1 Obstetrics and Gynecology Epidemiology Center, Brigham and Women's Hospital, Harvard Medical School, Boston, MA
2 Department of Epidemiology, Harvard School of Public Health, Boston, MA
3 Strangeways Research Laboratory, Institute of Public Health, University of Cambridge, Cambridge, United Kingdom
4 Medical Research Council Dunn Human Nutrition Unit, Cambridge, United Kingdom
Correspondence to Dr. Karin Michels at the Strangeways Research Laboratory, Institute of Public Health, University of Cambridge, Worts Causeway, Cambridge CB1 8RN, United Kingdom, or the Obstetrics and Gynecology Epidemiology Center, Brigham and Women's Hospital, 221 Longwood Avenue, Boston, MA 02115 (kmichels{at}rics.bwh.harvard.edu).
Received for publication August 13, 2004. Accepted for publication January 11, 2005.
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ABSTRACT |
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bias (epidemiology); data collection; diet; food; nutrition assessment; questionnaires; regression analysis
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INTRODUCTION |
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Our previous observations were based on nutrients, which are additionally correlated by being derived in part from the same foods. In the current paper, we examine the behavior of two food groups, fruit and vegetables, whose intakeand hence the error in their assessmentis correlated in most people. We were interested in exploring whether inclusion of these food groups plus total caloric intake in a model adequately captured their association with plasma levels of vitamin C.
Vitamin C is an essential nutrient that is circulated in the bloodstream. Vitamin C intake is the strongest predictor of plasma levels of vitamin C, and about 90 percent of dietary vitamin C in Western diets comes from consumption of fruits and vegetables, mainly citrus fruits and juices, green vegetables, tomatoes, and potatoes (3). Hence, plasma vitamin C is a biomarker of both fruit and vegetable consumption and vitamin C intake (4
, 5
). Absorption and clearance of vitamin C, as well as smoking habits, infections, and inflammation, affect plasma levels of vitamin C.
It is customary in analyses of epidemiologic data to compare outcomes among individuals with extreme consumption of specific food items of interest (i.e., comparing the highest and lowest categories of intake). It has been assumed that the ranking of individuals according to their levels of intake is preserved despite measurement error in diet assessment. We considered the effect of categorization of fruit, vegetables, and energy intake on the association with plasma vitamin C.
The European Prospective Investigation into Cancer and Nutrition (EPIC) in Norfolk, United Kingdom (EPIC-Norfolk), provides unique data on self-reported diet assessed with both a 7-day diary diet record and a food frequency questionnaire (FFQ) and plasma levels of vitamin C obtained from 4,487 women and men. This allowed us to compare the performance of these two assessment instruments in relation to the biomarker.
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MATERIALS AND METHODS |
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Diet assessment
7-day diet diary.
At the clinic visit, trained nurses, using the participant's diet of the previous day as an example, taught participants how to fill in the diary. Participants completed the second and subsequent 5 days of the diary at home, recording in as much detail as possible all foods and beverages they had consumed. The 7-day diary booklets included colored photographs of 17 foods, each with three different portion sizes to help participants estimate the portion size consumed. The diaries were mailed back to the coordinating center at the University of Cambridge. Diary data were coded and analyzed with a specially developed program for extraction of average daily nutrient intakes (7, 8
).
Food frequency questionnaire.
The self-administered semiquantitative FFQ was designed to measure the average consumption of 130 food items during the year preceding the baseline health check. The questionnaire was based on the FFQ developed by Willett et al. (2, 9
) and adapted as previously described (7
, 10
). For each food item, participants were asked to indicate their usual consumption from one of nine frequency categories ranging from "never or less than once per month" to "six or more times per day." The FFQ did not include specific questions on portion size but rather specified average portions and unit sizes (e.g., piece, slice) or household units (e.g., glass, cup, spoon). Nutrient intake was calculated with a specially developed program (11
).
