Ten year trends in the dietary habits of Danish men and women. Cohort and cross-sectional data

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1 European Journal of Clinical Nutrition (1997) 51, 535±541 ß 1997 Stockton Press. All rights reserved 0954±3007/97 $12.00 Ten year trends in the dietary habits of Danish men and women. Cohort and cross-sectional data M Osler 1,3, BL Heitmann 2,3 and M Schroll 3 1 Department of Social Medicine and psychosocial Health, University of Copenhagen, Blegdamsvej 3, 2200 Copenhagen N, Denmark; 2 Danish Epidemiology Science Center at the Institute of Preventive Medicine, Copenhagen Hospital Corporation, Copenhagen Municipal Hospital, 1399 Copenhagen K, Denmark; and 3 The Copenhagen County Centre of Preventive Medicine, Department of Medicine C, Glostrup University Hospital, 2600 Glostrup, Denmark Objective: To describe 10 year trends and individual changes in food habits of Danish men and women in relation to dietary recommendations using data from both a cohort and a repeated cross-sectional study, and to examine whether the two sampling methods give similar results. Design: Baseline data were collected in 1982±1984 and respectively repeated measurements for cohort and cross-sectional changes in food habits. Setting: The County of Copenhagen, Denmark. Subjects: and women aged 30±70y in 1982±1984, 1986±1988 and 1992±1994. The trend analyses included 3785 subjects for cohort and 7316 for cross-sectional study, respectively. Longitudinal changes were studied among 2430 individuals with food data from all three examinations. Methods: Food intakes were estimated using a short food frequency questionnaire. Results: During the study period both men and women reported a decreased intake frequency of animal and vegetable fats, milk, eggs, meat products, white bread and potatoes, while they had increased intakes of low-fat margarine, fruit, raw vegetables, coarse breads, oatmeal, pasta, rice, cakes and candy. In both men and women the decrease in the consumption frequency of, white bread and potatoes, and the increase in pasta, and candy, were higher in the younger than in the older age group. In contrast, the increased consumption frequency of coarse breads, and oatmeal were most pronounced in the older age groups. For most foods the cohort and the repeated cross-sectional surveys gave similar results. Conclusions: From 1982 through 1994 the food habits of middle-aged Danish men and women changed in the direction of a more healthy diet as recommended by health authorities. With the limitation of a possible reporting bias both the cohort and repeated cross-sectional study designs may be used for monitoring changes in food intake. Descriptors: food frequency; longitudinal; cross-sectional; cohort Introduction During the past three decades Danish health authorities have recommended that the adult population reduces intake of animal fats by eating less dairy and meat products. Furthermore, in order to increase the intake of carbohydrates and dietary bres a more frequent consumption of bread, other cereal products, vegetables and fruit has also been emphasized (Lyhne & Stubgaard, 1992). Dietary surveys of community residents at two or more points in time may be required to obtain direct evidence on whether such goals have been met. Such surveys include cohort designs in which a panel of individuals are surveyed repeatedly, and/or repeated cross-sectional designs in which a fresh sample of individuals is examined on each survey occasion. The cohort estimates can be seriously biased because dropout between measurements may be related to the diet intake and the repeated questioning may intervene with responses. Furthermore, the ageing of a cohort may also in uence results. The cohort estimate Correspondence: Dr M Osler. Received 10 January 1997; revised 24 March 1997; accepted 14 April 1997 may be more biased but has lower standard deviation than the cross-sectional estimate, so that it is not obvious which estimate to choose for measuring effects of communitywide interventions (trends) (Koepsell et al, 1992; Diehr et al, 1995). Repeated cross-sectional studies have been used for monitoring trends in dietary habits in for example Finland and USA (PraÈttaÈla et al, 1992; Popkin et al, 1996). These studies have indicated that the food intake has changed in the direction of a more healthy diet. Also cohort studies, with repeated measurements of dietary intake of the same individuals, are available from a number of countries, and have been used to describe time trends in dietary habits (Huijbreghts et al, 1995). Here, too, results indicate that food intake has changed in the direction of a more healthy diet. However, to our knowledge no studies have examined whether the different sampling methods applied on the same population give comparable results for change in food habits. The aims of the present study were therefore (1) to describe the 10 year trends and individual changes in dietary habits of Danish men and women in relation to dietary recommendations using data from both a cohort study and from repeated cross-sectional studies, and (2) to examine whether these two sampling methods reveal similar results.

