| Food Composition Data: A User's Perspective (1987) |
|Dietary assessment methods used by the national health and nutrition examination surveys (NHANES)|
Dietary assessment methods used by the national health and nutrition examination surveys (NHANES)
Design of NHANES II
Major nutrition-related components of NHANES II
Uses of dietary data
Plans for future NHANES
CATHERINE E. WOTEKI
National Center for Health Statistics, US Department of Health and Human Services,
Hyattsville, Maryland, USA
The National Center for Health Statistics has conducted health examination surveys for over 20 years. The National Health Survey Act of 1956 authorized the secretary of what is now the Department of Health and Human Services, acting through the National Center for Health Statistics, to collect statistics on a wide range of health issues. Among other topics, the centre is authorized to collect statistics on "determinants of health" and "the extent and nature of illness and disability of the population of the United States (or of any groupings of the people included in the population)..
As part of its response to this mandate, the centre fielded the first National Health Examination Survey in 1959. The target population for this survey was adults of 18 to 74 years of age. Two additional surveys were conducted during the 1960s, extending the age groups examined to include children of 6 to 11 years and adolescents of 12 to 17. In 1971, the range of topics included in the survey was extended to include nutritional status. Nutritional status was assessed through a fivefold approach including a medical history, a physician's examination, biochemical tests, body measurements, and a dietary interview. The first National Health and Nutrition Examination Survey (NHANES I), conducted from 1971 to 1974, examined a representative sample of persons between the ages of 1 and 74. An additional sample of adults aged 25 to 74 years, called the NHANES I Augmentation, was examined in 1974-1975. The second National Health and Nutrition Examination Survey (NHANES II) was conducted from 1976 to 1980. The age range extended to include infants of six months to one year. In December 1984, the centre completed data collection for the Hispanic Health and Nutrition Examination Survey (NHANES) of persons of Mexican American, Puerto Rican, and Cuban ancestry residing in the continental United States. We are beginning to plan the next survey, NHANES III, scheduled to begin in 1988.
Over the years, data generated by the health examination surveys have served a variety of uses. The surveys have provided estimates of the prevalence of characteristics or conditions in the American population. Normative or descriptive data have been published on topics such as weight and stature. Both types of estimates permit the monitoring or measurement of changes in health and nutritional status over time through successive surveys. Problems of public health importance have been identified. The survey data have also been used to study the interrelationships of health and nutrition variables in the general population.
My purpose is to describe the National Health and Nutrition Examination Surveys, paying particular attention to the dietary intake data. Using NHANES II as a reference point, I will discuss design considerations, the major components of the survey related to nutrition, uses of the dietary data, and plans for future surveys.
Design of NHANES II
In approaching the design of NHANES I and II, many factors had to be considered. I will discuss here the design specifications that were considered in planning the most recent national survey, NHANES II.
NHANES II was planned to be a profitability sample of the civilian, non-institutionalized population of the United States for persons of 6 months to 74 years. Three subgroups in the population were of special interest for nutritional assessment because it was thought they were at higher risk of malnutrition: pre-school children (6 months to 5 years), the aged (60 to 74 years), and the poor (persons below the poverty level as defined by the US Bureau of the Census). These groups were oversampled to improve the reliability of the statistics generated. Although women of child-bearing age were also considered to be at risk of malnutrition, no oversampling was necessary. The total sample size desired was 21,000 examined persons, and the number of sample persons selected in each primary sampling unit (PSU) was to be between 300 and 600.
The data collection mechanism used in NHANES I was used again in NHANES II with appropriate modifications. An initial interview was conducted in the household in which sociodemographic information and medical histories were collected. Sample persons were scheduled to visit mobile examination centres in which the physical examination, dietary interview, anthropometry, and other procedures and tests were conducted. At any time during the survey period, two centres were operating in different locations while a third was being serviced or relocated. The mobile examination centres provide a controlled, standardized environment for the examinations and tests. The examinations and tests were conducted by a small, well-trained staff which moved from site to site with the mobile examination centres.
Because of the small number of mobile examination centres, the logistical constraints involved in moving and setting up the centres, the large number of sample persons, and the length of the examination, the total period for data collection was planned to be three to four years. The average length of an individual examination was two to three hours, but it varied depending on the age of the examinee. The examination for pre-school children lasted no more than two hours, while the time for an adult did not exceed three hours.