Biomarker
Plasma vitamin C level was measured from blood samples taken in citrate-covered tubes by venipuncture. Plasma was stabilized in a standardized volume of metaphosphoric acid and stored at 70°C. The plasma concentration of vitamin C was measured with a fluorometric assay within 1 week of sampling (12). The coefficient of variation ranged from 4.6 percent to 5.6 percent across the distribution of plasma vitamin C concentrations.
Statistical analysis
EPIC-Norfolk participants were included in this analysis if they had plasma measures for vitamin C and if information on fruit and vegetable consumption from both dietary instruments was computerized (n = 5,067). Regular users of vitamin supplements containing vitamin C were excluded from this analysis (n = 446). We also excluded participants whose measured plasma levels of vitamin C were below the first percentile or above the 99th percentile (n = 86) and participants whose total caloric intake as assessed by either instrument was below 500 kcal or above 4,200 kcal (n = 14).
The distribution of individuals in quintiles of fruit consumption, vegetable consumption, and total caloric intake according to the 7-day diary and FFQ values was cross-tabulated. Agreement was evaluated using a weighted kappa statistic, which considers disagreement close to the diagonal less heavily than disagreement further from the diagonal.
Linear regression was used to model the association between plasma vitamin C levels and self-reported consumption of fruits and vegetables as assessed by 7-day diary and FFQ. We considered fruit consumption and vegetable consumption separately in our analyses and created a combined variable summing data across the consumption of fruits and vegetables. To permit comparability between the two diet-assessment instruments, we standardized fruit and vegetable consumption and energy intake by dividing them by their respective standard deviations. Regression coefficients for standardized dietary variables resulting from the linear regression models are interpretable as the µmol/liter change in vitamin C plasma levels per one-standard-deviation change in fruit, vegetable, or energy intake. Fruit and vegetable consumption was also modeled per 100 g of consumption of the food.
We also included quintiles of the fruit, vegetable, and energy intake variables in our models to explore how categorization affected the association with the dependent variable given the differences in classification of fruit and vegetable consumption obtained with the two diet assessment instruments. Note that creating quintiles is a form of standardization, since we compare the second, third, fourth, and highest quintiles of estimated intake with the lowest quintile as estimated with either instrument.
Regression models were adjusted for potential confounders assessed concurrently: age, sex, body mass index (weight (kg)/height (m)2), height, and current smoking. Participants with missing values for any of the covariates were excluded from this analysis (n = 34). This left a study population of 4,487 EPIC-Norfolk participants.
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RESULTS |
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Adding total caloric intake to the model did not affect estimates for dietary variables obtained with the 7-day diary, while regression coefficients for foods from the FFQ were marginally altered (models 4, 10, 16, and 17). Adding energy intake had a larger impact on coefficients of vegetable consumption (models 10, 16, and 17) than on coefficients of fruit consumption (models 4, 16, and 17). While energy intake calculated from either the 7-day diary or the FFQ was not related to plasma vitamin C levels in a univariate model (model 1), significant inverse associations emerged when energy was added to models with fruit and/or vegetable intake reported on the FFQ, but not for those reported in the 7-day diary (models 4, 5, 10, 11, 16, 17, and 22).
When fruit and vegetable consumption was categorized into quintiles of intake, the association with plasma vitamin C persisted (models 6, 12, and 18). Inclusion of quintiles of fruit consumption, vegetable consumption, and caloric intake in one model produced similar associations between fruit and vegetable consumption and plasma vitamin C for both the 7-day diary and the FFQ, but caloric intake was significantly related to plasma vitamin C only if caloric intake was estimated from the FFQ (model 19).
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DISCUSSION |
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Our analytic model regressed plasma vitamin C level on fruit consumption and vegetable consumption. Plasma vitamin C has been found to be the biomarker with the strongest relation to fruit and vegetable consumption (13, 14
).
