In the year 1999, 64 percent of the U.S. population was overweight or obese, while the prevalence of obesity among children and adolescents more than doubled during the previous two decades. Fifty-six percent of women over the age of fifty had low bone density, and 16 percent were suffering from the debilitating disease of osteoporosis. And while smoking prevalence hit an all-time low among adults, it has continued to increase among America’s youth.
Statistics like these sell newspapers, inspire public-policy initiatives, and provide topics for classroom discussions. But where do these statistics come from? How can scientists determine the percentage of the entire U.S. population that suffers from conditions such as obesity and osteoporosis or engage in unhealthy habits such as smoking?
Statistics such as those listed above, along with a whole host of other health and nutrition data, are derived from the National Health and Nutrition Examination Survey, or NHANES. NHANES is arguably the largest and longest-running national source of objectively measured health and nutrition data. Through physical examinations, clinical and laboratory tests, and personal interviews, NHANES provides a “snapshot” of the health and nutritional status of the U.S. population. Findings from NHANES provide health professionals and policymakers with the statistical data needed to determine rates of major diseases and health conditions (e.g., cardiovascular disease, diabetes, obesity, infectious diseases) as well as identify and monitor trends in medical conditions, risk factors, and emerging public health issues, so that the appropriate public health policies and prevention interventions can be developed.
History of NHANES
The current NHANES was born out of The National Health Survey Act of 1956. This particular piece of legislation provided for the establishment of a continuing National Health Survey to obtain information about the health status of individuals residing in the United States, including the services received for or because of health conditions. The responsibility for survey development and data collection was placed upon the National Center for Health Statistics (NCHS), a research-oriented statistical organization housed within the Health Services and Mental Health Administration (HSMHA) of the Department of Health, Education, and Welfare (now the Department of Health and Human Services). Since its inception in 1959, eight separate Health Examination Surveys have been conducted and over 130,000 people have served as survey participants.
The first three National Health Surveys—National Health Examination Survey (NHES) I, II, and III—were conducted between 1959 and 1970, each with an approximate sample size of 7,500 individuals. NHES I (1959–1962) focused on selected chronic diseases of adults between 18 and 79 years of age, while NHES II (1963–1965) and NHES III (1966–1970) focused on the growth and development of children (6–11 years of age) and adolescents (12–17 years of age), respectively.
Between the passage of the 1956 Act and the completion of NHES III, numerous nutrition-related studies were conducted that indicated that malnutrition remained a significant problem within certain segments of the U.S. population. This data, along with increasing scientific evidence linking dietary habits and risk for disease, prompted the Department of Health, Education, and Welfare to establish a continuing National Nutrition Surveillance System in 1969 (under the authority of the 1956 act) for the purposes of measuring the nutritional status of the U.S. population and monitoring the changes over time. Rather than conduct two separate surveys, which would require two separate samples and numerous additional hours of work, it was decided that the National Nutrition Surveillance System would be combined with the National Health Examination Survey, thereby forming the National Health and Nutrition Examination Survey, or NHANES.
Five NHANES have been conducted since 1970. NHANES I, the first cycle of the NHANES studies, was conducted between 1971 and 1975 and included a national sample of approximately 30,000 individuals between one and seventy-four years of age. Extensive dietary intake and nutritional status were collected by interview, physical examination, and a battery of clinical tests and measurements. NHANES II (1976–1980) included just slightly over 25,000 participants and expanded the age of the first NHANES sample somewhat by including individuals as young as 6 months of age. In addition, children and adults living at or below the poverty level were sampled at higher rates than their proportions in the general population (“oversampled”) because these individuals were thought to be at particular nutritional risk.
While NHANES I and II provided extensive data regarding the health and nutritional status of the general U.S. population, it was somewhat biased against other ethnic groups residing in the United States, particularly Hispanics, whose numbers had been steadily increasing since data collection began in the 1960s. Thus, a Health and Nutrition Surveillance Survey specifically targeting the three largest Hispanic subgroups in the United States—Mexican Americans, Cuban Americans, and Puerto Ricans—was conducted between 1982 and 1984. The Hispanic Health and Nutrition Examination Survey (HHANES) was similar in design (i.e., similar instrumentation and data collection procedures) to the first two cycles of NHANES and included 16,000 individuals residing in regions across the United States with large Hispanic populations.
NHANES III (1988–1994) included a total of 40,000 individuals and expanded the age range even further than previous NHANES by including infants as young as two months of age, with no upper age limit on adults. In addition, to ensure the representativeness of both ethnicity and age, African Americans, Mexican Americans, infants, children, and those over sixty years old were oversampled. NHANES III also placed a greater emphasis on the effects of environment on health than either of the two previous NHANES (I and II). For example, data were gathered examining the levels of pesticide exposure, and the presence of carbon monoxide and various “trace elements” in the blood.
Beginning in 1999, NHANES became a “continuous survey.” That is, unlike the previous NHANES surveys, which were conducted over a period of approximately four years with a “break” of at least one year between survey periods, the 1999–2000 survey was (and all subsequent surveys will be) National Health and Nutrition Examination Survey (NHANES) conducted without breaks, on a yearly basis. As the survey period is shorter in length, the subject sample will be smaller. The 1999–2000 survey included nutritional and medical data on approximately 8,837 individuals up to 74 years of age.
