One popular idea among public health advocates is that eating restaurant food causes obesity. Restaurant food is often rich and portion sizes tend to be large. Concerned policymakers are developing new regulations on restaurants in an effort to fight obesity. For example, in response to high obesity rates in low-income neighborhoods, the Los Angeles City Council unanimously approved a law in July 2008 banning the opening of new fast food restaurants in a 32 square-mile area containing 500,000 residents. “Calorie posting” laws are in effect in cities such as New York and Seattle, and the recent health care reform bill mandates calorie posting for all chain restaurants with 20 or more outlets.
If large portions and effective marketing lead people to eat more when they go to restaurants than when they eat at home, then these regulations may reduce obesity. But it is not obvious that the link between eating at restaurants and obesity is causal. The increasing prevalence of restaurants may in part reflect a greater demand for calories.
The case against restaurants centers on correlations showing that the frequency of eating out is positively associated with greater fat, sodium, and total energy intake, as well as with greater body fat. These correlations have been reproduced in a broad range of data sets and study populations. Furthermore, the number of restaurants and the prevalence of obesity have been rising for a number of decades. But simple correlations between restaurant visits and overeating may conflate the impact of changes in supply and demand. People choose where and how much to eat, leaving restaurant consumption correlated with other dietary practices associated with weight gain. A key question is whether the growth in eating out is contributing to the obesity epidemic, or whether these changes merely reflect consumer preferences. The interesting causal parameter is how much more an obese person consumes in total because he or she ate at a restaurant. If changes in preferences are leading consumers to eat out more, regulating restaurants may only lead consumers to shift consumption to other sources rather than to reduce total caloric intake.
Empirical Research Design
In a paper forthcoming in the American Economic Journal: Applied Economics, we reexamine the conventional wisdom that restaurants are making America obese. We assess the nature of the connection between restaurants and obesity by exploiting variation in the supply of restaurants and examining the impact on consumers’ body mass. In rural areas, interstate highways provide variation in the supply of restaurants that is arguably uncorrelated with local consumer demand. To serve the large market of highway travelers passing through, a disproportionate number of restaurants locate immediately adjacent to highways. For residents of these communities, we find that the highway boosts the supply of restaurants (and reduces the travel cost associated with visiting a restaurant) in a manner that is plausibly uncorrelated with demand or general health practices. To uncover the causal effect of restaurants on obesity, we compare the prevalence of obesity in communities located immediately adjacent to interstate highways with the prevalence of obesity in communities located slightly farther away.
The estimates suggest that restaurants — both fast food and full service — have little effect on adult obesity. The differences in obesity rates between communities adjacent to highways and communities farther from highways are close to zero and precise enough to rule out any meaningful effects. These results indicate that policies focused on reducing caloric intake at restaurants are unlikely to reduce obesity substantially, at least for adults.
But given that a typical restaurant meal contains more calories than a home-cooked meal, it may seem surprising that greater restaurant availability does not increase obesity. To understand why restaurants have little impact on obesity, we examine food intake data collected by the U.S. Department of Agriculture. These micro data contain information on all food items consumed by a large panel of individuals. We find that people who eat large portions in restaurants tend to reduce their calorie consumption at other times during the day; calories eaten in the restaurant substitute (at least in part) for calories eaten at other times that day.
These food intake results have broad implications for obesity policy and general health and safety regulation. Economic theory implies that regulating specific inputs in the health production function may not improve outcomes if consumers can compensate in other ways. For example, previous research has suggested that smokers react to cigarette taxes by smoking fewer cigarettes more intensively. In the case of obesity, consumers have access to multiple sources of cheap calories. Restricting a single source — such as restaurants — is therefore unlikely to affect obesity, as our findings confirm. This mechanism may underlie the apparent failure of many interventions targeted at reducing obesity. Despite their ineffectiveness, such policies have the potential to generate considerable deadweight loss. Our results suggest that obesity reductions are unlikely in the absence of more comprehensive policies.
Data The obesity data used in this study come from a confidential extract of the Behavioral Risk Factor Surveillance System (BRFSS). The BRFSS is an ongoing, large-scale telephone survey that interviews hundreds of thousands of individuals each year regarding their health behaviors. In addition to questions about demographic characteristics and health behaviors, the survey asks each individual to report his or her weight and height.
Although national BRFSS data are publicly available from the Centers for Disease Control, the CDC does not release geographic identifiers at a finer level than the county. To complete our study, we requested confidential BRFSS extracts from states that include a much finer geographic identifier: telephone area code and exchange (i.e., the first six digits of a 10-digit telephone number). Eleven states — Arkansas, Colorado, Iowa, Kansas, Maine, Missouri, North Dakota, Nebraska, Oklahoma, Utah, and Vermont — cooperated with our requests. Sample years vary by state and overall cover 1990 to 2005.
Our measures of obesity include body mass index (BMI), defined as weight in kilograms divided by the square of height in meters. A person is considered overweight if he has a BMI of 25; he is obese if his BMI is over 30. The average BMI in our sample is 26.6, the prevalence of overweight individuals is 58 percent, and the prevalence of obese individuals is 21 percent. These figures closely match national averages over the same time period. Restaurant establishment data are from the United States Census ZIP Code Business Patterns and include separate counts of full service (“sit-down”) and limited service (“fast food”) restaurants for every ZIP code in the United States. Ideally we would have individual-level data on frequency of restaurant consumption to document the relationship between restaurant consumption and proximity to an interstate highway. To our knowledge, however, no existing data sets with this information have the necessary sampling rates to provide a sample of meaningful size in our study areas. Instead, we conducted our own survey on frequency of restaurant consumption, described below.
Restaurant Proximity and Body Mass
Our goal is to measure the effect of restaurant consumption on body mass. In this section, we examine the effect of restaurant availability on body mass; in the next section, we confirm that restaurant availability affects restaurant consumption. An analysis that assumes restaurant placement is exogenously determined (i.e., uncorrelated with other factors that could affect obesity) is unattractive. Both restaurants and people choose where to locate, so restaurant availability is likely to be correlated with other factors that could affect weight. We address this issue by finding an instrumental variable that satisfies two essential properties: first, it affects restaurant availability, and second, it is uncorrelated with other determinants of weight.
Distance Our instrument exploits the location of interstate highways in rural areas as a source of exogenous variation in restaurant placement. We compare two groups of small towns: those directly adjacent to an interstate highway (0–5 miles away) and those slightly farther from an interstate (5–10 miles away). For convenience, we refer to these two sets of towns as “adjacent” and “nonadjacent,” respectively.
The interstate highways were designed in the 1940s to connect the principal metropolitan areas and industrial centers of the United States. As an unintended consequence, the highways lowered transportation costs for rural towns that happened to lie on highway routes running between major cities. Previous work has concluded that highways may affect county-level economic outcomes, which might in turn have some impact on obesity. To avoid this potential confounding factor, our study uses a much finer level of geographic detail: ZIP codes and telephone exchange areas. This geographic detail enables us to limit our study to ZIP codes and exchanges whose centers lie within 10 miles of an interstate highway. At this level, we expect all towns to benefit from the lower long-distance transport costs that highways provide. We therefore expect — and find — no systematic differences in economic outcomes between the two groups of towns in our sample.