Dietary supplement use and medical conditions:

Article Outline

• Abstract
• Introduction
• Methods
  • Study population and data collection
  • Assessment of current supplement use
  • Medical conditions
  • Participant characteristics
  • Statistical analyses
• Results
• Discussion
• Acknowledgements
• References
• Copyright

Abstract 

Background

Over half of U.S. adults use vitamin or mineral supplements, and some are likely using supplements to treat chronic diseases or risk factors for disease. Information on the relationship between supplement use and medical conditions is useful to health professionals to understand the self-medication behavior of their patients, and important for researchers because medical conditions may be potential confounding factors in observational studies of supplement use and disease risk.

Methods

The cross-sectional data in this report are from 45,748 participants, aged 50 to 75 years, who completed a self-administered, mailed questionnaire on current dietary supplement use (multivitamins plus 16 individual vitamins or minerals), medical history (cancer, cardiovascular-related diseases, and other self-reported medical conditions), and demographic characteristics.

Results

Supplement use (mean number used at least once a week) was higher among respondents who were older, female, highly educated, Caucasian, and of normal body mass index (all p<0.001). After controlling for these covariates, supplement use was higher among those with the condition for 13 of the 21 conditions examined (p<0.01); only having diabetes or high stress was associated with using fewer supplements. For specific supplements, the strongest associations were for cardiovascular disease and its risk factors with vitamin E, niacin, and folate, and for calcium with indigestion and acid reflux disease. For several conditions, the relative odds of using specific supplements were consistently higher for men than for women.

Conclusions

Supplement use was associated with many medical conditions in this cohort. However, these cross-sectional data do not permit inferences about the temporal sequence. Some associations appeared to be based on evidence for efficacy (e.g., folate with coronary artery disease), and others could be based on misinformation (e.g., selenium with benign prostatic hyperplasia).

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Introduction 

O ver 50% of U.S. adults use some type of vitamin, mineral, or other dietary supplement.1 Many do so because they believe that supplements can prevent or treat chronic diseases, such as cancer and cardiovascular disease,2, 3, 4, 5 despite limited scientific support for the efficacy of such use.6, 7, 8, 9, 10 Therefore, it is important for healthcare professionals to understand what conditions patients are self-treating and with what types of supplements.

Information on associations of supplement use with medical conditions is also of value to researchers for the design and analysis of observational studies examining supplement use and chronic disease risk. Although randomized clinical trials are considered the standard of study designs to determine causality, they have several limitations that preclude their use to test any but the most compelling hypotheses. These limitations include (1) relatively short follow-up, (2) limited generalizability to healthy populations, (3) high cost, (4) low power for outcomes that are not part of the original study design, and (5) inability to test agents that may potentially be harmful.5, 11 Further, it is unlikely that randomized trials will ever be able to address many important questions about supplement use and disease, such as the effects of very long-term use or effects on rare outcomes. Therefore, observational (e.g., cohort) studies are needed to fully understand the effects of supplement use on disease risk. However, a major drawback to observational studies of supplement use is that confounding can compromise the results, because supplement use is associated with many factors that also affect chronic disease risk.

Several studies have investigated factors that might confound associations between supplement use and chronic disease risk, including demographic characteristics,3, 4, 5, 12, 13, 14, 15 behavioral factors,4, 5, 12, 14, 15 and dietary intake.3, 5, 12, 14, 16, 17 These studies generally found that supplement users—compared to non–supplement users—are more likely to be female, Caucasian, nonsmokers, and physically active; have high level of education and incomes; have higher cancer-screening rates; and consume diets lower in fat and higher in fiber and some micronutrients.

