1 Department of Epidemiology and Welch Center for Prevention, Epidemiology and Clinical Research, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD
2 National School of Public Health, Institute of Health Carlos III, Madrid, Spain
3 Medical Department, AstraZeneca, Madrid, Spain
4 Division of Human Nutrition, Graduate School of Food Technology, Agrobiotechnology, Nutrition and Health Sciences, Wageningen University, Wageningen, the Netherlands
5 Interfaculty Reactor Institute, Delft University of Technology, Delft, the Netherlands
6 Cardiovascular Research Unit, College of Medicine, University of Edinburgh, Edinburgh, United Kingdom
7 Department of Medical Physiology, Faculty of Medicine, University of Tromsø, Tromsø, Norway
8 Department of Preventive Medicine, Faculty of Medicine, University of Málaga, Málaga, Spain
9 Nordic School of Public Health, Göteborg, Sweden
10 Epidemiology Unit, Hadassah Medical Organization, Jerusalem, Israel
11 Hadassah School of Public Health and Community Medicine, Hebrew University, Jerusalem, Israel
12 Department of Epidemiology, School of Public Health, University of California, Los Angeles, CA
13 Department of Preventive Medicine and Public Health, Faculty of Medicine and Odontology, Universidad de Valencia, Valencia, Spain
Correspondence to Dr. Eliseo Guallar, Welch Center for Prevention, Epidemiology and Clinical Research, 2024 East Monument Street, Room 2-639, Baltimore, MD 21205-2223 (e-mail: eguallar{at}jhsph.edu).
Received for publication January 10, 2005. Accepted for publication March 16, 2005.
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ABSTRACT |
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case-control studies; chromium; metals, heavy; myocardial infarction; neutron activation analysis
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INTRODUCTION |
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The association of chromium intake with cardiovascular endpoints is largely unknown. The few early studies available were limited by unreliable analytical methods (18, 19
) and/or the use of serum chromium measurements, which may not adequately reflect long-term chromium intake (1
, 3
, 19
, 20
).
To evaluate the hypothesis that long-term exposure to chromium is inversely related to the risk of coronary heart disease, we measured the toenail chromium concentrations of participants in the EURAMIC Study (EURopean Multicenter Case-Control Study on Antioxidants, Myocardial Infarction, and Cancer of the Breast) (2123
) and assessed their association with the risk of a first myocardial infarction.
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MATERIALS AND METHODS |
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Cases were men with a first acute myocardial infarction (International Classification of Diseases, Ninth Revision, code 410), confirmed by characteristic electrocardiogram abnormalities and elevated serum enzyme levels, who had been hospitalized within 24 hours of the onset of symptoms. Cases were recruited from the coronary care units of participating hospitals.
Controls were men without a history of myocardial infarction who were recruited from the case's population catchment area and frequency matched for age (in 5-year intervals). In Finland, Israel, Germany, Scotland, and Switzerland, random sampling from local population registers was used for control selection. In Russia and in the two Spanish study centers, population registries could not be used, because of the lack of complete census data or because of legal restrictions. Therefore, controls were selected from among hospitalized patients with disorders not known to be associated with dietary factors (renal colic, hernia, acute appendicitis or mesenteric adenitis, volvulus or subocclusion due to fibrosis, noninfectious prostatism, and rectal or anal disorders other than cancer, hemorrhoids, or chronic infections). When low participation rates from population samples were anticipated, controls were selected by random sampling from the catchment area of the patient's general practitioner (in the Netherlands) or by inviting apparently healthy friends and relatives of the patient to participate (in Norway) (2123
).
Cases and controls were recruited concurrently during 1991 and 1992. The overall response rate was 81 percent in cases and 64 percent in controls. Informed consent was obtained from study participants in accordance with the ethical standards of the responsible local committees on human experimentation.
Data collection
Toenail clippings from all 10 toes were collected within 8 weeks of inclusion in the study and were stored in small plastic bags at room temperature (22, 23
). A nonfasting venous blood sample was drawn for cholesterol analysis. In cases, blood samples were drawn within 24 hours of hospital admission.
Information on smoking habits, history of hypertension, angina pectoris, and diabetes was collected for all subjects by means of standard questionnaires (21). Socioeconomic status, alcohol intake, and family history of cardiovascular diseases were assessed through locally developed questionnaires.
Analysis of biologic samples
Toenail chromium concentrations were measured by instrumental neutron activation analysis at the Interfaculty Reactor Institute of Delft University of Technology in Delft, the Netherlands (24, 25
). The quality assurance system of this laboratory is accredited by the Dutch Council for Accreditation for compliance with European (EN 45001) and international (ISO/IEC Guide 25) standards. Toenail clippings were irradiated for 4 hours in a thermal flux of 5 x 1012/second/cm2. After a decay time of 21 days, the gamma radiation of chromium-51 was measured during 1 hour in a well-type germanium detector. Samples were rotated during irradiation. Irradiation of study samples was conducted from April 1998 through June 1999. The clippings of each subject were irradiated and measured together. For each center, samples from cases and controls were analyzed together and randomly distributed across batches. Personnel at the Interfaculty Reactor Institute were blinded with respect to the case-control status of the samples.
