The results of this study support an association between supplement use and meeting RDAs/AIs for calcium, magnesium and vitamin C in this multi-ethnic population of middle-aged and older adults and are consistent with an existing study exploring the impact of supplementation in Americans 51 and older (8
). In addition, our results suggest that the association between supplementation and meeting the RDAs/AIs for calcium is stronger for women than for men. Also, Chinese- and African-Americans tend to show the largest effects for calcium supplementation.
Some of this may be explained by differences in dietary intake between ethnicities and genders. indicates that median dietary intake for calcium tends to be less in women than in men and in less African Americans and Chinese Americans. Therefore, calcium supplementation in these populations may have a greater impact. In addition, women tended to take a higher dose of supplemental calcium than men (median supplemental intake in women users=600 mg; median supplemental intake in men users=200 mg) making it more likely that women calcium supplement-users would cross the AI threshold than men calcium supplement-users.
The differences in recommended nutritional requirements between men and women along with the differences in dietary habits between the two genders, has prompted some vitamin manufacturers to make multivitamins tailored to each gender. This type of customization may increase the effect of supplements in both men and women and future studies should assess the contribution of these types of vitamins. In addition, more studies analyzing differences in dietary habits between people from different backgrounds can help nutritionists and other health professionals target populations that are most likely to benefit from dietary supplements and tailor other dietary interventions to specific groups.
The most common type of vitamin supplement used in the US population is the multivitamin (30
). In this study, we found that multivitamin use was significantly associated with meeting the RDAs for magnesium and vitamin C regardless of gender/ethnic group (). However, for calcium, multivitamin use was only associated with meeting the AI in two of the eight groups; whereas, high-dose calcium use was associated with meeting the AI in all groups. For potassium, multivitamin use was not associated with meeting the AI for any of the groups. This is partially a reflection the amount of micronutrient typically found in a multivitamin and the proportion of the AI/RDA that amount represents. A typical multivitamin has 60 mg of vitamin C accounting for 80% of the RDA in women and 67% of the RDA in men. On the other hand, a typical multivitamin has 200 mg of calcium (accounting for between 17% and 20% of the AI depending on age) and less than 100mg of potassium (accounting for only 2% of the AI).
It would be ideal if most people could adequately supplement dietary intake of important nutrients by taking a single multivitamin. However, the amount of micronutrients in multivitamins is limited by the size of the tablet. Therefore, consideration is taken when determining the amount of each micronutrient in the multivitamin. Determining ways to reformulate multivitamins to more adequately meet the gaps in nutrition should be considered.
Despite the strong association between supplement use and meeting the RDA/AI for calcium and magnesium, still about half of supplement-users are not meeting the RDA/AI for these important nutrients. Vitamin C is the only micronutrient where supplementation results in close to 100% of supplement-users with intakes meeting or exceeding recommended levels. As previously mentioned, the amount of vitamin C in multivitamins is high in relation to the amount needed to meet the RDA. In addition, baseline dietary intake of vitamin C is relatively high in this population ().
Not surprisingly, supplement users were more likely than non-users to exceed ULs for calcium, magnesium and vitamin C. However, this association was confined to those taking high-dose vitamins and not in those taking multivitamins only. This is of concern for two reasons. First, micronutrient intake that exceeds recommended ULs may lead to poor health outcomes, such as kidney stones (21
). Second, many users do not discuss their decision to use supplements with their doctor, and therefore, may not receive advice concerning appropriate use of micronutrient supplementation (24
Our findings should be interpreted in light of a few limitations. First, we used an FFQ to assess dietary intake. Although FFQs are cost-efficient and allow direct assessment of nutrient intakes, they are prone to bias due to inaccurate recall. However, the FFQ used in this study was validated for use in a multi-ethnic population and any bias found to be similar between different ethnic groups (31
). In addition, we did not assess the contribution of antacids to supplemental calcium intake which can be a significant source of supplemental calcium in older populations (32
). Therefore, we may have underestimated the contribution of calcium supplementation in this population. Also, due to the nature of our analysis, we were unable to adjust for differences in meeting the RDA/AI through food intake alone between supplement users and non-users. Since we saw little difference in the percent meeting the RDA through dietary intake alone between these groups, any bias from this is minimal and would not change the overall results. In addition, this study examines an outcome (meeting RDAs/AIs) that is a proxy for nutrition in the population, and does not look longitudinally to determine if meeting RDAs/AIs through supplementation will result in better long-term health outcomes. Future studies should evaluate dietary supplementation in relation to cardiovascular disease, disability status and life-expectancy in longitudinal studies. Finally, our analysis of the association between exceeding the ULs for the micronutrients of interest and dietary supplementation is an unadjusted analysis that does not take into account potential confounding variables, such as age and education. However, since zero participants classified as non-users for potassium and vitamin C supplements exceeded the UL, we were unable to do an adjusted regression analysis to examine this relationship.
Despite these limitations, there are several strengths to this study. The large sample size and multiple ethnic groups in this population gave us enough power to examine interactions between supplementation and ethnicity. In addition, the large sample size and detailed dietary questionnaire allowed for a very specific analysis of the micronutrients of interest and dietary supplementation. Therefore, the associations between dietary supplementation and meeting recommended daily intakes for calcium, magnesium and vitamin C are robust and should be reproducible in future studies.
The present study indicates a clear association between meeting RDA/AIs and supplement use for calcium, magnesium and vitamin C. However, even with the assistance of dietary supplements many middle-aged and older Americans are not getting adequate nutrition, and there was no association between supplement use and meeting the AI for potassium. In addition, those taking high-dose vitamin supplements were more likely to exceed the UL for that nutrient. Future studies should explore dietary supplementation along with other methods to improve nutrition in middle-aged and older Americans.