Long-term dietary acrylamide intake and risk of endometrial cancer in a prospective cohort of Swedish women
Acrylamide has been found in foods heated at high temperatures and there is evidence of carcinogenicity of acrylamide in experimental animals. However, the potential health risks of dietary acrylamide intake in humans remain uncertain. We examined the association between dietary acrylamide intake and the incidence of endometrial cancer among 61,226 participants of the Swedish Mammography Cohort who were cancer-free at enrollment in 1987–1990 and completed a food frequency questionnaire at baseline and again in 1997. Cox proportional hazards models were used to estimate rate ratios with 95% confidence intervals, adjusted for endometrial cancer risk factors. During a mean follow-up of 17.7 years, a total of 687 incident cases of endometrial adenocarcinoma were diagnosed in the cohort. We found no association between long-term acrylamide intake and risk of endometrial cancer. The multivariate rate ratio of endometrial cancer for women in the highest quartile of acrylamide intake (mean, 33.8 μg/day) compared to those in the lowest quartile (mean, 15.9 μg/day) was 0.96 (95% CI, 0.76–1.21). The association did not vary materially by smoking status. In conclusion, these findings do not support the hypothesis that dietary acrylamide intake is positively associated with risk of endometrial cancer, at least not within the ranges of acrylamide consumed in this population. © 2008 Wiley-Liss, Inc.
Acrylamide is a small water-soluble molecule, classified in 1994 by the International Agency for Research on Cancer as a probable human carcinogen on the basis of its carcinogenicity in experimental animals.1 Exposure to acrylamide occurs primarily through tobacco smoke and occupational exposure, and to a minor extent through drinking water.1, 2 In 2002, Swedish researchers reported that some foods heated at high temperatures could contain relatively high levels of acrylamide.3 Shortly after this finding, it was shown that acrylamide may be formed in foods by Maillard browning reactions, in which amino acids (e.g., asparagine) reacts with reducing sugars (e.g., glucose) at high temperatures (>120°C).4, 5 High acrylamide levels have been found in foods such as potato crisps, French fries, fried potato, bread (especially crisp bread), breakfast cereals, cookies and coffee.2
In animal studies, positive dose-response relations have been found between acrylamide given in drinking water and cancer in several organs,6 particularly in hormone-sensitive organs such as the uterus and the mammary gland.7, 8 Epidemiologic studies of the association between dietary acrylamide intake and risk of various cancers are few and most studies have not found any association.9–14 However, a cohort study in the Netherlands showed a positive association between acrylamide intake and risk of postmenopausal endometrial and ovarian cancer, but not breast cancer, in never smokers.15
Given the paucity of data of the possible association between dietary acrylamide intake and endometrial cancer incidence, we prospectively examined this relationship in the Swedish Mammography Cohort, a large population-based cohort study of Swedish women.
Material and methods
The Swedish Mammography Cohort was established in 1987–1990 when all women who were born between 1914 and 1948 and residing in Uppsala and Västmanland counties in central Sweden received a mailed invitation to be screened by mammography. Enclosed with this invitation was a 6-page questionnaire that elicited information on diet, body size, education and reproductive factors; a completed questionnaire was obtained from 66,651 women, representing 74% of the source population. In the autumn of 1997, all cohort members who were still alive and residing in the study area received a second questionnaire; 39,227 women (70%) answered the questionnaire. This study was approved by the ethics committees at the Uppsala University Hospital (Uppsala, Sweden) and the Karolinska Institutet (Stockholm, Sweden).
From the baseline cohort of 66,651 women, we excluded women with an incorrect or a missing national registration number and those lacking date on the questionnaire, date of moving out of the study area or date of death. After additional exclusion of women with implausible values for total energy intake (i.e., 3 standard deviations from the mean value for loge-transformed energy intake) and women with a cancer diagnosis (except nonmelanoma skin cancer) or who had undergone a hysterectomy before baseline, the study cohort consisted of 61,226 women. Among these, 26,304 were premenopausal, 2,570 were perimenopausal and 32,352 were postmenopausal. For analyses using data from the second questionnaire, 36,369 women were included after excluding those with implausible energy intake and those diagnosed with cancer or had undergone a hysterectomy between baseline and January 1, 1998 (defined as baseline for these analyses).