Procedures: How is NHANES Data Collected?
When NHES was originally conceived, it was determined that data would come from three primary sources:
1. Direct Interview: directly interviewing the survey participant and those within their household about their health.
2. Direct Examination: conducting clinical tests, anthropometric, biochemical, and radiological measurements, and physical examinations.
3. Physician Inquiry/Medical Records: reviewing participant’s medical record.
In current practice, however, NHANES data are derived primarily from the first two sources; that is, via direct interview and direct clinical examination.
The NHANES data collection procedures have changed slightly over the years. These changes reflect not only the changing demographics of the United States over time, but also the changing nature of the survey (e.g., the inclusion of the nutrition component, the interest in the effects of environment upon health). Nonetheless, the basic tenets of data collection, particularly with regards to sampling, are similar.
The goal of NHANES is to obtain a nationally “representative” yet manageable sample of noninstitutionalized persons residing in the United States. To achieve this goal, a nationwide probability sample of the population is selected via a complex series of statistical techniques. In very basic terms, the country is divided into geographic areas, also known as “primary sampling units” (PSUs). The PSUs are then combined to form strata, and each strata is then divided into a series of neighborhoods. Households are chosen at random from these neighborhoods, and inhabitants of those households are interviewed to determine if they are eligible for participation in the survey. Theoretically, each selected survey participant represents approximately 50,000 other U.S. residents.
Data Collection Procedures.
Once a household has been identified, a trained interviewer conducts an initial in-home interview with the potential survey participant to determine his or her study eligibility. Eligibility is determined by the collective responses to two in-depth questionnaires (the NHANES Household Adult (or Youth) Questionnaire and the Family Questionnaire) and from a series of blood pressure measurements. If the potential participant is deemed eligible for the study, an appointment is scheduled at a mobile examination center for the complete battery of medical and nutritional tests and measurements. The mobile examination centers (MEC) consist of four large trailers that contain all of the diagnostic equipment and personnel necessary to conduct a wide range of both simple and complex physical and biochemical evaluations. Four types of data collection methods are employed in the MEC:
1. A physical examination (including body measurements, a variety of X-rays, audiometry, electrocardiography, bone densitometry, allergy testing, and spirometry.
2. A dental examination.
3. Specimen collection (for hematological and urinary analysis).
4. Personal interview (to collect nutrition-related information; data on sensitive subjects such as tobacco use among youngsters, sexual experience, and depression; and tests of cognitive development and learning achievement).
The nutritional assessment component of NHANES was designed to include a variety of data sources, including:
* Dietary Intake Interviews: Quantitative and qualitative dietary information is collected using a 24-hour recall and food frequency questionnaire (FFQ).
* Nutrition-Related Interview: Information that is not sufficiently obtained via the dietary intake interview is included in this interview (e.g., water intake, vitamin and mineral supplementation, meal and snack patterns, infant feeding practices, alcohol intake, and food sufficiency).
* Anthropometric Data: Height, body weight, body composition, and various body circumferences are measured in order to determine body weight-fat distribution.
* Hematological and Nutritional Biochemistries: Blood lipid levels, blood glucose levels, vitamin and mineral status measures (e.g., iron, calcium, sodium, potassium, chloride, folate, vitamins such as B12, A, and E), and protein status (total protein, albumin, and creatinine) are determined.
* Nutrition-Related Clinical Assessments: A combination of the above methodologies are used to assess risk for chronic diseases such as cardiovascular disease, diabetes, osteoporosis, and gallbladder disease.
Results: What Have We Learned from NHANES?
NHANES is probably best known for the prevalence data it provides on obesity. Indeed, as a result of data derived from NHANES, researchers, health professionals, and makers of public policy have been able to chart the increasing prevalence of obesity in the United States, as well as changes in obesity demographics (e.g., age, ethnicity, gender). Nonetheless, NHANES provides much more than just obesity prevalence data. NHANES issues vital data on the prevalence and correlates of chronic diseases such as arthritis, cardiovascular and respiratory diseases, diabetes, gallbladder and kidney diseases, osteoporosis, and cancer. In addition, NHANES supplies important information on the prevalence and trends of risk factors and other key health behaviors, including alcohol use, tobacco use and exposure, drug use, sexual experience, immunization histories, and physical activity. Data from NHANES has been instrumental in the development and implementation of a number of health-related guidelines and reforms and public-policy initiatives, including growth charts for children, folate fortification of grain products, and a reduction in the manufacturing and sales of lead-containing products.
Future Directions: Where Is NHANES Going from Here?
As previously mentioned, beginning in 1999, NHANES became a continuous, annual survey in order to provide more timely data on the health and nutritional status of the population. In addition, NHANES will eventually be linked with other related health and nutrition surveys of the U.S. population, including the National Health Interview Survey (NHIS) and the U.S. Department of Agriculture’s Continuing Survey of Food Intakes by Individuals (CSFII). By combining and integrating the data from these extensive surveys, a more comprehensive evaluation of the current health and nutritional status of the U.S. population can be made.