2 536 Material and methods Subjects The participants were drawn from three random agestrati ed samples of people aged 30, 40, 50 and 60y among the population in 11 municipalitites in the Western part of the Copenhagen County, which are considered fairly representative of the general population. In 1982 the sample comprised 4807 subjects of whom 3785 (79%) attended a general health examination, carried out from 1982±1984. The second sample drawn in 1986 included 2000 subjects of whom 1504 (75%) participated in a health examination carried out from 1986±1987. The third sample drawn in 1991 included 1912 subjects, and 2027 (70%) attended the health examination carried out from 1991±1992. This sample also included subjects aged 70 y. In 1987±1988, those who participated in the baseline study in 1982±1984 were invited to a follow-up examination. Of those invited 2987 (83%) attended. Five years later, 4130 of the 1982 sample, were still alive and lived in Denmark. These were invited to the second follow-up examination from 1993± 1994, where 2554 (65%) attended. Table 1 gives the age distribution of participants. The longitudinal analyses of individual changes were based on the 2430 subjects with food data from all three examinations. The study was a part of the Danish MONICA project and was carried out in collaboration with the Copenhagen County Centre of Preventive Medicine. The study was approved by the Ethical Committee of the County of Copenhagen, and all participants had provided informed consent. Questionnaire data An extensive questionnaire on socio-demographic variables, lifestyle, and health was completed before the general health examination. This questionnaire included questions about how often 26 food items usually were consumed. The alternatives used in the frequency scale were as follows: never, once a month or less, twice a month, once a week, 2±3 times a week, once a day, 2±3 times a day, and 4 times or more daily. In the statistical calculations the scale alternatives were converted into weekly frequencies. The FFQ did not include any quantitative assessments of portion sizes. The same questionnaire has been used by the WHO for monitoring food consumption patterns in the MONICA project (Boeing et al, 1989), and has furthermore been validated as a part of the present study (Osler & Heitmann, 1996). This validation showed that the mean food intake measured by the diet history increased with increasing frequency category for the FFQ, indicating that the FFQ was able to identify levels of food intake correctly. Furthermore, those who reported a less frequent intake by FFQ in 1993±1994 compared with Table 1 Age (y) Number of participants in relation to sex, age and study Cross sectional (1) study (1982±1984) (2) study (1986±1988) (3) study (1991±1992) Total Cohort Baseline (1982±1984) (1) follow-up (1987±1988) (2) follow-up (1993±1994) Total

3 1987±1988 also has lower mean daily intakes according to the diet history interview. Thus the FFQ may be used to monitor changes in food patterns at a group level. The method of data collection was exactly the same at the ve examinations. Statistical methods The FFQ data were measured on an ordinal scale, and differences in food intake over time were described using a non-parametric test for trend across ordered groups developed by Cuzick (1995), and a page test for the related samples (longitudinal analyses). The FFQ data was also dichotomised as daily and non daily intakes, and for each food item differences in the percentage of participants with a daily food intake were tested by chi-square tests for trend in proportions. Differences between men and women in food frequency intake at baseline were analysed using the Mann±Whitney test. Results At the examination in 1982±1984 women consumed lowfat margarine, cheese, fruit, vegetables, coarse ryebread and coarse white bread more often, and animal fats, vegetable margarine, meat products, light ryebread, white bread and potatoes less frequently than men (all P < 0.01). There were no differences between men and women in how frequently they consumed milk products, eggs, sh, pasta, rice, cakes, jam and candy (all P > 0.05). Older men and women consumed animal fats, low-fat margarine, light ryebread, white bread, potatoes, fruit, sh and jam more frequently and vegetable margarine, milk, coarse ryebread, coarse white bread, rice, pasta, meat for sandwiches, ice cream and candy less frequently than the younger age groups (data not shown). When data from baseline was compared with the rst and second follow-up study, there were decreasing trends in the consumption frequencies in both men and women for animal and vegetable fats, milk, eggs, meat products, white bread and potatoes, while the consumption frequencies of low-fat margarine, fruit, raw vegetables, coarse rybread, coarse white bread, oatmeal, pasta, rice, cakes and candy increased (all P < 0.05). The intake frequencies of sh, juice, and boiled vegetables increased only in men, while the consumption frequency of cheese decreased only in women (Table 2). For some foods, changes were associated with age. In both men and women the decrease in the consumption frequency of, white bread and potatoes, and the increase in pasta, and candy, were higher in the younger than in the older age group. In contrast, the increased consumption frequency of coarse breads, and oatmeal were most pronounced in the older age groups. As an example Figure 1 shows these changes for potatoes and coarse white bread, using the data from the repeated cross-sectional surveys. Except for intake frequencies of cheese, vegetable margarine and fruit in men and of fruit and ice cream in women, the cohort and crosssectional data showed similar trends over time (Table 2). Among the 2430 individuals with longitudinal data the values were very close to those reported when data from the cohort examinations were analysed as if they were independent samples (data not shown). The analyses of changes in the percentage of participants with a daily intake of the different foods established nearly the same variation in food intake, whether data from the cohort or the crosssectional study was used (Table 3). Discussion The present study shows that from 1982 through 1994 the food habits of middle-aged Danish men and women changed in the direction of a more healthy diet as recommended by health authorities. In accordance with the recommendation of eating less dairy and meat products, we found a decreasing trend in the consumption frequency of animal fats, milk, eggs and meat products. The study also suggested compliance with the recommended increased intake of bread, other cereals, vegetables and fruit because of the increasing intake frequencies of coarse breads, raw vegetables, oatmeal, pasta and rice. However, at the same time the consumption frequency of potatoes decreased. The increased consumption frequency of pasta and decrease in potatoe use may re ect cross-cultural changes in eating habits rather than the educational efforts. Observed changes in food consumption and other health-related behaviours may be biased, due to an increased awareness in the population on health-related matters, and a tendency to give socially acceptable answers. Hence, it can not be excluded that the changes found in the present study are the result of reporting errors rather than results from actual changes in food intake frequency. Furthermore, our study was based on analyses of food frequencies, however, it has been shown earlier that the frequency of use of a food item is directly proportional to the quantity eaten (Abrahamson et al, 1963; Osler & Heitmann, 1996), and for several foods the changes were supported by data from the National Food Supply Statistics (Fagt & Groth, 1992). During the period under study the National Food Supply Statistics showed a decrease in the use of butter, margarine, whole milk and potatoes, while the use of vegetables for salad, rice, chocolate and candy increased. The late increase in consumption frequency of pasta revealed by the present study was also supported by the statistics. Indeed, from 1988±1990 the yearly use of pasta products doubled from 0.9 kg/capita to 1.8kg/capita. For meat, and eggs the statistics showed an opposite trend to that reported in our study. However, the increase in egg supply is mainly due to an increased use of eggs for food production in the industry (Fagt & Groth, 1992), while the questionnaire data used in the present study most likely re ects changes in the consumption of eggs within households. The meat supply showed a marked increase for both pork and beef meat, and it is uncertain why the present study showed an opposite trend. Data for changes in the consumption of sh was not available in the supply statistics, and data for fruit not comparable with the data from the present study. In an earlier study in a subgroup of the participants in the longitudinal study, we showed, that the observed changes in food intake were associated with a decrease in median fat energy percent from 41±38 (Osler & Heitmann, 1997). Also, the mean level of total serum cholesterol has been reported to decrease by nearly 1% per year in the same study population (Sjùl et al, 1991). Hence, it is likely that the changes in food intake, revealed by the present study, re ect true changes. Conclusions In general, both the longitudinal study and the repeated cross-sectional surveys showed the same trend and individual difference in food intake between gender and over 537

