To our knowledge, this study is the first showing associations between herbal supplement use and blood lead levels in US women. After controlling for multiple factors, lead levels of women users of specific herbal supplements were 10% higher than women nonusers. When we examined herbal supplement use among reproductive age women, the relationship with lead levels was even stronger, with lead levels 20% higher overall, up to 40% higher among users of select herbal supplements compared to non-users. These findings were not observed for other dietary supplements and were restricted to types of herbal supplements that have been implicated in heavy metal contamination.
Consistent with recent reports among US adults,38
mean blood lead levels were generally low for women (1.3 mcg/dl) and men (2.0 mcg/dl). Diet is the main lead exposure for most US adults and is estimated to be ~10 mcg/day.39
Consuming one supplement containing 1.5 mcg of lead represents 15% of this amount. The contribution would be greater for supplements containing more lead and when using multiple supplements (reported by 30%). Because adults absorb about 10% of ingested lead,40
among women, regular use of a supplement containing 1.5 mcg of lead/serving would be expected to increase lead levels by ~11%, which approximates the mean difference found in our study.
Our findings are similar to epidemiological studies in Taiwan,22,41
showing elevated blood lead levels in those who use TCM herbs compared to nonusers and a study in China showing increased lead in breast milk of mothers using Chinese medicine herbs compared with non-users.42
In contrast to prior studies, we observed no statistically significant associations in men. This should be interpreted with caution, as among men, use of some supplements was very low, and we observed wide confidence intervals around many effect estimates. Although we controlled for several factors, we lacked data on occupation; thus, some degree of unadjusted confounding is probable, particularly in men who are most apt to be employed in occupations with high lead exposure. Alternatively, differences in the impact of lead exposure could be explained by women having greater susceptibility. For example, compared to men and older women, young women appear to retain lead more avidly29
and have periods of increased nutrient demands (e.g., pregnancy and lactation). Increased lead absorption has been associated with lower iron stores and may occur before frank deficiency.43
Although we did not see an association between lead and circulating iron (measured by serum iron), inclusion of ferritin (a better measure of iron stores) in future studies would be useful to explore if higher iron requirements of childbearing women might explain observed differences between women of childbearing age and older women or men. Finally, growing literature suggests that genetic polymorphisms that may modify lead kinetics 44,45
play a greater role in women compared to men.46
We adjusted for calcium supplement use both because some calcium products may contain excess lead47
and prior studies suggest an inverse correlation between calcium and lead levels.16,48–50
Our findings support clinical trial evidence demonstrating a protective effect of calcium supplements on lead levels in lactating women49
and suggests the effect is not limited to this group alone. It is hypothesized that this effect is likely due to suppression of lead absorption in the gut and suppression of bone turnover.15
We considered whether higher lead levels might be associated with herbs not included in our study or with less well-defined factors related to using any dietary supplement. We did not observe higher lead levels among garlic supplement users or among dietary supplement users in general, lending support to the specificity of the association between specific herbal supplement use and blood lead levels.
Lead is readily taken up from soil by plants.51,52
Soil and air pollution influences herb lead content.53
It is possible that excess lead found in some herbs may, in part, be due to herbs grown in less-regulated countries, such as China and India, which are major exporters of raw plant products for the supplement industry.54
Alternatively, uncontaminated herbs could acquire lead during manufacturing due to contaminated water, equipment, pipes, or storage.19
Saper et al. found US-manufactured Ayurvedic supplements contained a median lead content of 7.5 mcg/g compared to 11.0 mcg/g for Indian-manufactured supplements,25
demonstrating that choosing only US-manufactured supplements may decrease, but not assure a lead-free supplement, suggesting the problem is likely due to multiple factors.
The study does not allow determination of causality, many variables were self-reported, and the use of some herbal supplements was low, which consequently limited our power to detect associations among specific herbal supplements, particularly among men who infrequently used some herbs showing the strongest associations among women. Differences in use of “Ayurvedic/TCM” and “Other” herbs could be because of higher use among women for menstrual and menopausal symptoms.55
Unfortunately, we lacked information on frequency and total duration of supplement use. This information and additional information on occupational and non-occupational lead exposures, such as urban living, home renovations, and hobbies, would be of interest for future studies. In this study, we restricted our examination of herbal supplements to those that had previously been reported to contain high levels of lead. However, not all supplements have been analyzed for lead content, and it is possible that some with excess lead are missing from our study. Some forms of herbs may have been missed as participants were queried about “dietary supplement” use and may not have reported herbs purchased as raw herb mixtures, whole herbs, or teas, which are not labeled as dietary supplements and may be regarded as traditional foods or medicine.
We lacked sufficient information to assess supplements according to source and lacked power to assess them according to different brands, which is of interest for future studies, as the quality of a supplement may vary according to herb source and manufacturer. Finally, the NHANES is conducted only in English and Spanish and may not represent recent immigrants from other countries living in the US, who may be disproportionably affected both by higher lead levels56
and by lead poisonings due to use of folk herbal remedies.57
Our observations suggest that among women, including women of reproductive age, specific herbal supplement use is a significant contributor to circulating lead. At present, there are limited data supporting the clinical efficacy of herbal supplements included in this study.58–62
Unless health benefits can be clearly demonstrated by well-designed, well-executed studies, our findings suggest that the potential for harm from use of some herbal supplements may exceed the benefit. Unfortunately, US consumers are often misinformed about the regulation, safety, and effectiveness of dietary supplements.63
Moreover, 37% of physicians surveyed across the US were unaware that supplements do not require FDA pre-market approval for safety and efficacy,64
indicating increased training and CME on this topic are needed. It is hoped that the FDA’s current good manufacturing practices (CGMPs), which by 2010 require all manufacturers to evaluate supplement identity, purity, strength, and composition, will lead to safer supplements. However, CGMPs are based primarily on industry self-regulation and do not address premarket proof of safety or efficacy. Furthermore, without increased resources, the FDA is unlikely to have the capacity to widely inspect or enforce these rules.