Serum Carotenoids and Markers of Inflammation in Nonsmokers
Stephen B. Kritchevsky1,
Andrew J. Bush1,
Marco Pahor2 and
Myron D. Gross3
1 Department of Preventive Medicine, University of Tennessee, Memphis, Memphis, TN.
2 Sticht Center on Aging, Department of Medicine, Wake Forest University, Winston-Salem, NC.
3 Division of Epidemiology, University of Minnesota, Minneapolis, MN.
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ABSTRACT
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One explanation for discrepant results between epidemiologic studies and randomized trials of ß-carotene and cardiovascular disease may be a failure to consider inflammation as a confounder. To evaluate the potential for such confounding, the authors relate the serum concentrations of five carotenoids (
-carotene, ß-carotene, ß-cryptoxanthin, lycopene, and lutein/zeaxanthin) to levels of three inflammatory markers (C-reactive protein, fibrinogen, and white blood cell count) measured during the Third National Health and Nutrition Survey, 19881994. The analysis included 4,557 nonsmoking participants aged 2555 years. Adjusted concentrations of all five carotenoids were significantly lower in those with C-reactive protein levels above 0.88 mg/dl (p = 0.001). There was a trend toward lower adjusted ß-cryptoxanthin concentrations with increasing level of fibrinogen (p value test for trend = 0.01), but other carotenoids were not related. Many of the carotenoid concentrations were lower among participants with high white blood cell counts. After log transformation, only adjusted mean ß-carotene levels were significantly lower in those with white blood cell counts above 7.85 x 109/liter (p < 0.01). These cross-sectional data do not clarify the biologic relation between carotenoids and C-reactive protein but, to the extent that the carotenoids are associated with C-reactive protein levels, a carotenoid-heart disease association may be, in part, an inflammation-heart disease association.
acute-phase proteins; beta carotene; carotenoids; C-reactive protein; cross-sectional studies; fibrinogen; inflammation; leukocyte count
Abbreviations:
NHANES III, Third National Health and Nutrition Survey.
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INTRODUCTION
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Following articulation of the hypothesis that the oxidation of low density lipoprotein cholesterol is essential for the manifestation of its full atherogenic potential (1
), a number of dietary antioxidants, including ß-carotene and other carotenoids, have been examined as possible preventive factors. Several epidemiologic studies have found serum, plasma, or adipose levels of various carotenoids, including ß-carotene, to be inversely associated with the risk of cardiovascular disease (2




8
). In contrast to this encouraging evidence, four randomized placebo-controlled clinical trials of ß-carotene have shown no reduction and possibly an increase in cardiovascular events in the groups receiving supplementation (9

12
). In two trials, baseline levels of serum ß-carotene were strongly and inversely associated with cardiovascular disease occurrence (9
, 13
). One of several explanations that can be advanced to reconcile these contradictory findings is that unmeasured confounders in the epidemiologic studies are responsible for the inverse associations found in those studies. Most of the epidemiologic studies have included "traditional" risk factors of cardiovascular disease, such as cholesterol levels, smoking, body mass index, and blood pressure (2

5
, 8
). As important as these risk factors are, it has been increasingly recognized that there is an inflammatory component to the atherogenic process, and several markers of inflammation, such as C-reactive protein, fibrinogen, and white blood cell count, have been linked to clinically manifest heart disease (14
, 15
).
Many serum antioxidants decline during the "acute-phase response" to injury or infection, which is characterized by increased levels of markers of inflammation (16
). It is unclear, however, whether low grade inflammation, such as has been linked to cardiovascular disease risk, is associated with reduced carotenoid levels and, thereby, could serve to confound epidemiologic analyses. To address this issue we analyzed data from the Third National Health and Nutrition Survey (NHANES III) to determine whether there is a relation between serum carotenoid levels and three markers of inflammation (C-reactive protein, white blood cell count, and fibrinogen levels) among relatively young nonsmoking individuals.
