In this prospective cohort of self-reported regularly menstruating women, we observed poorer lipid profiles during anovulatory cycles; however, there were no statistically significant associations between baseline lipoprotein cholesterol levels and risk of incident anovulation. It does not appear as though higher lipoprotein cholesterol levels predict anovulation. Although we cannot completely rule out the steroid hypothesis, the slight increased risk of anovulation associated with increased levels of TC observed when using multiple measures of TC raises the possibility of an underlying weak association that may be detected in a larger study with greater study power, or in women who are more likely to be anovulatory. The women who experienced at least one anovulatory cycle displayed several endocrine or metabolic disturbances. Furthermore, we observed that several of these endocrine characteristics—increased Day 2 LH:FSH ratio, presence of acne and decreased levels of SHBG—were predictive of sporadic anovulatory cycles. These results together suggest that an endocrinologic disturbance may lead to, or be the result of, sporadic anovulation.
As there is a lack of comparable research on this topic in normal, healthy women, we compared our results with prior studies of women diagnosed with ovulatory disorders, such as PCOS (Zawadski and Dunaif, 1992
; Rotterdam, 2004
; Azziz et al., 2009
). Like our group of women with anovulatory cycles, despite regular menstrual cycles, women with PCOS have a poorer lipid profile (Rajkhowa et al., 1997
; Pirwany et al., 2001
; Yilmaz et al., 2005
; Macut et al., 2008
; Valkenburg et al., 2008
; Moran and Teede, 2009
; Akram et al., 2010
). However, we did not observe lipid abnormalities as severe as those often reported for women with a PCOS diagnosis (Macut et al., 2008
). This is likely a consequence of our strict study inclusion/exclusion criteria which purposefully excluded women at high risk or diagnosed with PCOS. However, we did not observe the differences in HDL cholesterol previously reported among women with a PCOS diagnosis compared with women without a PCOS diagnosis (Rajkhowa et al., 1997
; Yilmaz et al., 2005
; Macut et al., 2008
; Valkenburg et al., 2008
). This could in part be related to the fact that full metabolic disturbances may not be present in women with only sporadic anovulation (Rizzo et al., 2009
Biologically, an association between lipid levels and ovulation might be expected because of the role of cholesterol in steroid biosynthesis (Strauss, 2004
). Furthermore, in studies where women with PCOS were treated with cholesterol-lowering medications, not only did their cholesterol levels decline, but also their testosterone levels, LH:FSH levels, and hirsutism scores declined (Duleba et al., 2006
; Banaszewska et al., 2007
; Kodaman and Duleba, 2008
). The results of the current study do not rule out the steroid hypothesis; rather, they suggest that an underlying association may exist (albeit weak), the detection of which will require greater study power. It is possible that perhaps the poorer lipid profiles observed among women with PCOS are a consequence, rather than a cause, of endocrine disturbance and anovulation. The observed associations between lipoprotein cholesterol levels and anovulation might be explained by changing androgen levels, for which we unfortunately did not capture data. Alternatively, the observed associations between lipoprotein cholesterol levels and anovulation may not be the result of the steroid biosynthesis pathway, but some heretofore unrecognized biological mechanism.
In this study, women with at least one anovulatory cycle demonstrated higher levels of several non-specific markers for androgen bioactivity, including higher LH:FSH ratios (Homburg, 2002
) and higher follicular phase LH levels (Balen et al., 1995
), all consistent with, but not diagnostic of, androgen excess. Increased LH:FSH ratios are indicative of hypothalamic–pituitary–ovarian dysfunction, characteristic of women with PCOS (Azziz et al., 2009
). We also observed lower SHBG levels among the anovulatory women, likely secondary to hyperinsulinemia and associated with excess androgen activity (Pugeat et al., 1996
). Insulin levels were increased to a small degree among anovulatory women, an observation consistent with the insulin resistance that also frequently accompanies PCOS (Dunaif, 2006
). These endocrine parameters were found to be significant predictors of sporadic anovulation, an apparently reversible condition that may be more common among certain groups of eumenorrheic women than originally thought (De Souza et al., 1998
). Other investigators, however, suggest that anovulation occurs among eumenorrheic women only on a very infrequent basis (Malcolm and Cumming, 2003
; Chatterton et al., 2005
). This is the first study, to our knowledge, to prospectively identify endocrine markers of sporadic anovulation in regularly menstruating women.
Our study offers several advantages over previous studies of lipids and anovulation, in particular the prospective evaluation of the association between baseline lipoprotein cholesterol levels and incident anovulation. By preserving temporality, we were able to further evaluate whether the detected associations were a result of the biological effects of lipoprotein cholesterol on anovulation, rather than being symptomatic of an underlying condition leading to both ovulatory dysfunction and a more atherogenic lipid profile. In addition, we were able to evaluate the possibility of confounding by underlying endocrine disturbances by adjusting for several markers of endocrine function. This study is further distinguished from previous research by its strict inclusion/exclusion criteria, which ensured a sample of eumenorrheic women without a prior clinical diagnosis of PCOS.
While the current study allowed us to expand upon previous studies in this area, we were limited by several factors. Importantly, we were unable to directly measure androgen levels, and had to rely on several non-specific markers to assess androgen activity. In addition, daily measures of progesterone and transvaginal ultrasounds (the gold standard) were not available to assess ovulation. However, multiple well-timed serum hormone measurements (on average, 2, 7 and 13 days before, and 2, 5, 8 and 12 days after the mid-cycle LH surge), along with the use of fertility monitors measuring LH daily in urine, were used to aid in classifying ovulatory cycles. We also employed a conservative definition for anovulation to avoid potential misclassification. Despite these measures, misclassification of anovulation is possible. The small number of anovulatory cycles limits the statistical power available to detect subtle effects, if such an effect exists.
In conclusion, we detected higher TC (P = 0.01), LDL (P = 0.06) and triglyceride (P = 0.0002) levels in regularly menstruating women with anovulatory cycles when multiple measurements across the cycle were considered; however, baseline lipoprotein cholesterol levels were not significantly associated with incident anovulation. Only when multiple measures of TC were included did we observe a weak and marginally significant association with risk for anovulation. While these results do not rule out the steroid hypothesis, they do not support a strong association between lipoprotein cholesterol levels and anovulation. Regularly menstruating women with at least one anovulatory cycle tended to exhibit endocrine and metabolic disturbances similar to, but less severe than, women with PCOS. Collectively, these findings further underscore the possibility of a gradient of severity of endocrine and metabolic disturbances proportional to the degree of ovulatory dysfunction. These data are preliminary and more research is needed to identify the pathophysiology of incident anovulation among eumenorrheic women, particularly given the absence of measured androgens here.