Chromium concentrations were reported in µg/g. For each sample, a limit of detection was defined as the concentration at which chromium could be detected with 97.5 percent certainty. The limit of detection for a sample of average weight (54 mg) was 0.4 µg/g. In samples with chromium concentrations below the detection limit (n = 96), the chromium concentration was set at one half the reported detection limit.
For quality control, a sample of the certified reference material (BCR-CRM 414, plankton; Institute for Reference Materials and Measurements, Geel, Belgium) was included in each analytical batch (25). The interassay coefficient of variation based on 49 measures of this reference material was 17.3 percent (6.2 percent for log-transformed values).
Serum cholesterol level was determined enzymatically using kits obtained from Boehringer-Mannheim (Boehringer-Mannheim GmbH, Mannheim, Germany). High density lipoprotein cholesterol level was determined after precipitation with dextran sulfate and magnesium chloride. Cholesterol determinations were performed at the National Public Health Institute in Helsinki, Finland.
Statistical methods
Because of the marked right skewness of the distribution of toenail chromium values, logarithmic transformations were used. The distribution of chromium in controls was used to compute cutoff points and medians for quintiles of exposure. Geometric mean chromium concentrations by participant characteristics among controls were compared by linear regression and 2 tests.
For multivariate analysis, the association of chromium with the risk of myocardial infarction was estimated with multiple logistic regression. Adjusted odds ratios in quintiles 25 were calculated using the lowest quintile as the reference category, and trend tests were computed by including log chromium concentration in the logistic models as a continuous variable. All p values reported are two-tailed. Statistical analyses were performed in S-Plus (26).
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RESULTS |
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The concentration of chromium in myocardial infarction cases was 1.10 µg/g (95 percent CI: 1.10, 1.18). After adjustment for age, study center, smoking, alcohol drinking, body mass index, high density lipoprotein cholesterol, diabetes, history of hypertension, and family history of coronary heart disease, chromium was 13 percent lower in cases (95 percent CI: 1, 24) (p = 0.04). In risk analyses, there was an inverse trend in the odds ratio for myocardial infarction with increasing chromium concentration (table 3). This inverse trend persisted after adjustment for cardiovascular disease risk factors; odds ratios were 0.87, 0.72, 0.54, and 0.65 for quintiles 25 of chromium, respectively, in comparison with the first quintile (p for trend = 0.04). Adjustment for adipose tissue -tocopherol, ß-carotene, lycopene, linoleic acid, and toenail selenium did not materially modify the risk estimates (table 3). Further adjustment for levels of other metals in toenails did not materially alter the results (not shown).
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DISCUSSION |
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Trivalent chromium, the reduced form of the element, is required for insulin action (17
). A chromium-containing oligopeptide present in insulin-sensitive cells binds to the insulin receptor, markedly increasing the activity of the insulin-stimulated tyrosine kinase (7
, 29
, 30
). Overt chromium deficiency has been demonstrated in patients receiving total parenteral nutrition without chromium supplementation (4
, 10
). It is characterized by hyperglycemia, glycosuria, and weight loss that cannot be controlled with insulin (9
, 10
, 31
); as a consequence, total parenteral nutrition solutions are regularly supplemented with chromium (32
). Intraperitoneal injections of potassium chromate also reversed atherosclerotic plaques in rabbits (33
, 34
).
The main route of exposure to chromium in the general population is dietary intake. Most chromium in the diet is trivalent chromium, and any hexavalent chromium in food or water is reduced to trivalent chromium in the acidic environment of the stomach (1, 3
). Foods with high chromium concentrations include whole grain products, green beans, broccoli, and bran cereals (35
). The chromium content of meats, poultry, and fish varies widely, since chromium may be introduced during transport, processing, and fortification of foods (35
). Foods rich in refined sugars not only are low in chromium but promote chromium loss (36
). The use of stainless steel equipment in food processing adds measurable amounts of chromium (37
), especially in acidic foods, salted foods, and foods that are in close contact with stainless steel when processed. The chromium content of grain products, fruits, and vegetables also varies extensively, possibly for the aforementioned reasons (37
) or because of soil properties (38
).
Based on the chromium content of well-balanced diets (35), Adequate Intake values for chromium in adults have been established at 35 µg/day in men and 25 µg/day in women (3
). Although there are no national survey data on chromium intakes (3
), a study of self-selected diets of US adults indicated that the chromium intake of a substantial proportion of subjects may be well below the Adequate Intake (11
); similar results have been shown in the United Kingdom, Finland, Canada, and New Zealand (39
). Thus, subclinical chromium deficiency may be a contributor to glucose intolerance, insulin resistance, and cardiovascular disease, particularly in aging populations or populations that have increased chromium requirements because of high-sugar diets (6
, 9
).