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MATERIALS AND METHODS
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A detailed description of NHANES III can be found elsewhere (17
). Briefly, NHANES III, conducted from 1988 to 1994, was a national probability sample designed to provide national estimates of the health and nutritional status of the US civilian, noninstitutionalized population aged 2 months and older. The adult portion of the sample included 20,050 individuals from which we selected individuals who were 2555 years of age, nonpregnant, and with nonmissing laboratory values for C-reactive protein and ß-carotene and other carotenoids. Smokers are known to have both lower circulating ß-carotene levels and increased levels of inflammatory markers (5
, 18
). Given these associations, analyses including both smokers and nonsmokers would tend to show an association between carotenoids and inflammation. Since it may be that smoking leads to lower carotenoid levels through the inflammatory process, we deemed that an evaluation including only nonsmokers would be a better test of the link between inflammation and carotenoid levels. Therefore, current smokers and former smokers who had quit for fewer than 5 years were excluded. In addition to smoking, prevalent chronic disease also has the potential to cloud the link between carotenoids and inflammatory markers. So, to reduce the influence that prevalent chronic disease might have on the observed relations, we restricted the age range. Application of the inclusion and exclusion criteria produced a sample of 4,557 participants. Of the 9,502 age-eligible participants, 1,372 were excluded because of missing lab measures, 3,468 because of smoking exclusions, and 105 because of pregnancy. Fibrinogen measurements were obtained only in participants 40 years of age and older. For these analyses the sample size is 2,025. This report is based on data released for public use in 1997 (19
).
Laboratory methods and quality control procedures are described in detail elsewhere (17
, 20
). The carotenoids were measured using isocratic high-performance liquid chromatography-based methods (Waters HPLC System; Waters Chromotography Division, Millford, Massachusetts) that adapted a method described by Sowell et al. (21
). The carotenoids measured were
-carotene, ß-carotene, ß-cryptoxanthin, lycopene, and lutein/zeaxanthin. The method of analysis does not discriminate between lutein and zeaxanthin, and the combination of these carotenoids is presented as a single value. External standards were run to ensure calibration of the method (lutein/zeaxanthin and ß-cryptoxanthin from Hoffman-LaRoche, Inc., Nutley, New Jersey;
-carotene, ß-carotene, and lycopene from Sigma Chemical Company, St. Louis, Missouri). Internal quality control was maintained using pooled serum at both high and low concentrations. The range of the coefficient of variation over the nine pools was 6.432.6 percent (median, 9.4 percent) for
-carotene, 5.710.5 percent (median, 7.0 percent) for ß-carotene, 6.629.4 percent (median, 8.7 percent) for ß-cryptoxanthin, 5.98.8 percent (median, 7.7 percent) for lycopene, and 8.515.8 percent (median, 11.0 percent) for lutein/zeaxanthin (20
). Serum C-reactive protein was quantified using latex-enhanced nephelometry (Behring Nephelometer Analyzer system; Behring Diagnostics, Westwood, Massachussetts). The assay was standardized using the World Health Organization's international reference preparation of C-reactive protein. Both within- and between-assay quality control procedures were used, and the coefficient of variation of the method was 3.216.1 percent (median, 6.3 percent) through the study period (20
, 22
). The white blood cell count was obtained using a Coulter Counter (Coulter Counter model S-Plus JR; Coulter Electronics, Hialeah, Florida). The coefficient of variation for this assay was no more than 3 percent (20
). Fibrinogen was measured in citrated plasma using an automated coagulation analyzer (Coagmate XC Plus; Organon Teknika, Durham, North Carolina). The total coefficient of variation across normal pooled plasma was 3.9 percent. Serum total cholesterol and high density lipoprotein cholesterol were measured enzymatically (23
, 24
). Standing height and weight were measured after participants put on foam slippers and paper shirt and pants. Age, ethnicity, alcohol consumption, dietary supplement use, and general health status were ascertained by self-report. Alcohol, fruit, and vegetable intakes was derived from a food frequency questionnaire that asked about intake over the previous 30 days. Alcohol intake is the number of self-reported drinks of beer, wine, and liquor. Servings of fruits and vegetables were the sum of servings from a list of 18 fruits (including juices) and vegetables (excluding white potatoes). The poverty index is a measure of household income presented as the ratio of household income to the inflation-adjusted poverty threshold (17
).