Assessment of diet
A food-frequency questionnaire with 67 and 96 food items was used to assess diet at baseline and in 1997, respectively. In these questionnaires, women were asked to report how often, on average, they had consumed each food item over previous 6 months (1987–1990 FFQ) or the previous year (1997 FFQ). The questionnaires had 8 mutually exclusive predefined categories for frequency of consumption, ranging from “never/seldom” to “3 or times per day” (1997 FFQ) or “4 or more times per day” (1987–1990 FFQ). Information on the acrylamide content in Swedish foods was obtained from the Swedish National Food Administration16 and Svensson et al.17 In 2002, more than 130 food samples were collected from supermarkets in Uppsala, Sweden (the study area), for analysis of acrylamide concentrations.17 Dietary acrylamide intake was calculated by multiplying the frequency of consumption of each food item by its acrylamide content per age-specific serving. The age-specific serving sizes were based on mean values obtained from 213 randomly selected women from the study area who weighed and recorded their food intake for an average of 27.8 days.
The validity of the baseline dietary questionnaire was assessed previously by comparing responses from the FFQ with responses from four 1-week dietary records among 129 women randomly chosen from the cohort (Wolk A, unpublished data). Although the validity of values for acrylamide intake could not be directly tested, we examined correlations between the questionnaire and the dietary records for the major food sources of acrylamide. The corrected Pearson correlation coefficients were 0.6 for coffee, 0.5 for whole grain bread and 0.6 for breakfast cereals/muesli.
Assessment of nondietary exposures
Nondietary exposure data obtained at baseline and in 1997 (second questionnaire) included education, weight, height, menstrual and reproductive history, and use of oral contraceptives and postmenopausal hormones. We calculated body mass index (BMI) as weight in kilograms divided by the square of height in meters. On the second questionnaire, participants also provided information on history of diabetes, cigarette smoking and physical activity at work and leisure-time.
Case ascertainment and follow-up
We ascertained incident cases of endometrial cancer by computerized record linkages of the study population with the national and regional Swedish Cancer registers, both of which have been estimated to be almost 100% complete.18 Only primary invasive adenocarcinomas of the endometrium were included as cases in these analyses. Dates of death for deceased women were ascertained by linkages to the Swedish Death register. We obtained information on dates of hysterectomy through linkage to the Swedish Inpatient Register.
Person-time of follow-up was calculated from the date of enrollment (or January 1, 1998 for subanalyses using data from the second questionnaire only) until the date of diagnosis of endometrial cancer, the date of hysterectomy, the date of death from any cause, or December 31, 2007, whichever occurred first. Acrylamide intake was adjusted for total energy using the residual method19 and categorized into quartiles.
To account for changes in diet during follow-up and to better represent long-term dietary intake, we used a cumulative average approach.20 Specifically, the incidence of endometrial cancer from baseline through 1997 was related to acrylamide intake reported on the baseline dietary questionnaire, and the incidence of endometrial cancer from 1998 through December 2007 was related to the average acrylamide intake at baseline and in 1997; for women who did not complete the second questionnaire, only information from the baseline questionnaire was used for the whole follow-up. We also related dietary intake in 1997 to endometrial cancer incidence from 1998 through December 2007.
We used Cox proportional hazards regression models21 to estimate incidence rate ratios (RRs) and 95% confidence intervals (CIs) for the relation between acrylamide intake and risk of endometrial cancer. To control as finely as possible for age and calendar time, and possible two-way interactions between these 2 time scales, we stratified the models by age in months at start of follow-up and the year of enrollment. In multivariate models, we further adjusted for education (primary school, high school, university), BMI (<18.5, 18.5–24.9, 25–29.9, ≥30 kg/m2), parity (nulliparous, 1–2, ≥3), age at first birth (nulliparous, <26, 26–30, ≥31 years), age at menarche (≤12, 13, ≥14 years), age at menopause (<51, ≥51 years), use of oral contraceptives (ever/never), use of postmenopausal hormones (ever/never) and intakes of carbohydrate (in quartiles) and total energy (kcal/day). In subanalyses using data from the second questionnaire, we further controlled for history of diabetes (yes/no), smoking (never, past, current smoker) and total physical activity (MET-hours/day, in quartiles). We tested the proportional hazard assumption using the likelihood ratio test and found no departure from the assumption.