The survey was designed to produce statistics for the four broad geographic regions of the United States and for the total population by age, sex, race, and income classifications. In the end, a total of 20,322 individuals were interviewed and examined in NHANES II in 64 primary sampling units. Because not all individuals underwent all aspects of the interview and examination, appropriate non-response adjustments were made. These non-response adjustments bring the sum of the final weights into close alignment with the age, sex, and race estimates of the Census Bureau at the mid-point of the survey.
Major nutrition-related components of NHANES II
The five major components of NHANES II were a household questionnaire, a medical history questionnaire, a dietary questionnaire, examination by a physician, and special procedures and tests. The household questionnaire consisted of questions about family relationship; age, sex, and race of family members; housing information; occupation, income, and education level of each family member; and participation in the food stamp programme and school breakfast and lunch programmes. Separate medical history questionnaires were used depending on the age of the sample person, one questionnaire for children of 6 months to 11 years and another for persons of 12 to 74 years. Both the household questionnaire and the medical history questionnaire were administered in the respondent's home.
When individuals arrived at the mobile examination centre, they were scheduled through the dietary interview, physician's examination, and special procedures and tests. The procedures and tests included body measurements for all, allergy tests for persons of 6 to 74, X-rays of persons of 25 through 74 except pregnant women (cervical spine, lumbar spine except women under 50 years, and chest), and urine and blood tests. From blood samples taken in the centre, a number of nutrition-related assays were done. These included serum albumin, serum vitamins A and C, serum lipids (cholesterol, triglycerides, and high-density lipoproteins), protoporphyrin, serum iron, total iron-binding capacity, serum zinc, and serum copper. Red cell folates, serum folates, serum ferritin, and serum vitamin B12 were determined on blood samples with abnormal complete blood count, haemoglobin, haematocrit, or MCV, and on a subsample of normals.
The dietary questionnaires consisted of a 24-hour recall, a food-frequency questionnaire, a dietary-supplement questionnaire, and specific questions on medication, vitamin, and mineral supplement usage. All interviews were conducted by trained interviewers who had at least a bachelor's degree in home economics.
In the 24-hour recall, respondents were asked to report all foods and beverages consumed on the previous day. Respondents estimated the size of the portions consumed by referring to food models. In addition to foods and portion sizes, interviewers asked about what time of day the food was eaten and its source. The time of day was coded as one of five ingestion periods: morning, noon, between meals, evening, or total day. The source of the food was coded as home, school, restaurant, or other.
Each food item was coded by the interviewer within 72 hours of the interview. The food code book developed for the survey contained five-digit food codes for approximately 2,500 food items. Each food item was identified by name (including brand names if appropriate), by whether it was raw, dry, or frozen, by how it was prepared, and, for mixed dishes without food codes, by major ingredients. A food composition data base updated from NHANES I was used to calculate the energy, vitamin, and mineral content of the reported foods. Modifications to the NHANES I data base included new data from VSDA's revised Handbook No. 8, and food composition data from food companies on new products and brand-name products of unique formulation.
The food-frequency questionnaire elicited information about the consumption of 18 food groups over the previous three months. Frequency was given one of four possible codes: a whole number, never, less than once a week, or unknown. The interval at which the food was usually eaten was also given one of four possible codes: never, daily, weekly, or less than weekly. One question was asked about use of vitamin and mineral supplements, and one about how often the salt shaker was used at the table. Responses to this last question could be assigned to one of three codes: rarely or never, occasionally or seldom, frequently or always.
The dietary-supplement questionnaire contained questions about whether the respondent was on a special diet, what type, and for how long. One question asked about the possible use of nine medications in the previous week. These were commonly prescribed medications that might interfere with test results or affect interpretation of results. Another question related to problems preventing the respondent from obtaining needed groceries. The final question asked about trouble swallowing, pain, nausea and vomiting following eating, and loss of appetite.
The medication, vitamin, and mineral usage questionnaire requested specific information about brand name, manufacturer's name, and reason for using vitamin or mineral supplements and medications.
The quality of the dietary component was controlled at several levels. Before the survey began, the dietary interviewers were trained in interview techniques and in how to code the 24hour recall. A manual was issued to each interviewer which described the procedures to be followed. Periodically, the forms were reviewed and evaluated, and instructions were issued to the interviewers to promote consistency. Interviewers exchanged coded 24-hour recall forms to check each other's work, and forms were also reviewed by the field staff before being forwarded to headquarters. At every location, each interviewer tape-recorded two interviews with randomly selected subjects. The recordings were evaluated at headquarters for adherence to procedures. Comparisons were made at headquarters of the mean values and frequency distributions by stand location and by interviewer to detect unusual results by location and systematic errors by interviewers. Foods for which no appropriate food codes existed were forwarded to headquarters for assignment of new code numbers.