For statistical analysis, we used analysis of covariance for an evaluation of the relations between inflammatory markers and mean carotenoid concentrations, using the GLM procedure in SAS version 6.12 (SAS Institute, Inc., Cary, North Carolina). Mean levels were calculated by quartile of white blood cell count and fibrinogen. The distribution of C-reactive protein was skewed; only 30 percent of the population had levels above the assay's detection limit. Therefore, C-reactive protein was categorized in three levels, the cutpoints corresponding to the lower detection limit (the 70th percentile) and the 85th percentile. Modeled covariates have been related to plasma antioxidant concentrations in previous investigations (25
27
). Specifically, we included the following variables in all adjusted models: gender; ethnicity (African American, Hispanic, and White/other); age; body mass index; alcohol consumption defined as the total number of drinks of beer, wine, and alcohol over the past month; servings of fruits and vegetables over the past month; serum total and high density lipoprotein cholesterol; the use of vitamin supplements in the past 24 hours and the use of supplements in the past month; decile of household income as measured by the poverty income ratio; and self-reported health status in three levels (very good and excellent health, good health, and fair or poor health). A pairwise test of differences in the least-squares mean carotenoid concentration by levels of inflammatory marker and a test of linear trend in mean carotenoid concentrations across levels of the markers are reported for fibrinogen and white blood cell count. Given the large number of participants with uncertain C-reactive protein levels, there was little basis for assessing a linear trend with increasing C-reactive protein levels, so no trend statistic is given. To more closely examine the relations at very high levels of white blood cell count and fibrinogen, we also ran models comparing those in the top decile of the distribution with those in the lower three quartiles. The distributions of all carotenoids were skewed, and values were log transformed in one set of regression models. Interactions between the level of inflammatory markers and supplement use and self-reported health status were hypothesized a priori, but neither was statistically significant (p > 0.10) for any combination of carotenoid and marker. Because of the large number of comparisons made, a nominal p value of 0.01 was used as the criterion for statistical significance.
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RESULTS
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The study sample is described in table 1. The average age of the sample was 38.6 (standard deviation, 8.6) years. Participants tended to be overweight with the median body mass index being 26.9 kg/m2. The sample also included a majority of women and nondrinkers. Approximately one fifth of the participants had previously smoked cigarettes. By design, the NHANES III sampled African-American and Hispanic residents out of proportion to their representation in the country. The sample is 30 percent African American, 33 percent Hispanic, and 37 percent non-Hispanic White or other. The "other" category comprised less than 5 percent of the study sample. Dietary supplement use was common, with nearly 40 percent of the sample reporting some supplement use in the past month and 21 percent having taken a supplement in the previous 24 hours. Most participants reported being in good or excellent health (47.2 percent), and 17.5 percent of the participants reported themselves to be in fair or poor health.
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TABLE 1. Characteristics of nonsmoking participants aged 2555 years (n = 4,557), Third National Health and Nutrition Survey, 19881994
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The mean concentration of specific carotenoids varied markedly. Lycopene was present in the serum in the highest concentrations followed by lutein/zeaxanthin, ß-carotene, ß-cryptoxanthin, and
-carotene. Carotenoid concentrations were correlated with one another, the strongest correlation being between
- and ß-carotene (r = 0.6, p < 0.0001), and the weakest being between lycopene and
- and ß-carotene (r = 0.11 and 0.13, respectively, p < 0.001).
Levels of fibrinogen and white blood cell count are also given in table 1. Only 30 percent of the participants had a detectable C-reactive protein level (>0.21 mg/dl), so the mean and median values are not reported. The markers of inflammation were correlated with one another. The Spearman rank correlation between C-reactive protein and fibrinogen was 0.39 (p < 0.001); between C-reactive protein and the white blood cell count, 0.28 (p < 0.001); and between fibrinogen and the white blood cell count, 0.18 (p < 0.001).
Table 2 shows unadjusted mean carotenoid concentrations by category of inflammatory markers. When contrasting the high and low C-reactive protein categories, ß- and
-carotene exhibited the largest relative differences (34 percent and 27 percent, respectively), but significantly lower concentrations of all the carotenoids were found in those with C-reactive protein levels above 0.21 mg/dl. There was not a striking inverse relation between carotenoid levels and fibrinogen, though there was borderline statistical evidence of an inverse linear trend with ß-crypoxanthin and lutein/zeaxanthin. Participants with white blood cell counts above 7.85 also had lower levels of all carotenoids except ß-cryptoxanthin.