Few studies have reported on the relationship between supplement use and medical conditions. However, as noted above, if medical conditions influence both choice of supplements and future disease risk, then medical conditions would be important potential confounding factors in studies of supplement use and chronic disease. Specifically, medical conditions that are risk factors for, precursors of, or comorbid with diseases of interest may be indications for supplement use and could distort observed associations between supplement use and disease. For example, dyslipidemia, hypertension, and elevated insulin levels have been associated with regular use of vitamin E, magnesium, niacin, folate, and other B vitamin supplements3, 18, 19, 20, 21, 22; also, patients with rheumatoid arthritis sometimes use calcium, vitamin D, folate, or multivitamin supplements.23 These conditions may be comorbid with and/or increase risk for coronary artery disease, cancer, and death.22, 23, 24, 25, 26, 27 Thus, if supplements are used by persons with less serious, preclinical, or comorbid conditions (e.g., hypertension or diabetes), supplement use may appear to increase the risk for a more serious disease or outcome (e.g., coronary artery disease or death) when, in truth, such a relationship does not exist. Clearly, studies investigating associations of supplement use with chronic disease risk must carefully assess medical conditions associated with supplement use, because failing to control for these conditions during statistical analyses could result in spurious associations.

The purpose of this report was to describe cross-sectional associations of vitamin and mineral supplement use with participant characteristics and a range of medical conditions in a large cohort of older adults. We hypothesized that supplement use would be associated with many medical conditions, because consumers may believe that supplements are helpful in treating the condition or preventing future disease. We examined associations of major diseases, important cardiovascular risk factors, other medical conditions, and health complaints with both the total number and specific types of supplements used by participants in a large cohort study.

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Methods 

Study population and data collection 

Data in this report are from the Vitamins and Lifestyle (VITAL) study, a cohort investigation of 75,000 men and women, aged 50 to 75 years, in western Washington State. The objectives of the VITAL study are to investigate associations of supplemental vitamin C, vitamin E, calcium, multivitamins, and other supplements with cancer incidence. Using names obtained from a commercial list, 330,000 men and women were contacted by mail with the goal of recruiting 35,000 men and 40,000 women, of whom 75% had used at least one dietary supplement in the last 10 years. Recruitment of the VITAL cohort began in October 2000 and continued through October 2002.

The mailing sent to potential participants included a recruitment letter targeting supplement users and a 24-page questionnaire. The questionnaire collected detailed information on vitamin, mineral, and herbal supplement use over the previous 10 years, as well as information on other cancer risk factors, including diet, physical activity, medical history, and demographic characteristics. At the time of these analyses (in December 2001), 45,748 men and women had completed and returned the VITAL baseline questionnaire (21.2% of eligible persons contacted). The analyses in this report are based on cross-sectional data obtained from the VITAL baseline questionnaire.

Assessment of current supplement use 

The VITAL supplement questionnaire assessed total intake of ten vitamins and six minerals from all types of supplements. Details on the design of this instrument are described elsewhere.28 Briefly, participants first provided information on use of multivitamins by selecting from a list of 16 brand names or providing information on the composition of their brand (i.e., specific doses of vitamins and minerals contained in the multivitamin) if it were not listed. Next, participants reported use of ten vitamins and six minerals from single supplements or from other mixtures not classified as multivitamins (e.g., “stress” or antioxidant mixtures). Responses were in closed-ended categories; for frequency of use, response options were 1 to 2, 3 to 4, 5 to 6, or 7 days per week, and dose per day was based on the most common formulations for each supplement (e.g., for the vitamin E response, options were 30, 100, 200, 400, 600, or 800 IU). Participants were asked to look at their supplement bottles when completing the form.

For the analyses reported here, current supplement use was defined in two ways. Total number of supplements was computed as the sum of multivitamins, ten vitamins, and six minerals (i.e., 17 total supplements) on the VITAL baseline questionnaire, based on self-reported current use of these supplements at least once a week. Use of specific supplements was determined as follows. First, we identified nutrients that have been associated with various diseases in published studies18, 19, 20, 21, 22, 23, 29, 30 and for which single supplements were used by at least 5% of the VITAL cohort. Next, we summed across intakes from multivitamins, mixtures, and single supplements to determine the average total daily dose for each nutrient (Σ = dose per day × days per week ÷ 7). Finally, cut-points to be considered a “user” were set to levels that were not likely to be obtained from regular use of multivitamins alone, based on the supplement use patterns for that nutrient in the VITAL cohort. Based on these criteria, use of specific supplements was defined as ≥5 days per week for multivitamins and by average total daily intake for vitamin C (≥150 mg); vitamin E (≥180 mg alpha-tocopherol); folate (≥600 mcg); niacin (≥40 mg); calcium (≥250 for men, ≥400 for women); selenium (≥75 mcg); and zinc (≥25 mg).