Although there is a biologic basis for a protective effect of chromium on cardiovascular disease, few epidemiologic studies have addressed this hypothesis. This is due in part to the lack of food composition data and to the variability in the chromium content of foods as a function of preparation (1, 3
). In addition, the low absorption and bioavailability of chromium offer little confidence in dietary assessment-based estimates of intake and exposure. For all of these reasons, biomarkers of chromium intake offer compelling alternative exposure assessment approaches. However, early studies used unreliable analytical methods for chromium assay (18
, 19
) and/or serum chromium measurements (19
), which do not adequately reflect tissue chromium concentrations (1
, 3
).
In our study, we used toenail chromium concentrations to obtain a time-integrated measure reflecting chromium exposure over the past several weeks that has been previously used in epidemiologic studies (28, 40
). In a paired comparison of toenails collected 6 years apart, the reproducibility of toenail chromium measurements was moderate (40
). This suggests that toenail chromium can be used as a biomarker of long-term exposure, although attenuation of the potential associations would be expected to occur because of measurement error. As an additional indicator of the validity of toenail chromium measurements, we found in toenails the pattern of decreasing chromium concentrations with age that was previously found in hair, sweat, and serum (41
). Both hair and toenail chromium determinations reflect trivalent chromium, and their biologic similarity makes it likely that chromium concentrations in nails, as in hair, parallel tissue chromium concentrations (40
).
Our study presented some additional strengths. We used instrumental neutron activation analysis for chromium determination, a highly sensitive technique that is considered the preferred method for determination of chromium concentrations in biologic materials (4, 24
). The international character of the study, including cases and controls from countries with widely different habits, also adds relevance to our findings. Finally, the use of incident cases of myocardial infarction makes it unlikely that toenail chromium could have been affected by disease development. For practical purposes, the toenail measurements used in our study represent predisease biomarkers of chromium exposure.
Sample contamination is a major concern when measuring chromium in biologic materials (4, 42
). In the EURAMIC Study, toenails were clipped using conventional manicure scissors, which could have resulted in the transfer of chromium-containing stainless steel to the toenails. However, using irradiated clippers, Anderson and Morris (43
) determined that the transfer of such material to toenails was very small (<100 pg) and that most of the transferred material was removed during washing prior to neutron activation analysis. Thus, toenail contamination during clipping is likely to have been small and nondifferential and is unlikely to account for the observed case-control differences in our study. We further minimized chromium contamination by storing the toenails in plastic bags (42
) and by using neutron activation analysis, which does not involve preparatory steps (such as sample homogenization or dissolution) prior to analysis (4
, 24
).
Although our study provides evidence of an inverse association of chromium with the risk of myocardial infarction, some limitations need to be considered. Our analyses, based on single measurements of toenail chromium concentration, were subject to random measurement error that added to the analytical error. Since random errors tend to attenuate risk estimates, we can assume that our analyses underestimated the inverse association of chromium with myocardial infarction. The validity of control selection is also a key issue in case-control studies. Although we aimed at obtaining population-based controls, this approach was only feasible at five of the 10 study centers. However, results were similar for centers with population controls and centers with other types of controls and were also independent of the response rate at each center, reducing the possibility of selection bias.
A further limitation of our study was the lack of data on the sources of toenail chromium. Most toenail chromium in the general population is likely to be derived from trivalent chromium intake (1, 3
), but we could not separate it from the effect of occupational or environmental inhalation of hexavalent chromium, which is present in industrial fumes and dust (44
) and which may produce different cardiovascular effects. In addition, since we lacked data on dietary intake and on use of multivitamin or mineral supplements, we could not rule out the possibility that the inverse association between chromium and nonfatal myocardial infarction could be explained by dietary determinants of chromium intake or other lifestyle characteristics. Finally, our cases were patients with nonfatal myocardial infarction who survived until hospitalization; chromium may show a different association for fatal events. We note, however, that in the Health Professionals Follow-up Study (27
), toenail chromium concentration was inversely related to both fatal and nonfatal events.
In summary, our study provides evidence of an inverse association between long-term exposure to chromium and the risk of myocardial infarction. Despite the scarcity of information on the effects of chromium on cardiovascular disease prevention, chromium supplements are actively used and promoted as improving glucose control, weight loss, exercise capacity, and longevity, particularly in the United States (12, 13
), while the use of these supplements is low in Europe and Israel, where this study was conducted. Considerably more evidence is needed to substantiate many such claims, as well as to show the long-term safety of chromium supplementation in humans (7
, 8
, 14
17
). Nevertheless, our study indicates that intake of chromium-containing foods may be inversely related to the risk of nonfatal myocardial infarction, and it supports the increasing body of evidence pointing to the importance of chromium for cardiovascular health.
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ACKNOWLEDGMENTS |
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This paper was presented at the American Heart Association's 41st Annual Conference on Cardiovascular Disease Epidemiology and Prevention, San Antonio, Texas, March 2, 2001.
Conflict of interest: none declared.
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References |
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