Because cigarette smoke is an important source of acrylamide,22 we conducted stratified analyses by smoking status (never/ever) using data from the second questionnaire. To test for trend, we assigned the median value to each quartile of acrylamide intake and treated this value as a continuous variable in the Cox model. All statistical analyses were performed using SAS version 9.1 (SAS Institute, Cary, NC). All p-values were 2-sided.
In this cohort of Swedish women, the mean (± SD) daily intake of acrylamide at baseline was 24.6 ± 7.6 μg, which corresponds to 0.38 ± 0.17 μg acrylamide per kg body weight per day. The major food sources of acrylamide were coffee (29%), whole grain bread (13%), crisp bread (8%), breakfast cereals/muesli (7%), cookies/buns (6%) and fried potato (5%). The baseline distributions of risk factors for endometrial cancer by quartiles of acrylamide intake are shown in Table I. The mean daily acrylamide intake in the lowest and highest quartile was 15.9 μg and 33.8 μg, respectively. Women with a high acrylamide intake were, on average, younger than those with a low acrylamide intake. Other characteristics did not vary materially across quartiles of acrylamide intake.
Table I. Mammography Cohort by Quartiles of Acrylamide Intake in 1987–19901
|Postsecondary education (%)||11.9||13.0||12.9||13.2|
|Body mass index (kg/m2)||24.9||24.8||24.6||24.6|
|Age at menarche (years)||13.3||13.2||13.2||13.2|
|Age at menopause (years)||50.6||50.7||50.7||50.7|
|Age at first birth (years)3||23.9||24.1||24.2||24.2|
|Number of children3||2.4||2.4||2.4||2.4|
|Oral contraceptive use (%)||53.1||54.2||54.7||53.9|
|Postmenopausal hormone use (%)||41.9||44.4||44.6||46.8|
|Ever smoker (%)4||45.1||44.9||45.0||48.9|
|Total physical activity (MET-hours/day)4||42.2||42.4||42.6||42.8|
| Total energy (kcal/day)||1616||1609||1582||1535|
| Alcohol (g/day)||2.4||2.6||2.6||2.5|
| Carbohydrate (g/day)||227||228||229||231|
| Cereal fiber (g/day)||18.0||17.8||17.7||18.0|
During 1,080,747 person-years of follow-up (mean = 17.7 years), from 1987–1990 through December 2007, a total of 687 incident cases of endometrial adenocarcinoma were diagnosed. We observed no association between long-term acrylamide intake (assessed at baseline and in 1997) and risk of endometrial cancer after adjusting for age only or in multivariate analysis adjusting for endometrial cancer risk factors (Table II). Excluding cases diagnosed during the first 2 years of follow-up did not change the results materially, multivariate RR for highest versus lowest quartile, 1.01 (95% CI, 0.79–1.30). In analyses stratified by menopausal status, the multivariate RRs of endometrial cancer comparing the highest with the lowest quintile of acrylamide intake were 1.10 (95% CI, 0.71–1.72) among premenopausal women and 0.90 (95% CI, 0.67–1.20) among postmenopausal women.
Table II. Rate Ratios (95% Confidence Intervals) of Endometrial Cancer by Quartiles of Long-Term Acrylamide Intake (Assessed in 1987–1990 and 1997) Among 61,226 Women in the Swedish Mammography Cohort, 1987–2007
|Age-adjusted RR (95% CI)||1.00||1.07 (0.87–1.32)||1.05 (0.85–1.29)||0.94 (0.75–1.18)||0.59|
|Multivariate RR (95% CI)1||1.00||1.10 (0.89–1.36)||1.08 (0.88–1.34)||0.96 (0.76–1.21)||0.72|
We conducted a subanalysis using exposure data from the second questionnaire only. During 344,580 person-years of follow up, from January 1, 1998 through December 31, 2007, 273 incident cases of endometrial cancer were diagnosed. We observed no overall association between acrylamide intake and risk of endometrial cancer (Table III). Likewise, stratification by smoking status revealed no association among never smokers or among ever smokers.