Medical conditions 

All medical history information was based on self-report. For the purposes of these analyses, medical conditions were grouped into four categories:

Participant characteristics 

Data were collected on several demographic and health-related characteristics, including age, gender, education, race/ethnicity, height, and weight. Height and weight were used to compute body mass index (BMI) as weight (in kilograms) divided by height (in meters) squared.

Statistical analyses 

Linear regression models were used to examine associations of mean number of supplements used with demographic and health-related characteristics and medical conditions. In these models, the number of supplements used was log transformed [ln (x + 0.5)] and then back-transformed; covariates included age, gender, education, race/ethnicity, BMI, and the interaction of age by gender for the combined gender models. Student–Newman–Keuls tests for multiple comparisons were used to assess which demographic subgroup mean values differed significantly from each other. We calculated odds ratios (ORs) and 95% confidence intervals (CIs) from logistic regression models to determine the associations of medical conditions with use of each specific supplement, controlling for the covariates listed above (i.e., age, gender, education, race/ethnicity, and BMI); use of each of the other seven dietary supplements in the table; and the interaction of age by gender for the combined gender models. For both sets of analyses, results are given separately for men and women if associations were statistically significantly different by gender, based on the significance of the interaction term of medical condition by gender. Because of the large sample size, only p values <0.01 were reported as statistically significant. All analyses were performed using SAS 8.0 (SAS Institute Inc., Cary NC, 1999).

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Results 

Table 1 gives distributions of demographic characteristics and obesity in the study sample, as well as the associations of these factors with the mean number of supplements used. The mean age of participants was 61.8 years (standard deviation, 7.4); 49% were male; 92% Caucasian; 41% college graduates; and 23% obese (defined as BMI ≥30 kg/m²).31 Over 75% of participants reported regular use of at least 1 of the 17 dietary supplements examined, and the mean number was 3.15 (geometric mean, 1.77). Fifty-seven percent reported currently using a multivitamin, and the most commonly used single supplements were vitamin E (42%), vitamin C (38%), calcium (41%), folate (12%), and selenium (12%) (data not shown).

Table 1. Participant characteristics by mean number of dietary supplements used among VITAL study participants (N=45,748)

Note: Percentages may not add up to 100 because of rounding and missing data.

^1–3Means with different superscripts are statistically significantly different from each other (p<0.01).

SD, standard deviation; VITAL, Vitamins and Lifestyle.

After controlling for other characteristics (i.e., age, gender, education, race/ethnicity, and/or BMI), supplement use was higher among respondents who were older, female, highly educated, Caucasian or Native American, and who had a normal BMI (all p<0.001). The largest difference in supplement use was between men and women; women used a (geometric) mean of 2.25 supplements compared to 1.36 for men. Among supplement users only, the number of supplements used increased linearly with age in men, but not in women, after controlling for participant characteristics. For example, among male supplement users, mean use increased from 2.91 in the 50- to 55-year-old age group to 3.12 in the 70- to 75-year-old group; however, among women, mean use for these age groups was 3.35 and 3.30, respectively (data not shown).

Associations of medical conditions with number of supplements used are given in Table 2. In 13 of the 21 conditions examined, supplement use was higher among those with than without the condition, after adjusting for participant characteristics (p<0.01). Only having diabetes or a high amount of stress was associated with use of fewer supplements. There were some differences by gender, as coronary artery disease, cardiovascular disease risk factors (hypercholesterolemia and hypertension), and depression were associated with statistically significant higher supplement use among men only.

