This is the first study to examine the relationships between phthalate exposure and reproductive hormones in fertile men. Our results suggest that among fertile men, increasing environmental exposure to DEHP is associated weakly with hormonal alterations that would tend to decrease free testosterone (as reflected by the FAI). Urinary concentrations of other phthalate metabolites were not strongly associated with serum hormones in this population of fertile men.
Our results are similar to those reported by Meeker et al. (2009b)
who described these associations in men recruited at an infertility clinic, although they reported a significant inverse association between urinary MEHP concentration and both testosterone and E2
levels, not seen in our population. Although we observed a significant inverse correlation between testosterone and MEHHP and MEOHP concentrations, this association disappeared after covariate adjustment. Furthermore, although we saw a suggestive negative association between MEHP and E2
levels, it was weak and did not reach traditional levels of statistical significance.
However, as in Meeker et al. (2009b)
, we found significant inverse associations between FAI levels and three of the DEHP urinary metabolite (MEHP, MEHHP and MEOHP) concentrations and their sum. The similarity of this finding in these two populations is of interest because our two populations were quite different. Meeker et al. (2009b)
studied male partners of couples living in Boston undergoing infertility investigation; we studied fertile men from five US cities whose female partners were mid-pregnant at the time they gave their samples. Moreover, these two populations were dissimilar in their reproductive hormone levels and phthalate metabolite concentrations. The median values for FSH was higher, serum testosterone lower and MEHP higher in men studied by Meeker et al. (2009b)
compared with our population, suggesting that at least some of the infertile men had primary testicular dysfunction associated with higher FSH and lower testosterone. However, the outcomes of both studies are not inconsistent, and suggest that DEHP metabolites may act similarly on the male reproductive axis in these two populations.
We saw a decrease in calculated FT and FAI levels (markers of biologically active testosterone) with increasing DEHP exposure. Decreases in the free fractions of total testosterone may be related to higher SHBG levels, as discussed next. Although all men in our study had serum testosterone, FT and FAI within the reference range of our laboratory, these data [consistent with the known anti-androgenic effect of DEHP (Parmar et al., 1986
; Heindel et al., 1989
; Srivastava et al., 1990
; Li et al., 1998
; Higuchi et al., 2003
; Talsness et al., 2009
; Thompson et al., 2009
)], suggest that current environmental levels of DEHP exposure may exert a negative effect on the endocrine function of healthy men. This finding is consistent with data suggesting that phthalates may inhibit expression of genes involved in steroidogenesis (cholesterol transport and the biosynthesis of testosterone) in rat foetal testis after in utero exposure to large doses of DEHP (Borch et al., 2006
However, two recently published studies did not find any anti-androgenic effects of phthalates on human foetal Leydig cells activity (Huang et al., 2009
; Lambrot et al., 2009
). Huang et al. (2009)
studied prenatal exposure to phthalates in infants and reported a significant negative correlation between amniotic fluid MBP and the anogenital index adjusted by birth weight in female infants only. Lambrot et al. (2009)
investigated the effects of MEHP on the development of human foetal testis. Their main results demonstrated that phthalates alter the development of the germ cell lineage, but in contrast to results observed in the rat, phthalates did not affect steroidogenesis. Nonetheless, earlier studies (Swan et al., 2005
; Main et al., 2006
; Swan, 2008
) suggest that prenatal phthalate exposure may adversely affect male reproductive function in infant humans. Our data can only address adult exposures and not reflect any effects of prenatal exposure given the short half-life of DEHP metabolites (Koch et al., 2006
We also found a significant positive association between serum SHBG levels and urinary MEHP concentrations, but not with the other measured DEHP metabolites. Increased serum SHBG levels have been described as an indirect sign of reduced androgen activity (Belgorosky & Rivarola, 1985
). Increased serum SHBG levels lead to a reduction in FT, as SHBG is the major specific binding protein for testosterone, whereas testosterone is loosely bound to albumin. The fact that FAI and SHBG are statistically associated with DEHP urinary metabolite concentrations whereas testosterone individually is not suggests that the associations we observed in these measures of free testosterone are driven by changes in SHBG. The decrease in the FAI /LH ratio with increasing MEHP urinary concentrations suggests that the anti-androgenic effect of that metabolite – at least to some degree – is mediated via a negative effect on the function of Leydig cells in the testes. In addition, LH levels usually increase to compensate when testosterone or FT decreases because of a compromised testicular production, and we did not see that augmentation. However, considering the clear association between serum SHBG levels and MEHP, it is worth considering that DEHP may also affect the liver and SHBG synthesis, especially as rodent studies have shown that DEHP, DEHP metabolites and DBP significantly affect the liver (Lapinskas et al., 2005
; Wyde et al., 2005
; Eveillard et al., 2009
Unadjusted urinary concentrations of DEHP metabolites in our subjects were very similar to concentrations measured in US men from the 2003–2004 National Health and Nutrition Examination Survey (NHANES; CDC, 2009
). For example, the median and 75th percentiles values for MEHP in men from NHANES 2003–2004 were 2.2 and 6.0 ng /mL, respectively, compared with 3.2 and 7.7 ng /mL in our study.
Our data were limited by the use of a single urine sample to assess phthalates exposure and a single serum sample to describe hormone function. However, several studies have shown that although the urinary concentrations of phthalate metabolites are variable, exposure can be adequately described by a single sample (Hoppin et al., 2002
; Hauser et al., 2004
; Teitelbaum et al., 2008
). Similarly, a single sample can be used to classify men’s reproductive hormones (Bjornerem et al., 2006
In summary, our results suggest that among fertile men, increasing environmental exposure to DEHP is associated weakly with hormonal alterations that would tend to decrease markers of free testosterone.