Table III. Rate Ratios (95% Confidence Intervals) of Endometrial Cancer By Quartiles of Acrylamide Intake (in 1997) Among 36,369 Women in the Swedish Mammography Cohort, 1998–2007
|Q2 (20.5–24.6)||67||86,309||1.10 (0.78–1.56)||1.08 (0.76–1.53)||47||1.31 (0.85–2.04)||19||0.77 (0.42–1.42)|
|Q3 (24.7–29.1)||74||86,242||1.23 (0.88–1.73)||1.20 (0.85–1.69)||46||1.30 (0.83–2.02)||27||1.09 (0.63–1.91)|
|Q4 (≥29.2)||71||86,677||1.21 (0.86–1.70)||1.12 (0.79–1.59)||41||1.20 (0.76–1.90)||29||1.02 (0.59–1.78)|
|p for trend|| || ||0.24||0.47|| ||0.52|| ||0.69|
In this prospective cohort study of Swedish women, we found no association between long-term dietary acrylamide intake and risk of endometrial cancer. The association did not differ by menopausal or smoking status.
To date, only one previous study has reported on the relation between acrylamide intake and risk of endometrial cancer. In that prospective cohort study of 63,572 Dutch women, with 11.3 years of follow-up and 327 endometrial cancer cases, no overall significant association between acrylamide intake and endometrial cancer was observed.15 However, among never smokers, women in the highest quintile of acrylamide intake (mean intake, 40.2 μg/day) had a statistically significant ∼2-fold increase in risk of endometrial cancer compared to women in the lowest quintile (mean intake, 8.9 μg/day).15 The inconsistent findings from our study and the Netherlands cohort may be due to different ranges of exposure. The mean intake in the lowest acrylamide intake category was lower in the Netherlands cohort than in our cohort of Swedish women (mean intake, 15.9 μg/day). The mean acrylamide intake in the highest category was slightly higher in the Netherlands cohort than in our cohort (mean intake 33.8 μg/day). Another possibility is that the observed positive association between acrylamide intake and endometrial cancer risk in the subgroup of never smokers in the Netherlands cohort is a chance finding.
The major strengths of this study include its prospective and population-based design, a large sample size, and detailed information on diet. The prospective design precluded recall bias and the virtually complete follow-up of the study population through linkage to various population-based registers largely minimizes the concern that our results have been affected by differential loss to follow-up. Another strength is that dietary acrylamide exposure information was updated during follow-up, which reduces measurement errors and provides a better estimate of long-term diet. Moreover, we used acrylamide concentrations of foods from the Swedish market (in the study area), foods that were relevant for the study population.
A limitation of our study is that dietary intake was assessed with a self-administered food-frequency questionnaire, which will inevitably lead to some error in the measurement of diet and in the calculation of acrylamide intake. Furthermore, large variations in acrylamide levels have been found between single foodstuffs (different brands) within food categories as well as in different food categories.17 However, estimated acrylamide intake from foods has been found to significantly correlate with hemoglobin acrylamide adduct levels in Swedish women and men.23 Although we cannot rule out the possibility that measurement errors explain the lack of association between acrylamide intake and risk of endometrial cancer in the present study, it is unlikely that acrylamide in foods is an important risk factor. Another limitation is that although we adjusted for risk factors for endometrial cancer, we cannot exclude the possibility of residual confounding from these variables or from other uncontrolled dietary or nondietary factors. We were unable to control for physical activity and smoking at baseline. However, adjustment for physical activity and smoking in the subanalysis using data from the second questionnaire did not change our results.
In summary, results from this prospective study do not support the hypothesis that dietary acrylamide intake is positively associated with endometrial cancer risk, at least not within the ranges of acrylamide consumed in this Swedish population.