4 538 Table 2 Mean food frequencies (time/week) in 1982±1984, 1986±1988 and 1991±1994 in Danish men and women. Data from cohort and repeated crosssectional studies Food item Cohort Cross-sectional 1982± ± ± ± ± ±1993 (n ˆ 1950) (n ˆ 1463) (n ˆ 1289) (n ˆ 1950) (n ˆ 748) (n ˆ 1010) Animal fats * * Vegetable m * Low-fat m * * Cheese * Milk, yoghurt * * Eggs * * Fish * * Meat * * Meat for sandwich * * Fruit * * Juice * * Vegetables, boiled * * Vegetables, raw * * Coarse white bread * * Coarse ryebread * * White bread * * Light ryebread Oatmeal * * Pasta * * Potatoes * * Rice * * Cakes, biscuits * * Candy, chocolate * * Jam, honey Ice cream, soda Animal fats * * Vegetable m * * Low-fat m * Cheese * * Milk, yoghurt * * Eggs * * Fish Meat * * Meat for sandwich * * Fruit * * Juice * Vegetables, boiled Vegetables, raw * * Coarse white bread * * Coarse ryebread * * White bread * * Light ryebread * * Oatmeal * * Pasta * * Potatoes * * Rice * * Cakes, biscuits * * Candy, chocolate * * Jam, honey Ice cream, soda * Animal fats ˆ butter, non-vegetable margarine, lard; m ˆ margarine; soda ˆ carbonated beverages. *P < 0.05 test for trend across ordered alternatives.

5 539 Figure 1 Change in intake frequencies of potatoes and coarse white bread in men and women aged 30, 40, 50 and 60y in 1982±84, 1986±88 and 1992± Cross-sectional data.

6 540 Table 3 Percentage of men and women with a daily intake of selected food items from 1982±1994. Data from cohort and repeated cross-sectional studies Food item Cohort Cross-sectional 1982± ± ± ± ± ±1993 (n ˆ 1950) (n ˆ 1463) (n ˆ 1289) (n ˆ 1950) (n ˆ 748) (n ˆ 1010) Animal fats * * Vegetable m * Low-fat m * * Cheese * Milk, yoghurt * * Eggs * * Fish a * * Meat * * Meat for sandwich * * Fruit * Juice * * Vegetables, boiled * * Vegetables, raw * * Coarse white bread * * Coarse ryebread * * White bread * * Light ryebread Oatmeal * * Pasta a * * Potatoes * * Rice * * Cakes, biscuits * * Candy, chocolate * * Jam, honey * Ice cream, soda * Animal fats * * Vegetable m * * Low-fat m * * Cheese * * Milk, yoghurt * * Eggs * * Fish a * * Meat * * Meat for sandwich * * Fruit * * Juice Vegetables, boiled Vegetables, raw * Coarse white bread * * Coarse ryebread * * White bread * * Light ryebread * * Oatmeal * * Pasta a * * Potatoes * * Rice * * Cakes, biscuits * Candy, chocolate * * Jam, honey * Ice cream, soda * Animal fats ˆ butter, non-vegetable margarine, lard; m ˆ margarine; soda ˆ carbonated beverages. *P < 0.05 test for trend in proportions. a Percent with weekly intake.

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