Table 2. Mean number of dietary supplements used by VITAL study participants with various medical conditions (N=45,748)a

Note: Only p values ≤0.01 are shown: *p<0.01; **p<0.001; ***p<0.0001 for comparison of those who reported the condition versus those who did not report the condition.

F, female; M, male; VITAL, Vitamins and Lifestyle.

Table 3 gives associations (adjusted odds ratios [ORs]) of medical conditions with the use of specific dietary supplements. Among the strongest and most consistent associations was the use of vitamin E among respondents with coronary artery disease, hypercholesterolemia, or hypertension; for instance, those with coronary artery disease were 1.5 times as likely to use vitamin E supplements as those without coronary artery disease. Calcium use, most likely as antacids, was strongly associated with both acid reflux disease and frequent indigestion. For both cardiovascular-related conditions and reflux/indigestion, associations with supplement use were stronger for men than for women (e.g., the OR for calcium use with frequent indigestion was 2.90 for men and 1.20 for women). Men with BPH were more likely to use vitamin E, selenium, and zinc. History of cancer was significantly associated with vitamin E use, but only in women.

Table 3. Use of specific dietary supplements among VITAL study participants with various medical conditions (N = 45,748)^a

Note: Only ORs for p values ≤0.01 are shown: *p<0.01; **p<0.001; ***p<0.0001.

F, female; M, male; OR, odds ratio; VITAL, Vitamins and Lifestyle.

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Discussion 

In this large sample of older adult participants in a cohort study of supplement use and cancer risk, higher use of supplements was associated with older age, female gender, higher education, Caucasian or Native American race/ethnicity, and normal BMI. Participants with chronic diseases or other self-reported medical conditions tended to use more supplements than those without the conditions. Finally, there were associations between several medical conditions and use of specific supplements, and many—but not all—are consistent with popular beliefs about the effects of these supplements on health.

The association of supplement use with demographic and health-related characteristics is consistent with several published studies.3, 4, 5, 12, 13, 14, 15 The trend for age was weaker than in previous reports, probably because other studies included a wider range of ages. African Americans and Asians used fewer dietary supplements than other racial/ethnic groups, while Caucasians and Native Americans had the highest levels of use, similar to findings reported by Vitolins et al.32 in a study of 130 older rural adults in North Carolina. We are not aware of any studies that have explored possible reasons for these race/ethnic group differences in supplement use. However, since supplement use has been shown to be associated with healthful behaviors,3, 4, 5, 14, 15, 16, 17 it is possible that there may be a greater emphasis on preventive health among Caucasians and Native Americans32, 33, 34 that may partly explain higher levels of supplement use among these subpopulations.

In general, respondents with medical conditions used a higher mean number of supplements than those without the conditions. Our findings are consistent with the few published reports available in the literature. Data from the 1986 National Health Interview Survey35 indicated that individuals with one or more self-reported physical conditions were more likely to use supplements than those with none. In a study of 2152 middle-aged to elderly adults in Wisconsin, Lyle et al.14 reported that supplement use was more common among middle-aged men with a history of heart disease compared to those without this medical condition. High supplement use has also been reported to be associated with arthritis23, 36, 37 and stomach problems.37, 38 Collectively, these data suggest that the public may be using dietary supplements to treat illnesses.

The premise that consumers may be using vitamin and mineral supplements to treat disease is bolstered by the fact that, among our participants, the specific types of supplements used differed by medical condition. Specifically, respondents with certain conditions were more likely to use nutrient supplements that have been reported in the scientific and lay press to be potentially efficacious for these conditions. For example, folate use was associated with having coronary artery disease, and participants with hypercholesterolemia were more likely to use niacin. There were also similar findings for other, less serious conditions. Respondents who frequently had low energy or felt depressed/anxious used folate and zinc, respectively, which are typically found in B-vitamin complexes advertised as “energy boosters.” Surprisingly, some medical conditions were not associated with use of specific supplements. Although possibly protective associations have been previously reported in the scientific and lay press, there were no significant associations of prostate cancer with selenium (data not shown), osteoporotic fracture with calcium, or stress with any B vitamin (i.e., niacin or folate).

