Joey Chestnut likely wasn't counting calories when he wolfed down a record 68 hot dogs at Saturday's annual eating contest in Nathan's Famous in Coney Island. But thanks to a new law requiring New York City restaurants to post nutritional information, it's easy to calculate that Mr. Chestnut's meal amounted to nearly 20,200 calories, or 297 per frankfurter.  Menu labeling is increasingly popular. Laws mandating the practice have been adopted in about a dozen places, and many more are under consideration as policy makers search for ways to bring down America's high obesity rates. Legislation being considered in the U.S. Senate that would pre-empt a hodgepodge of local laws has won the backing of health advocates and many restaurant chains.  Whether the laws are based on reliable science is less clear. The handful of studies published looked only at what effect nutritional labels have on diners' choices for a single meal. The studies don't examine the impact on overall diet, even though other research suggests healthy, lower-calorie choices at one meal are often undermined by unhealthy, more fattening selections at a subsequent meal.  There are other open questions, including whether laws will affect enough restaurants to make a difference to public health and how accurate the posted counts are.  "There have been great attempts to provide Americans with healthier food," says Harry Balzer, food analyst at the NPD Group, citing the introduction of nutrition labels on packaged foods 15 years ago and a move by fast-food chains to offer healthier menu items. But, he says, "we've still become heavier."  New York City, which pioneered these laws, also has done the most to study their effects. The city's health department has claimed that menu labeling will reduce the number of obese New Yorkers by 150,000 over the next five years, and prevent 30,000 cases of diabetes.  But those numbers rest on some shaky assumptions. Before the menu-labeling law took effect last year, the health department surveyed 7,318 New York City diners as they left 275 fast-food restaurants during weekday lunch hours. The restaurants were selected because they already posted nutritional data in one way or another, though only one, Subway, revealed calorie counts prominently.  Nearly one-third of Subway customers reported noticing the nutritional information, compared with fewer than 5% of customers at the other chains. Among the Subway customers who spotted the numbers, more than one-third said it affected their orders -- and those individuals' meals averaged 99 fewer calories.  To calculate the projected decline in the number of obese New Yorkers, the city assumed diners in all chain restaurants would respond similarly to Subway customers in the study. But that rationale failed to take into account the rather unique nature of Subway. Much of the chain's advertising focuses on health and weight loss, so its customers might be more inclined to notice and act on caloric information.  On the other hand, relying on the Subway study also risks underestimating the benefits of disclosing caloric content, says Margo Wootan, director of nutrition policy for the Center for Science in the Public Interest and a leading advocate of menu labels in restaurants. She points out that there is less opportunity to cut calories at Subway because of the menu's narrow calorie range. At Nathan's, by comparison, a large order of fries has nearly 800 more calories than a hot dog, meaning customers can reduce their intake more sharply.  Another problem with the findings is that calorie counts are subject to dispute. Last year, Scripps television stations sent several menu items to testing labs and found some big deviations from posted calorie content, most of them making menu items appear healthier than they are. For example, two tests of Applebee's Cajun-lime tilapia meal found about 400 calories, compared with the posted total of 310.  Representatives for several of the restaurants whose items were tested responded by pointing out that not every meal will be the same. "There will be variation in the restaurant as there will be in the home," says Patrick Lenow, an Applebee's spokesman. He adds that the menu notes this possibility.  Despite the variation, many chain restaurants send just one menu item to be tested for their published counts, according to Sandy Koch, vice president of Analytical Laboratories Inc. in Boise, Idaho, which conducted the tests for Scripps.  Projections about consumer behavior in restaurants also might be overstating the number of calorie-labeled meals. According to NPD Group, one in five meals or snacks consumed in the U.S. are produced by restaurants, and 59% of restaurant traffic is at chains. That means about one in eight meals would be affected. J. Justin Wilson, a senior analyst at the Center for Consumer Freedom, which opposes menu labels, cites older data from the U.S. Department of Agriculture that put the figure closer to one in 15 in 1995 -- but didn't account for takeout meals eaten at home.  A further complication is that the New York study didn't follow its subjects over a full day, let alone a week. Some food researchers have found that consumers compensate for an unhealthy meal with a healthy one, and vice versa. "This balancing is particularly strong within meals, but also takes place across meals," says Pierre Chandon, a marketing professor at Insead, the international business school in France and Singapore.  This isn't a nail in the coffin of food labeling. Visible calorie counts could prompt consumers to be more aware of their overall calorie consumption, and to be more cautious about packing on additional, "compensation" calories following more spartan meals.  A number of surveys indicate that most Americans support menu labeling in restaurants. But health advocates acknowledge calorie counts won't solve the obesity problem. "It's not like we're going to pass menu labeling and then we can retire," Dr. Wootan says.  How menu labeling influences consumer habits won't be clear without further research, which New York City's public-health department and other institutions are pursuing.  What is clear is that surveying people about their eating habits is problematic. Food-research company Technomic polled 299 New Yorkers after the menu-labeling law took effect and found that 82% said calorie disclosures are affecting what they order.  Bob Goldin, Technomic's executive vice president, advised taking the results with a grain of salt. Surveys about healthy eating are a lot like those about sex, Mr. Goldin says: "People report doing it a lot more than they really do."  Write to Carl Bialik at numbersguy@wsj.com
Laws requiring restaurants to post nutritional data lean on slim research and overlook the more fattening meals that follow lower-calorie choices.