a Department of Community Medicine, University Hospital MAS, Lund University, S 205 02 Malmö, Sweden. E-mail: Martin.Lindstrom{at}smi.mas.lu.se
b International Centre for Health and Society, Department of Epidemiology and Public Health, University College London, 119 Torrington Place, London WCIE 6BT, UK.
c Department of Medicine, Surgery and Ortopedics, University Hospital MAS, Lund University, S 205 02 Malmö, Sweden.
Reprint requests to: M Lindstrom, Department of Community Medicine, University Hospital MAS, Lund University, S 205 02 Malmö, Sweden. E-mail: Martin.Lindstrom{at}smi.mas.lu.se
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Abstract |
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Methods The Malmö Diet and Cancer Study is a prospective cohort study. The baseline examinations used in the present cross-sectional study were undertaken in 19921994. Dietary habits were assessed using a modified diet history method consisting of a 7-day menu book and a 168-item questionnaire. A subpopulation of 11 837 individuals born 19261945 was investigated. This study examined high fat intake, defined as >35.9% among men and >34.8% among women (25% quartile limit) of the proportion of the non-alcohol energy intake contributed by fat. The subfractions saturated, mono-unsaturated and poly-unsaturated fatty acids and the P:S ratio (polyunsaturated/saturated fatty acids) were analysed in the same way. The uppermost quartile (75%) of total and subgroup fat intake was also studied. Socioeconomic differences before and after adjustment for low energy reporting (LER), defined as energy intake below 1.2 x Basal Metabolic Rate, were examined.
Results No socioeconomic differences in fat intake were seen between the SES groups, except for self-employed men, and male and female pensioners. Approximately 20% in most SES groups were LER. The LER and body mass index were strongly related. The SES pattern of fat intake remained unchanged after adjustment for age, country of origin and LER in a logistic regression model. The results for the subfractions of fat and the P:S ratio did not principally differ from the total fat results.
Conclusions This study provides no evidence that fat intake contributes to the inverse socioeconomic differences in cardiovascular diseases.
Accepted 1 December 1999
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Introduction |
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The Black Report proposed several possible explanations for socioeconomic differences in health, including theories of social selection, materialist or structuralist explanations, and cultural or behavioural explanations.10 Behaviours like smoking, leisure-time physical activity and dietary habits have become increasingly socially patterned.11 The mechanisms by which a high fat intake may cause cardiovascular diseases, especially ischaemic heart disease, include an increase in the level of plasma cholesterol, change in the lipoprotein profile,1215 a direct effect on blood pressure16,17 and an increase in (body mass index) BMI.18 Higher intake of saturated fat is associated with an increased risk of coronary heart disease, whereas a higher intake of polyunsaturated fats is associated with a decreased risk.19 In Sweden, the National Board of Health and Welfare previously recommended a reduction in total dietary intake of fat to below 30% of total energy intake including all energy-yielding nutrients.20 However, the new Nordic nutrient recommendations state that total fat intake should not exceed 30% of non-alcohol energy intake. The new recommendations also state that the consumption of saturated fat should not exceed 10% of total non-alcohol energy intake, that the desirable consumption of monounsaturated fat should amount to 1015% and the desirable intake of polyunsaturated fat to 510% of total non-alcohol energy intake.21 Studies have shown differences between socioeconomic groups in the compliance with dietary fat recommendations,22,23 which could be one explanation for the socioeconomic differences in cardiovascular disease and mortality. However, such socioeconomic differences could also be due to errors in dietary assessment, such as reporting bias.2429 For instance, the Whitehall II study shows that after excluding low energy reporters the positive socioeconomic gradient in dietary fat intake disappeared, because of a significant socioeconomic gradient in low energy reporting. The proportion of low energy reporters was approximately four times higher in the lowest compared to the highest socioeconomic group.30
The aim of this paper is to investigate whether there are socioeconomic differences in total, saturated, monounsaturated and polyunsaturated fat intake in a Swedish population, and if low energy reporting might affect such a socioeconomic pattern as it did in the Whitehall II study.
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Material and Methods |
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The present study population consists of all 11 837 people aged <65 years who participated in the MDCS during the 2-year period from the spring of 1992 until the summer of 1994 born between 1926 and 1945. The study sample consists of approximately one-quarter of the whole population aged 4564 in Malmö. People aged 65 (n = 2168), homeworkers (mostly women) (n = 340) and students (n = 45) were excluded. This study sample was selected because the first version of the questionnaire used in 19911992 did not include the psychosocial variables used to investigate socioeconomic differences in other analysis projects by the same research group. Also, dietary data was from September 1994 to October 1996 assessed with a second version of the diet history method.