Although we did not collect information on motives for using supplements, our data also suggest that consumers may be self-treating conditions with supplements based on erroneous or incomplete information on their efficacy. Men with BPH were more likely to use selenium than those without this condition. Even though preliminary results from randomized trials show that selenium may reduce the risk of prostate cancer,39 this nutrient has not been shown to have an effect on BPH, nor is BPH associated with prostate cancer risk. These participants are likely using these supplements because they are packaged into “prostate health” formulas. Likewise, respondents with coronary artery disease were more likely to use vitamin E, although the role of vitamin E in coronary artery disease remains unresolved.40, 41 These findings suggest that physicians, pharmacists, and other healthcare providers need to be aware of the conditions their patients may be self-treating, in order to make sure that patients know what supplements they should be using and to ensure that they are not taking harmful doses.42

There were interesting differences by gender in the associations of medical conditions with supplement use. Although men used fewer supplements than women overall, for several conditions, the number of supplements used and relative odds of using specific supplements were consistently higher for men than for women. For example, some medical conditions (e.g., coronary artery disease and depression) were associated with statistically significantly higher supplement use among men only, and the relative odds of using vitamin E, niacin, and calcium were generally larger among men. This suggests that women may tend to use supplements more broadly and for disease prevention, while men may choose to use supplements only after diagnosis of a condition. These results highlight the importance of examining gender strata when studying associations between supplement use and risk of disease.

There are several limitations to the interpretation of these results. A principal limitation is that the study sample was not representative of the general population. Participants were primarily Caucasian, generally affluent, had a high prevalence of supplement use, and were willing to complete a 24-page questionnaire. Information on medical conditions was from self-report and could not be verified by medical records; however, for some conditions that have variable definitions (e.g., hypertension), we relied on use of prescription drugs or medical procedures rather than simple self-report of a diagnosis. Nonetheless, participants with diabetes may potentially have been misclassified, because many individuals with this condition may not be taking insulin or oral diabetes medications.43 Most importantly, because our analyses are cross-sectional, we cannot determine whether supplements were first used before or after development of associated medical conditions. More specifically, we cannot conclusively establish whether participants in our study are using supplements to treat illnesses or prevent future disease. To obviate this problem in future studies, information on the initiation of supplement use (frequency, duration, and usual doses of vitamins and/or minerals); motivations for using supplements; and dates of disease diagnoses should be collected either retrospectively at baseline or prospectively. This information would permit the ascertainment of temporal relationships between supplement use and diagnosis of diseases, and it can also provide insight into whether supplement use may affect the natural history of a disease (e.g., if users of a particular supplement are found to have more advanced disease compared to nonusers who were diagnosed with the same preclinical condition).

In conclusion, we found many relationships between medical conditions and use of dietary supplements. Some of these relationships are consistent with scientific hypotheses that relate nutrients to disease risk, such as folate and niacin with cardiovascular-related conditions. However, others are possibly based on misinformation or on earlier findings that have not been confirmed (e.g., selenium and BPH). While physicians prescribe some supplements, a number of these supplement–disease associations are likely due to self-treatment of conditions with supplements, of which healthcare providers need to be aware.

It is also important to identify these associations in order to control for them in observational studies of supplement use and disease. This is especially true if supplements are used to treat medical conditions that are precursors to more serious disease, because in this situation supplement use will spuriously appear to increase disease risk. Observational cohort studies of supplement use and disease risk will be important for understanding the public health impact of widespread supplement use, and these studies need to carefully collect information on medical history and comorbid diseases at baseline so that potentially confounding effects of medical conditions can be controlled in statistical analyses.

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Acknowledgements 

This study was supported by National Cancer Institute grant R01 CA74846.

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