Dependant variables

Nutrition epidemiologist encourage that where possible multiple methods be utilized to quantify diet and nutritional status (Bates et al, 1997; Willett, 1998b; Willett and Buzzard, 1998; Hunter, 1998; Sempos, Liu and Ernst, 1999) as this approach will provide the best possible composite picture of relationships given the inherent strengths and weaknesses of nutrition epidemiology methods. Therefore food items that were thought to be of particular importance to health and that were potentially difficult to eat were selected along with corresponding biochemical analytes of import. In addition, dietary fibre was also an outcome measure. While many analytes were available for analysis this thesis focused on those that were the best correlated with recent intake of nutrient rich foods (Hunter, 1998; Bates et al, 1997) thus strengthening the plausibility of temporal relationships. Body Mass Index was utilized as a more long term measure of the relationship of dental status and nutritional status.

4.13.1 Specific foods

Carrots were chosen as they were a primary source of beta carotene in the U.S. diet (Willett & Buzzard, 1998) and potentially difficult to chew in their raw state. Data regarding the numbers of times per month carrots were eaten were derived from the Dietary Food Frequency File of the Food Frequency Questionnaire. Sample persons were asked:

□ “How often during the past month did you have carrots and other vegetable mixtures containing carrots?”

4.13.1.2 Tossed salads

Tossed salads are typically composed of green leafy vegetables and a vast array of raw vegetables (carrots, broccoli, cabbage, cauliflower) and fruits (tomatoes, green peppers) that are rich in folate, vitamin C and beta carotene. Data regarding the numbers of times per month tossed salad was eaten were derived from the Dietary Food Frequency File of the Food Frequency Questionnaire. Sample persons were asked:

□ “How often during the past month did you have tossed salad?”

4.13.1.3 Citrus fruits

Citrus fruits were chosen as a source of vitamin C in the adult population of the United States (Block & Sorenson, 1987). The data from the Dietary Food Frequency File of the Food Frequency Questionnaire were in response to the question:

□ “How often during the past month did you have citrus fruits, including oranges, grapefruits, and tangerines?”

4.13.1.4 Whole wheat and whole grain breads

The Food Frequency Questionnaire did not contain a question regarding the intake of whole grain or whole wheat types of breads. As these bread items were considered to be a major source of fiber and had in previous studies been indicated as difficult to eat by people with impaired dental health a variable was constructed to quantify their intake. The Individual Food Frequency File contained nearly 7500 variables that sample persons identified by type and brand during the 24 hour dietary recall interview. All the types of whole wheat and whole grain breads were assessed and recoded into a single variable. The constructed variable identified those who reported that they had eaten a whole wheat or whole grain bread product during the previous 24 hours and those who had not.

4.13.2 Nutrient intake

The DDC system, utilized during the 24 hour dietaiy recall interview, was preprogrammed with a nutrient computation system. That is, the software automatically computed from the type, brand and quantity of items reported by the sample persons the specific nutrient intakes (Feskanich et al, 1988). This thesis used the constructed variable of total grams of dietaiy fiber intake that was computed and found in the Individual Food Frequency Data File.

4.13.3 Nutritional status

This thesis focused on the biochemical assays of serum beta carotene, serum folate and serum vitamin C. The NCHS convened a panel of the Federation of American Societies for Experimental Biology to advise them on the best methods to use (Senti & Pilch, 1984; Klasing & Pilch, 1985;

Raiten & Fisher, 1995). Detailed reports of the specific laboratoiy methods and quality assurance mechanisms employed are found in the “Plan and Operations Manual for NHANES IIF (NCHS, 1994). A brief description of the each method is present below.

4.13.3.1 Serum beta carotene

Several researchers reported that beta carotene was derived almost entirely from vegetable and fruit sources (Block, Sinha & Gridley, 1994) and was veiy sensitive to recent intake from food sources and from supplements (Willett et al, 1983; Micozzi et al, 1992). Serum levels of beta carotene were measured in NHANES HI by isocratic high performance liquid chromatography HPLC (NCHS, 1994).

4.13.3.2 Serum folate

Hunter (1998) reported that the principal sources of folacin from the diet are leafy vegetables, fruits, fortified cereals and tea. Serum folate level correlates well with both dietary intake (Bates, Black & Phillips, 1982) and with supplemental folate intake (Selhub et al, 1993). Folate levels were determined using the Bio-Rad Laboratories “Quantaphase Folate” radioassay kit (NCHS, 1994).

4.13.3.3 Serum vitamin C

When recent intake of ascorbic acid is of interest, it was recommended (Hunter, 1998) that serum vitamin C level be used as the serum level increases monotonically with dietaiy intake (Basu & Schorah, 1982; Jacob, Skala & Omaye, 1987). Serum vitamin C concentrations were measured in NHANES in using a total vitamin C, fully reduced method using high- performance liquid chromatography with electrochemical detection (HPLC-

EC) analysis (NCHS, 1994).

4.13.4 Anthropometric measure

The Body Mass Index (BMI, weight/height^) was selected as the anthropometric measure of relative body composition as it appeared to be at least as good as other measures of relative adiposity (Willett, 1998c) with the caveat that care must be taken in its use and interpretation with respect smoking status, race/ethnicity, gender, age (Ulijaszek, 1997). Bioelectrical impedance was performed in the NHANES IB but researchers report that problems existed with the calibration of technicians (personal communication). In addition, it is unclear whether bioelectrical impedance is substantially better in estimating the percent body fat in epidemiologic studies than BMI (Willett, 1998c).