Subjects were recruited by postal invitation at random. Some respondents (25.2%) came to the examination spontaneously.31
All participants gave informed consent. Height and weight were assessed by trained project staff to the nearest 10 mm and 0.1 kg. The baseline demographic health questionnaire, the menu book and the food questionnaire were completed at home and controlled during the diet history interview by the diet assistants at the second visit to the MDCS project office a few weeks later.
Diet assessment
We used a modified diet history method, specifically designed for the MDCS.3234 The choice of methodology was guided by the need to assess total diet in a middle-aged and elderly urban population. The eating habits of this group were expected to be fairly regular and commonly include cooked sit-down meals. It consisted of two parts: a 7-day menu book for cooked meals, cold beverages (including alcoholic beverages), drugs, natural remedies and dietary supplements, and a 168-item questionnaire for collecting frequency information on regularly consumed foods, including hot beverages, sandwiches, edible fats, breakfast cereals, yoghurt, milk, fruits, cakes, candies and snacks during the past year. The usual portion sizes in the frequency questionnaire were estimated by the participant at home using a booklet with 48 black and white photographs. Portion sizes of dishes in the menu book were estimated during the dietary interview using a separate and more extensive book of photographs.
Energy and nutrient intakes were computed from the reported food intake of the dietary assessment method, and the food and nutrient reference values of the PC Kost2 '93.35 The method measures the entire diet, including cooking methods. It overestimates the absolute value for energy intake by 18% when compared with the reference method, 18 days of weighed food records.34 The correlations with the reference method are of the order of 0.5 to 0.6 for most of the nutrients. Compared to other usual diet methods this indicates a good concordance between the diet history method and food records. The relative validity thus ranks with the best reported in previous studies.36,37
Definitions
High fat intake was defined as 35.9% for men and
34.8% for women of non-alcohol energy intake contributed by total fat (triglyceride fatty acids, glycerol, phospholipids and sterols). The values 35.9% and 34.8% represent the lower limit (25% quartile limit) of the three uppermost quartiles of fat intake, for men and women respectively, in this study. High intakes of saturated, monounsaturated and polyunsaturated fatty acids were also defined as those above the first quartile of total non-alcohol energy intakes (14.1%, 12.6% and
5.3% for men and 14.1%, 12.1% and 5.0% for women). A low P:S (polyunsaturated/saturated fatty acids) ratio was defined as a ratio <0.30 for men and <0.29 for women, which was the lower quartile limit (a quarter of the individuals below this value) of the P:S ratio. The corresponding upper quartile limits (75% quartile) of total fat (men 43.9%, women 42.7%), saturated fat (19.1%, women 18.9%), monounsaturated fat (men 15.6%, women 14.9%), polyunsaturated fat (men 7.4%, women 7.0%) intake as well as the P:S ratio (men 0.48, women 0.46) were also analysed.
Low energy reporters (LER) were defined as those individuals reporting a total energy of <1.2 times their individual basal metabolic rate (BMR).38 This cutoff was chosen based on previous estimations of the lowest physically possible energy intakes required for weight maintenance in this sedentary population,39 and to make comparisons with other studies30 possible.
Country of originall those born in other countries than Sweden were merged into a single category.
Classification of socioeconomic status (SES) was based on data concerning job title, tasks and position at work, obtained in the questionnaire. The procedure was identical to the one used in the Swedish population census.40 The SES groups IV and V include qualified and unqualified manual workers, respectively, the SES groups II and III non-manual employees on a medium and low level, respectively, and the SES I group comprises non-manual employees in leading positions and employees with university degree. The five socioeconomic groups already defined (I, II,III, IV and V) are considered to be ordinally related to each other, which makes it possible to estimate not only socioeconomic differences but also a socioeconomic gradient for these five groups. The group self-employed people and business owners (group VI) is very heterogenous, including academically trained physicians, dentists, big company employers and also small shopkeepers, self-employed carpenters etc. Pensioners below age 65 (VII) and the unemployed (VIII) were included as two separate categories outside the active work force, thus making a total of eight socioeconomic categories. The category pensioners below age 65 partly consists of people that receive disability pensions.
