsed script when ordering.

Still, Schwartz says many people think restaurant portions are too big. And the U.S. Department of Agriculture's latest dietary guidelines recommend smaller portion of grains like rice and noodles than what was served at the Chinese restaurant in the study. For example, the USDA recommends that a man (like me) between 19 and 30-years-old eat no more than 8 ounces of grains a day. The full serving size of rice or noodles at the Chinese food restaurant was 10 ounces – and that's just the side for lunch.

Schwartz parsed other nuances of decisions around portion size in several small experiments in the study. The researchers first looked to see if diners would compensate the loss of starches with more of the main entrée (they didn't) and if a 25-cent discount for a smaller portion had more of an influence (it didn't).

Next they looked to see if displaying calorie content made a difference. Interestingly, about 21 percent of people chose to downsize before they knew how many calories were in a meal, as opposed to 14 percent who chose to downsize after they had that information. Lastly, researchers weighed the leftovers of 30 percent of the customers. The group that purchased a full order left an average of 2 ounces on the plate, as did the group that downsized.

So why did so many people choose a smaller side dish? And why did this tactic seem to work when others seemed to fail?

The first key was telling people that they could reduce their calorie intake by just having a little bit less food — "less of things they didn't even come in for," Schwartz says.

The second key was to ask customers before the servers started piling food onto the plate. Exercising self-control early on to restrict portion size is much easier than when already faced with a table full of temptation, she says. Basically, it's harder to decide to stop eating when you're knee-deep in a meal and choose to throw away or wrap up the leftovers. Diners often feel obligated to clean their plates as if their grandmothers were watching.

So what about just getting rid of the starches all together and offering a healthy alternative like fruits or veggies? Well, Schwartz says those options tend to test well in consumer studies, but don't translate to sales in the real world. How often have you picked the apples over the salty fries?

"We're trying to get people to think about not just what they eat, but how much they eat," she says. "What this study brings to the table is an actual reduction of calorie intake, something that just labeling foods with a calorie count hasn't done."

The New York City Department of Health launched its own effort to regulate portion sizes in January. The ad didn't go over well with some, though, because the unidentified, overweight man in the photo has a leg missing. The limb was digitally removed to depict a possible effect of type 2 diabetes — a chronic disease that overeating can lead to. [Copyright 2012 National Public Radio]