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TABLE 2. Mean (SD*) serum carotenoids by level of inflammatory marker among nonsmokers, Third National Health and Nutrition Survey, 19881994
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Table 3 shows the mean levels of serum carotenoids after adjustment for multiple covariates. After adjustment, those in the upper 15 percent of the C-reactive protein distribution had statistically significantly lower concentrations of all five of the carotenoids as compared with those without detectable C-reactive protein concentrations (p < 0.01). As in the unadjusted model, ß- and
-carotene showed the largest difference, each being about 20 percent lower in the highest category of C-reactive protein. There was no difference in the carotenoid concentration by the category of fibrinogen, but there was a statistically significant inverse trend in the mean ß-cryptoxanthin level with an increasing fibrinogen level. There was no association between the carotenoid concentration and the category of white blood cell count after adjustment, though the trend statistic suggested an inverse association between the white blood cell count and ß-carotene and lycopene. Those in the highest decile of white blood cell count or fibrinogen did not have significantly lower carotenoid concentrations compared with those in the lowest quartile. However, those in the highest decile of white blood cell count had 12 percent lower levels of ß-carotene compared with those in the lowest quartile (0.34 µmol/liter vs. 0.39 µmol/liter, p = 0.011). Gender and ethnicity interactions were examined for all inflammatory marker-carotenoid combinations, and none were found with one exception. There was a significant interaction between fibrinogen and Hispanic ethnicity in predicting lutein/zeaxanthin levels (p = 0.004). Hispanics tended to have higher lutein/zeaxanthin levels at the lowest quartile of fibrinogen level compared with participants of other ethnicities. The pattern of carotenoid levels across the categories of inflammatory markers held when only nonsupplement users were considered, though the absolute levels of carotenoids tended to be lower in this subset of participants (data not shown).
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TABLE 3. Adjusted mean (SE*) serum carotenoid concentrations by levels of inflammatory marker among nonsmokers, Third National Health and Nutrition Survey, 19881994
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Table 4 shows the covariate-adjusted associations between carotenoids and inflammatory markers after a logarithmic transformation of the carotenoid levels. The concentrations shown are after back-transformation to the original scale of measurement. After transformation, the strong inverse association between all of the carotenoids and C-reactive protein remained evident. Fibrinogen was inversely associated with the log of ß-cryptoxanthin but not with other carotenoids. The log ß-cryptoxanthin-fibrinogen relation was attenuated to nonsignificance after C-reactive protein was entered into the model, suggesting that this relation could have been secondary to the C-reactive protein's relation with ß-cryptoxanthin. Finally, after transformation, the mean ß-carotene levels were significantly lower in those with higher white blood cell counts. There was also a suggestion of an inverse trend between white blood cell count and the other transformed carotenoid values. Adding C-reactive protein to the white blood cell count model did not affect the relation between white blood cell count and ß-carotene, but it did diminish the associations between white blood cell count and the other carotenoids (data not shown).
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TABLE 4. Adjusted mean log-transformed serum carotenoid concentrations by levels of inflammatory marker among nonsmokers, Third National Health and Nutrition Survey, 19881994*
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DISCUSSION
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In a younger nonsmoking subset of the NHANES III population, serum levels of five carotenoids,
- and ß-carotene, ß-cryptoxanthin, lycopene, and lutein/zeaxanthin, were statistically significantly lower in participants with the elevated inflammatory marker, C-reactive protein. The association was relatively specific in that neither fibrinogen nor white blood cell count was related to the concentration of most of the carotenoids studied. This is intriguing given the correlation between the inflammatory markers and may imply that there are differing expressions of the inflammatory response. Alternatively, white blood cell count and fibrinogen may be less specific indicators of the inflammatory state, and thus the lack of association may reflect misclassification.