Statistical methods
The prevalences of the country of origin and the SES variables were compared to the prevalences in the same age brackets in another investigation with a higher participation rate (2-tests). Crude odds ratios (OR) and 95% CI were calculated in order to examine the risk of being a high fat consumer in relation to underreporting of energy (LER), age, country of origin, BMI and SES. Multivariate logistic regression analysis was performed to investigate the importance of potential confounders (age, country of origin, LER) of the socioeconomic differences in fat intake. Socioeconomic gradients were calculated as tests for trend for the five socioeconomic groups that were ordinally related to each other (I, II, III, IV and V). Finally, LER status was included in the logistic regression analysis to estimate the importance of LER on the socioeconomic patterns of dietary fat intake. The SPSS computer package was used in all the statistical analyses.41
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Results |
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Table 1 shows that men were self-employed, non-manual employees in higher positions and qualified manual workers to a higher extent than women, while women more often than men were non-manual employees in lower and middle positions and unqualified manual workers. The proportion of people of foreign origin was the same for men and women, 13.5% and 12.2%, respectively. Men (21.8%) and women (21.4%) were LER to the same extent. Men had generally higher BMI than women.
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Table 3 shows that a fifth of both men and women were LER. Low energy reporting was more common in older age groups. People born abroad had a higher OR of being LER than people born in Sweden. Both male and female non-LER had twice as high an OR of having a high fat intake compared to LER. For both males and females, there was a large difference in low energy reporting according to BMI. Above BMI 30.0 an OR 10.7 (95% CI : 4.923.4) was obtained for men; an OR of 6.1 (95% CI : 4.38.9) for women. However, no differences in LER between SES groups were seen, except for female disability pensioners who had a significantly higher proportion of LER compared to the SES group I (OR = 1.7, 95% CI : 1.32.3).
When country of origin and LER were included in the final multivariate model (together with age and country of origin), no change in the OR appeared. Thus, the LER variable did not alter the socioeconomic patterns in fat intake already apparent. The same patterns were observed for the upper quartile (75%) limit for total non-alcohol fat intake for both men and women (Table 4).
When the lower quartile limits (25%) of the three subfractions of fat were analysed separately in the multivariate model, an OR of 1.4 (95% CI : 1.11.8) in the intake of saturated fat and an OR 1.3 (95% CI : 1.01.7) in the intake of monounsaturated fat were seen for the self-employed (SES group VI) among men. Both male (OR = 0.6, 95% CI : 0.50.8) and female (OR = 0.6, 95% CI : 0.50.7), disability pensioners had a lower proportion of people with high intake of saturated fat. Male disability pensioners (OR = 0.6, 95% CI : 0.50.8), and the male unemployed (OR = 0.7, 95% CI : 0.50.9) had a lower intake of polyunsaturated fat. When age, country of origin and LER were included in the multivariate logistic regression model for men and women respectively, the SES pattern did not change. Finally, when the OR of having a low P:S ratio was analysed in the model, an OR of 1.5 (95% CI : 1.22.0) for male self-employed and business owners (SES group IV) and an OR of 1.9 (95% CI : 1.42.5) for unemployed men was seen. No SES differences were seen for women. When age, country of origin and LER were included in the multivariate logistic regression model for men and women respectively, the SES pattern did not change (Table 5).
The distribution of total fat intake, subgroups of fatty acids and the P:S ratio did not show any important SES differences at the upper quartile (43.9% of total energy intake for men and 42.7% for women) level. The results of the multivariate analyses for the upper (75%) quartile limits for saturated, monounsaturated, polyunsaturated fatty acids and the P:S ratio did not differ from the results at the lower quartile levels (Table 6).
No significant (P < 0.05) SES gradients (analysis including SES groups I, II, III, IV and V) were seen for either men or women in any of the total fat, saturated, monounsaturated, polyunsaturated or P:S ratio models.
The multivariate models (Tables 4, 5 and 6) were also calculated with the exclusion of the spontaneously appearing participants. These analyses yielded the same results as the results already illustrated in Tables 4, 5 and 6
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Body Mass Index (BMI) was not included in the multivariate analyses. A multivariate logistic regression model including BMI in the analysis did not change any of the results already shown.
When the mean fat intake proportions were calculated for each of the SES groups using multivariate ANOVA analysis, the same SES patterns as those illustrated in this study were observed.