We found an approximate twofold difference in fruit consumption and vegetable consumption estimated from the two diet assessment instruments. Fruit and vegetable consumption reported in the 7-day diary was approximately consistent with reports of an average intake of three servings per day by the United Kingdom population (15). A 24-hour dietary recall administered in a subgroup of the United Kingdom EPIC population found a mean fruit consumption of about 160 g/day (172.9 g among women and 148.7 g among men) and a mean vegetable consumption of about 160 g/day (165.4 g for women and 157.4 g for men) (16
). Hence, it is possible that fruit and vegetable consumption is overreported on the FFQ and captured more accurately by the 7-day diary, but we cannot be certain which instrument provides the more accurate assessment. Furthermore, since the 7-day diary was proximal to the time of blood drawing and recorded fruit and vegetable consumption during a 1-week period while the FFQ asked the respondent to recall habitual diet during the year prior to blood drawing, the 7-day diary intake levels would be expected to be more closely correlated with plasma vitamin C levels than the FFQ intake levels. It is unlikely, however, that the population mean in fruit and vegetable consumption decreased considerably during this 1-year time interval.
A few studies have attempted to validate self-reported intake of foods. The validity of food intake measurements obtained by means of an FFQ was evaluated among 173 participants in the Nurses' Health Study (17) and among 127 participants in the Health Professionals' Follow-up Study (18
) by comparison with reports from 7-day diaries. In both studies, self-reported consumption of fruits and vegetables was higher on the FFQ than in the 7-day diary.
In the present study, correlations between fruit and vegetable consumption from the FFQ were higher than those from the 7-day diary, probably indicating a higher degree of correlated error in the FFQ values. The change in regression coefficients for FFQ-derived foods when energy was included in the model also suggests a higher error correlation for FFQ foods.
The measurement error in the assessment of fruit and vegetable consumption also introduced substantial differences in classification of individuals into categories of intake. When individuals were grouped in quintiles of intake, their ranking differed according to whether intake values from the 7-day diary or the FFQ were considered. For a substantial proportion of the population, classification differed by more than one quintile.
When quintiles of fruit and vegetable consumption were included in a linear regression model, regression coefficients were similar for intakes estimated with the two dietary assessment instruments. However, quintile mean values differed, leading to different interpretations of the comparisons made. Whereas comparing individuals in the highest quintile of vegetable consumption with those in the lowest quintile indicated an increase in plasma vitamin C levels of approximately 10 µmol/liter with both dietary instruments, the 7-day diary required an increase from an average of 25 g/day to 195 g/day, whereas the FFQ required an increase of 86 g/day to 369 g/day. Respective dietary recommendations based on the FFQ would prescribe twice the level of consumption of fruit and vegetables as the 7-day diary to achieve a comparable health benefit (e.g., a change in plasma vitamin C level translating into reduced mortality and/or morbidity).
In our previous analyses, error correlations between nutrients derived from the same questionnaire distorted estimates in a statistical model (1). This distortion was particularly pronounced if nutrient values were untransformed and energy was entered into the model as a separate term. In the food model presented here, error correlations between foods on a questionnaire did not seem to result in distorted estimates in analytic models, even if energy was introduced as a separate term. Nutrients may be more affected than foods by correlated measurement error, since nutrients are additionally correlated through their shared food sources.
In summary, we found substantial differences in classification of fruit consumption and vegetable consumption assessed with a 7-day diary and an FFQ, with differences in the ranking of individuals according to their intakes estimated from the 7-day diary and the FFQ. The difference in ranking did not have a substantial impact on estimation of the effect of fruit and vegetable consumption when intake was categorized into quintiles, but the errors in the assessment of fruit and vegetable consumption with the 7-day diary and the FFQ resulted in different estimates of their effect size. We did not find distortions of effect estimates due to correlated errors in the estimation of fruit consumption, vegetable consumption, and energy intake.
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ACKNOWLEDGMENTS |
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EPIC-Norfolk is supported by program grants from the Cancer Research Campaign and the Medical Research Council, with additional support from the British Heart Foundation, the Stroke Association, the United Kingdom Department of Health, the United Kingdom Food Standards Agency, the Europe Against Cancer Program, the World Health Organization, and the Wellcome Trust.
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References |
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