Uses of dietary data
NHANES dietary data have been put to four types of uses: relating diet and demographic characteristics, relating diet and health characteristics, determining interactions of diet and nutritional status indicators, and tracking trends in diet and nutrient intakes over time.
In relating diet to the demographic characteristics of the population, the major question to be asked is: What are the food consumption patterns and nutrient intakes of subpopulations of the United States by such characteristics as age, race, sex, income, occupation, and education? The NHANES dietary data can answer questions such as: How do nutrient intakes and food consumption patterns of persons differ by level of education? What are the regional differences in consumption of certain food groups?
NHANES data have been used to relate the food consumption patterns and nutrient intakes of United States subpopulations to indicators of health status. Specific questions that have been addressed include: How do nutrient intakes compare with the Recommended Dietary Allowances and other dietary guidelines? What dietary patterns are associated with higher levels of tooth decay? What dietary and health variables are associated with iron-deficiency anaemia?
Examining interactions between nutrition-related variables, NHANES data can compare dietary intake, biochemical status, anthropometry, and presence or absence of health conditions. Questions that can be addressed by the data include: What are the relationships between dietary intake and biochemical status for persons who smoke, use vitamin/mineral supplements, or use oral contraceptives? Are those who take vitamins and other dietary supplements the ones who need them? Are subpopulations with high serum cholesterol and other evidence of cardiovascular disease consuming foods high in cholesterol and saturated fats?
Changes over time in food and nutrient intakes can be tracked and correlations made with health variables. Examples of questions that can be posed to the data include: What changes in obesity and diet have taken place in the last ten years? Are serum cholesterol values declining among men and women?
Plans for future NHANES
The next National Health and Nutrition Examination Survey (NHANES III) is scheduled to begin in 1988. We have already begun planning for the survey. Among the topics being considered are the content, sample design, data processing, co-ordination with other surveys, addition of a longitudinal component, and the possibility of continuous monitoring of special groups.
The needs of government agencies, including the Food and Drug Administration, the Environmental Protection Agency, and the National Institutes of Health, and researchers in industry and academia will be considered. As the content of the survey is being developed, consideration will also be given to which topics should be considered core components. The core components would be administered to all sample persons while the non-core components would be administered to a subsample. In addition, the core components would be repeated in future surveys.
Suggestions for content will be solicited from federal agencies, the legislative branch, the public health and nutrition communities, researchers, foundations, and associations. A variety of mechanisms are being considered to gather recommendations from these groups including letters, meetings, and advertisement in journals.
Concurrent with decisions about survey content, preliminary decisions must be made about sample design, data processing, co-ordination with other surveys, addition of a longitudinal component or continuous monitoring of high-risk groups. Some of the questions to be answered include: whether it is feasible to include primary sampling units from the Health Interview Survey or the Nationwide Food Consumption Survey as primary sampling units in NHANES III; where automation can improve turn-around time, cut costs, and decrease errors; and whether NHANES III can use the same food composition data base that was used in the Nationwide Food Consumption Survey. It is conceivable that the dietary interview could be automated in NHANES III. Coding and edit checks would be accomplished during data entry while the interview is conducted. Changes could also be made to the current format, increasing the number of 24-hour recalls per person, for example.
The last 15 years have been a period of unparalleled interest in the relationship of diet to health. The dietary component of NHANES together with the clinical and biochemical assessment form a unique data set on a nationally representative sample of people. NHANES data have been used to monitor changes in health, nutritional status, and dietary intake over time. Interrelationships among dietary and health variables in the general population have been studied. NHANES III will continue to build on this information foundation.
A problem in planning the dietary component for NHANES III is that conflicting demands are being made. Regulatory agencies and researchers want more detail about the food people eat, how it is packaged and prepared, and what nutrients, additives, and toxic substances it contains. Demands for more rapid publication of data would lead us to simpler interviews with less detail about the foods consumed.
While this dilemma probably cannot be resolved immediately, we would like to hear discussion of the pros and cons of shortened, simplified interviews and data bases for use with NHANES. We would also like to hear recommendations on how to make our national surveys more compatible while extending their usefulness to policy makers and researchers.