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Discussion |
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The risk of misclassification of fat intake is related to the concern that self-reported energy intakes often are too low for the habitual energy consumption. A difference in the measurement of fat intake between the SES groups might produce a differential misclassification that would not be compensated for by increasing the sample size. Differences in literacy skills, the ability to estimate portion sizes and frequencies, dietary memories, social desirability expectations etc. between the SES or educational groups might contribute to this source of misclassification. The finding that the LER are evenly distributed in all SES groups seems to make this possibility less plausible. Non-differential misclassification is a problem of principal interest in nutrition epidemiology, since it always works in the direction towards the null. This problem may have been present in this study, because the main results were negative. However, the risk of misclassification is affected by the reproducibility and validity of the dietary assessment method used. The diet history method used in this study has been among the best obtained.3235
Objections can also be raised to the definition of LER as subjects with a total energy intake/BMR of <1.2. This cutoff only identifies underreporters by comparison with a sedentary physical activity level. Studies have shown that there is underreporting at all levels of energy expenditure and that a cutoff around 1.2 identifies only about 50% of them.42 The situation would be improved if a more appropriate higher mean physical activity level was used in groups that are more active, or if each individual was evaluated against a physical activity level appropriate to him/herself.43 This problem has been the basis for the recommendation that all dietary studies should incorporate assessments of physical activity.44 However, such an inclusion of physical activity level has not been performed in this study since one of our main objectives was a comparison with the British studies cited above, where such an adjustment for physical activity level had not been performed.
In Sweden, the National board of Health and Welfare previously recommended a reduction of fat to below 30 % of total energy intake.20 Similar recommendations have been made by governments in other Western countries.45 The new recommendations state that fat intake should not exceed 30% of non-alcohol energy intake.21 However, the main resultsno SES differences in fat intakeremained the same even when the old recommendation (30% of total energy including alcohol) was used in the model.
The reason for using the lower (25%) and higher (75%) quartile cutoff limits instead of the 30% limit was that the 30% limit resulted in a 94.3 % and 92.9 % risk population among men and women respectively.
The new recommendations from the Swedish National board on Health and Welfare also state that the intake of saturated fat should not exceed 10% of total non-alcohol energy, that the intake of monounsaturated fat should be within the limits 1015% of total non-alcohol energy intake and that the intake of polyunsaturated fat should range within the limits 510%.21 In this study, the lower quartile cutoff limit 14.1% for both sexes for saturated fat is way above the recommended upper limit. The lower quartile cutoff, 12.6% for men and 12.1% for women for monounsaturated fat is in the middle of the recommended 1015% range, while the upper (75%) quartile value indicates that a quarter of the population has an intake above the recommendations.
Overreporting has been defined as a total energy intake above 2.82 x BMR.42,43 This appears to be a negligible problem in our study, since only 0.4 % of the participants were overreporters according to this definition.
Studies of the relation between dietary fat and chronic disease commonly use different forms of energy adjustment to isolate the effect of high fat intake from that of dietary energy.4649 In this study only one kind of energy adjustment was performed by defining fat intake as a proportion of non-alcohol energy intake.
Only a few socioeconomic differences in dietary fat intake were found in this study. No SES gradients were found for either total fat intake or the fatty acid subgroups and the P:S ratio in the models. This result does not differ from the results of the Whitehall II study. However, the Whitehall II study found very strong differences in the distributions of LER. Lower SES groups had a much higher proportion of LER than higher groups. These socioeconomic differences profoundly affected the results concerning the intake of dietary fat.30 No such effects were seen in this study. Consequently, our study provides stronger evidence than the Whitehall II study for the notion that there are no socioeconomic differences in fat intake. Furthermore, the MDCS cohort represents the whole range of SES groups in Swedish society, from the white collar workers in higher positions to the unskilled blue collar workers and the unemployed, while the Whitehall II study comprises only civil servants working in offices.
The very strong relationship between BMI and low energy reporting is consistent with the findings of Stallone et al. This finding also supports their conclusion that the higher proportion of low energy reporters among lower socioeconomic groups and among persons with higher BMI are two independent phenomena.30
The almost complete absence of socioeconomic differences between the eight socioeconomic groups examined in this study seems to make differences in dietary fat intake a less plausible explanation to the socioeconomic inequalities in cardiovascular morbidity and mortality among individuals 4564 years of age living in the city of Malmö and other similar urban populations.
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Acknowledgments |
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References |
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