Our findings are in general agreement with those of other studies of antioxidants and markers of the acute-phase response. However, previous studies have been limited by sample size, sample generalizability, or the breadth of inflammatory markers and carotenoids examined. In a study of 22 patients with non-small cell lung cancer and 13 controls, Talwar et al. (28
) found that C-reactive protein levels were significantly inversely correlated with
-tocopherol and lutein. Cholesterol levels were lower in the cancer patients, but lipid-adjusted differences were not presented (28
). Chang et al. (29
) found cholesterol-adjusted carotenoids and vitamin E levels to be lower in acute stroke patients compared with matched controls, and Boosalis et al. (30
) found C-reactive protein levels to be inversely correlated with lycopene, ß-carotene,
-carotene, and total carotenoids in a study of 85 elderly nuns (mean age, 86 years). Cryptoxanthin was only weakly correlated with C-reactive protein levels and zeaxanthin/lutein was not correlated. In a study of correlates of serum ß-carotene and
-tocopherol, Iribarren et al. (31
) found sialic acid, which is elevated during the acute phase response, and white blood cell count, but not fibrinogen level, to be inversely correlated with serum ß-carotene levels. In a multiple regression analysis including a number of correlates of ß-carotene levels, sialic acid remained inversely associated with ß-carotene levels.
The link between serum carotenoids and markers of inflammation may shed new light on two features of the epidemiologic literature in this area. Several studies have found not just one but a number of different carotenoids to be inversely associated with the risk of atherosclerotic disease (3


7
, 9
). In two studies, the inverse associations were only evident in cigarette smokers (3
, 6
). The relation between the carotenoids and C-reactive protein demonstrated here may help to explain both the lack of a specific association with a particular carotenoid and the stronger association among smokers. Smokers have higher C-reactive protein levels in general (18
) and are likely to have a range of inflammatory responses to smoking. It might be that those smokers with the largest C-reactive protein response have the greatest risk of heart disease. To the extent that the carotenoids are associated with C-reactive protein levels, carotenoid-heart disease associations may be, in part, inflammation-heart disease associations.
The lower detection limit of the C-reactive protein assay used in NHANES III corresponds to the highest quartile of baseline C-reactive protein concentration reported by the Physicians Health Study (32
). In that study, physicians in the highest quartile had nearly a threefold risk of having a myocardial infarction compared with those in the lowest quartile of C-reactive protein concentration. Interestingly, aspirin blunted the relation between C-reactive protein and myocardial infarction. Those in the highest quartile of C-reactive protein in the placebo group had a 4.16 relative risk of myocardial infarction, while those in the aspirin group who were in the upper quartile had a relative risk of only 1.8.
The physiologic basis for the observations reported here is unclear. The carotenoid associations differed little after adjusting for the intakes of fruits and vegetables, suggesting that differences in carotenoid intake do not explain the associations. ß-Carotene has been found to influence leukocyte function (33
), and lower levels of ß-carotene among those with higher white blood cell counts may reflect an increased demand for either this carotenoid or for its metabolite, vitamin A. We observed ß-cryptoxanthin and fibrinogen to be inversely associated, particularly after log transformation. ß-Cryptoxanthin is plentiful in foods that also tend to be high in vitamin C, which itself has been shown to be inversely related to fibrinogen levels (34
).
It is not possible to deduce from the present data whether C-reactive protein leads to lower carotenoid levels or whether low carotenoid levels lead to increased levels of C-reactive protein. It seems unlikely that carotenoids lead to lower levels of C-reactive protein, since the carotenoid-C-reactive protein association is not specific to any one carotenoid. While it is conceivable that all of the measured carotenoids could affect C-reactive protein levels, such a broad response seems unlikely. It is more likely that inflammatory insults lead to both an elevation in C-reactive protein and a reduction in serum carotenoids in some fashion. Of course, both may be linked to a third unmeasured factor. For example, obesity has recently been shown to be associated with high C-reactive protein levels in the NHANES III population, and high body mass index has been consistently related to lower levels of some carotenoids (22
, 25
27
). Our analysis includes adjustments for body mass index and other major determinants of serum carotenoid levels, but relatively little is known about the correlates of elevated C-reactive protein in the non-acutely ill population.
