Good grief. Is it possible that Americans are eating less? (Please forget the marathon hot dog fests you witnessed over the weekend.) Tiny signs of this sea change are popping up, a reversal that could have a serious impact on the food service industry and especially restaurants.

A hint of this notion comes from Levi, a young Hungarian who has been waiting on tables at one of Nantucket’s top restaurants for the past seven summers. Each year he has returned to school with less cash in his pocket. His employer is busy — it’s as hard as ever to get reservations to the island’s favorite watering holes — but revenues are in a slump.

Levi and others report that diners are simply spending less and, believe it or not, eating less. They are ordering appetizers only and sharing entrées, or doing without dessert and coffee.

Laurel Strauch from Nantucket’s Chamber of Commerce confirms that the number of visitors is up this year, with more than a 6% jump in the number of people streaming into the island by ferry and through the airport over last year. Why, then, are the restaurants hurting?

Like so many other economic patterns just now emerging, it appears that the aging of the population is having an impact. The reality is that older people simply don’t eat as much. Certainly, in high priced tourist venues like Nantucket, economics also play a role. Restaurants in this New England community have to earn an entire year’s income in just four months, so prices are high. And when revenues fall short, the bill of fare tends to get even pricier, creating a nasty cycle.

However, it also appears that the incessant alarms about obesity and overeating have resonated and that people of all ages and economic backgrounds are trying to cut back. Older people are leading the charge. “There is definitely a tendency for older people to eat less and order less,” Bob Goldin of the food consulting firm Technomic says. “It’s a function of health and budgetary concerns.” Given the trend in recent years toward ever larger portion sizes, eating less while eating out is a challenge to the consumer. For restaurants, a hankering for more modest portions throws many long-cherished practices out the window.

Restaurants have been reluctant to react to this new reality, Mr. Goldin says: “Unfortunately, portion size is how a great many people judge the quality of a restaurant.” Not only are restaurants afraid that cutting their serving size will hurt their image, but “they will have to charge less, and they’re not sure the numbers will work out,” he says.

Last spring TGI Friday’s broke new ground by offering customers a series of smaller entrees. Mr. Goldin says the jury is still out on the success of the program.

Harry Balzer, a vice president at NPD Group, a consumer-marketing outfit, say she has been “watching people eat since 1980.” He has reached the remarkable conclusion (you heard it here first) that concerns about eating more healthfully not only are cutting into restaurant revenues but also are beginning to pinch the national waistline. “We are finally getting a handle on the obesity issue,” he says. One aspect of that seismic shift, according to Mr. Balzer, is that “there is a movement towards portion control.”

Mr. Balzer notes that the percentage of Americans classified as overweight has increased for decades, but more recently has hovered around 62%, actually lower than the 64% reported in 2002. “For instance, the consumption of carbonated soft drinks has been declining,” he says, which is one indication that Americans are making some healthy choices.

In a new study due out later this fall, which may uncover some of the causes for the change, Mr. Balzer will report: “Americans are back in their kitchens. They may not be making dinner from scratch, but they are serving their families ready to eat and frozen items.”

This development may not seem startling to some, but it is a dramatic departure for the restaurant industry. For 50 years, restaurants have benefited from the growing number of women in the work force. As women left their homes, beginning in the 1950s, families began to eat out, taking advantage of higher two-salary incomes and giving the woman of the house a convenient way to feed her family.

“Since 2000, the percentage of women working has been flat,” Mr. Balzer says. “It’s going to be very difficult for restaurants to grow.”

For the past several years, takeout and delivery food has been the driving force for the restaurant industry, but Mr. Balzer says that trend has peaked. He ascribes this shift as flowing from a generational change: “Today’s young women have a different view. They don’t have to be supermoms and do it all. The percentage of young women dropping out of the work force is greater than the number of older women retiring.”

One big beneficiary of these trends could be supermarkets, which have lost shares of the dining budget for years. These stores are gearing up to provide families with simple-to-assemble dinners and more prepared foods, supplanting takeout and delivery options in some cases.

So what will restaurants do? “Where’s the agent for change?” Mr. Balzer asks. “It’s going to be convenience and how they deal with the need for fresh foods.”

Mr. Goldin points out that despite these issues, restaurant revenues have been increasing, in part because rising food costs have prompted price increases. Higher prices usually mean less consumption, which could be another factor depressing the national appetite.

Over the next several years, it is conceivable that higher food prices — combined with the aging of the population, more women feeding their families at home, and concern about the health effects of obesity — could begin to rein in the American figure. Perhaps instead of apple pie being the iconic national food, one day we’ll hear “as American as tofu.” Heaven forbid.