The role of inflammation in determining cardiovascular disease risk is an area of great interest, and a direct causal relation has yet to be proven. Similarly, the role of the carotenoids in determining disease risk is uncertain. Nevertheless, the associations noted here suggest that future studies of carotenoid levels and cardiovascular disease should include measures of C-reactive protein to determine whether inflammation acts to confound observed associations.
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NOTES
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Reprint requests to Dr. Stephen Kritchevsky, Department of Preventive Medicine, University of Tennessee, Memphis, 66 N. Pauline, Suite 633, Memphis, TN 38105 (e-mail: skritchevsky{at}utmem.edu).
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REFERENCES
|
---|
-
Steinberg D, Parthasarathy S, Carew TE, et al. Beyond cholesterol: modifications of low-density lipoprotein that increase its atherogenicity. N Engl J Med 1989;320:91524.[ISI][Medline]
-
Iribarren C, Folsom AR, Jacobs DR Jr, et al. Association of serum vitamin levels, LDL susceptibility to oxidation, and autoantibodies against MDA-LDL with carotid atherosclerosis: a case-control study. Arterioscler Thromb Vasc Biol 1997;17:11717.[Abstract/Free Full Text]
-
Kardinaal AF, Kok FJ, Ringstad J, et al. Antioxidants in adipose tissue and risk of myocardial infarction: the EURAMIC Study. Lancet 1993;342:137984.[ISI][Medline]
-
Kohlmeier L, Kark JD, Gomez-Garcia E, et al. Lycopene and myocardial infarction risk in the EURAMIC Study. Am J Epidemiol 1997;146:61826.[Abstract]
-
Morris DL, Kritchevsky SB, Davis CE. Serum carotenoids and coronary heart disease: the Lipid Research Clinics Coronary Primary Prevention Trial and Follow-up Study. JAMA 1994;272:143941.[Abstract]
-
Street DA, Comstock GW, Salkeld RM, et al. Serum antioxidants and myocardial infarction: are low levels of carotenoids and
-tocopherol risk factors for myocardial infarction? Circulation 1994;90:115461.[Abstract]
-
Gey KF, Stähelin HB, Eichholzer M. Poor plasma status of carotene and vitamin C is associated with higher mortality from ischemic heart disease and stroke: Basel Prospective Study. Clin Investig 1993;71:36.
-
Klipstein-Grobusch K, Geleijnse JM, den Breeijen JH, et al. Dietary antioxidants and risk of myocardial infarction in the elderly: the Rotterdam Study. Am J Clin Nutr 1999;69:2616.[Abstract/Free Full Text]
-
Greenberg ER, Baron JA, Karagas MR, et al. Mortality associated with low plasma concentration of beta carotene and the effect of oral supplementation. JAMA 1996;275:699703.[Abstract]
-
Hennekens CH, Buring JE, Manson JE, et al. Lack of effect of long-term supplementation with beta carotene on the incidence of malignant neoplasms and cardiovascular disease. N Engl J Med 1996;334:11459.[Abstract/Free Full Text]
-
Omenn GS, Goodman GE, Thornquist MD, et al. Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease. N Engl J Med 1996;334:11505.[Abstract/Free Full Text]
-
The Alpha-Tocopherol Beta Carotene Prevention Study Group. The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers. N Engl J Med 1994;330:102935.[Abstract/Free Full Text]
-
Virtamo J, Rapola JM, Ripatti S, et al. Effect of vitamin E and beta carotene on the incidence of primary nonfatal myocardial infarction and fatal coronary heart disease. Arch Intern Med 1998;158:66875.[Abstract/Free Full Text]
-
Danesh J, Collins R, Appleby P, et al. Association of fibrinogen, C-reactive protein, albumin, or leukocyte count with coronary heart disease: meta-analyses of prospective studies. JAMA 1998;279:147782.[Abstract/Free Full Text]
-
Ridker P, Glynn R, Hennekens C. C-reactive protein adds to the predictive value of total and HDL cholesterol in determining risk of first myocardial infarction. Circulation 1998;97:200711.[Abstract/Free Full Text]
-
Louw JA, Werbeck A, Louw ME, et al. Blood vitamin concentrations during the acute-phase response. Crit Care Med 1992;20:93441.[ISI][Medline]
-
National Center for Health Statistics, US Department of Health and Human Services. Third National Health and Nutrition Examination Survey, 199894. Reference manuals and reports (CD-ROM). Hyattsville, MD: Centers for Disease Control and Prevention, 1996.
-
Das I. Raised C-reactive protein levels in serum from smokers. Clin Chim Acta 1985;153:913.[ISI][Medline]
-
National Center for Health Statistics, US Department of Health and Human Services. National Health and Nutrition Examination Survey III, 19881994. Hyattsville, MD: Centers for Disease Control and Prevention, 1997.
-
Gunter EW, Lewis BG, Koncikowski SM. Laboratory procedures used for the Third National Health and Nutrition Survey (NHANES III), 19981994. Atlanta, GA: National Center for Environmental Health, Centers for Disease Control and Prevention, Public Health Service, US Department of Health and Human Services, 1996:754.
-
Sowell AL, Huff DL, Yeager PR, et al. Retinol,
-tocopherol, lutein/zeaxanthin, ß-cryptoxanthin, lycopene,
-carotene, trans-ß-carotene, and four retinyl esters in serum determined simultaneously by reversed-phase HPLC with multiwave length detection. Clin Chem 1994;40:41116.[Abstract/Free Full Text]
-
Visser M, Bouter LM, McQuillan GM, et al. Elevated C-reactive protein levels in overweight and obese adults. JAMA 1999;282:21315.[Abstract/Free Full Text]
-
Boehringer Mannheim Corporation. Procedural inserts: Hitachi 704 cholesterol, triglycerides, and HDL-cholesterol HP. Indianapolis, IN: Boehringer Mannheim Corporation, 1991.
-
Bachorik P, Kwiterovich P. The measurement of plasma cholesterol, low density lipoprotein- and high density lipoprotein cholesterol. In: Fa H, ed. Techniques in diagnostic human biochemical genetics: a laboratory manual. New York, NY: Wiley-Liss, Inc, 1991:4259.
-
Herbert JR, Hurley TG, Hsieh J, et al. Determinants of plasma vitamins and lipids: the Working Well Study. Am J Epidemiol 1994;140:13247.[Abstract]
-
Russel-Briefel R, Bates M, Kuller L. Relationship of plasma carotenoids to health and biochemical factors in middle-aged men. Am J Epidemiol 1985;122:7419.[Abstract]
-
Brady WE, Mares-Perlman JA, Bowen P, et al. Human serum carotenoid concentrations are related to physiologic and lifestyle factors. J Nutr 1996;126:12937.[ISI][Medline]
-
Talwar D, Ha TK, Scott HR, et al. Effect of inflammation on measures of antioxidant status in patients with non-small cell lung cancer. Am J Clin Nutr 1997;66:12835.[Abstract]
-
Chang C, Lai Y, Cheng T, et al. Plasma levels of antioxidant vitamins, selenium, total sulfhydryl groups and oxidative products in ischemic-stroke patients as compared to matched controls in Taiwan. Free Radic Res 1998;28:1524.[ISI][Medline]
-
Boosalis MG, Snowdon DA, Tully CL, et al. Acute phase response and plasma carotenoid concentrations in older women: findings from the Nun Study. Nutrition 1996;12:4758.[ISI][Medline]
-
Iribarren C, Folsom AR, Jacobs DR Jr, et al. Patterns of covariation of serum ß-carotene and
-tocopherol in middle-aged adults: the Atherosclerosis Risk in Communities (ARIC) Study. Nutr Metab Cardiovasc Dis 1997;7:44558.[ISI]
-
Ridker PM, Cushman M, Stampfer MJ, et al. Inflammation, aspirin, and the risk of cardiovascular disease in apparently healthy men. N Engl J Med 1997;336:9739.[Abstract/Free Full Text]
-
Meydani SN, Wu D, Santos MS, et al. Antioxidants and immune response in aged persons: overview of present evidence. Am J Clin Nutr 1995;62(suppl):1462S76S.[Abstract]
-
Khaw K, Woodhouse P. Interrelation of vitamin C, infection, haemostatic factors, and cardiovascular disease. BMJ 1995;310:155963.[Abstract/Free Full Text]
Received for publication September 13, 1999.
Accepted for publication February 22, 2000.