In this study, we aimed to assess whether increased uBPA was associated with angiography defined coronary artery stenoses (coronary atherosclerosis). We studied a relatively small sample from those undergoing first angiography in a regional cardiac centre from 2001 to 2004. The centre in question serves the EPIC Norfolk cohort study population in which we have previously shown that higher uBPA concentration was predictive of incident cardiovascular disease 
. In EPIC-Norfolk, diagnoses were clinically confirmed from hospital episode data and mortality records. In the current study, we found evidence of higher uBPA concentrations in those with intermediate or severe stenoses compared to those graded as having no coronary artery disease, in independently read angiograms.
These results are important as they suggest that associations between uBPA and CAD may be specific to coronary artery stenosis. Associations between uBPA and cardiovascular disease have now been reported in three previous studies, in which disease diagnosis data was drawn from questionnaire self-report 
or identified from clinical notes 
. In NHANES 2003/2004 and again in NHANES 2005/2006, higher uBPA concentrations were associated with heart disease (pooled p value <0.001). A major limitation of these studies is their cross-sectional nature, leaving open the possibility of reverse-causation, e.g. that patients with CAD may have changed their behaviour or diet in a way that incidentally led to increased exposure to BPA. A prospective design was therefore adopted with 758 incident CAD cases and 861 controls from the European Prospective Investigation of Cancer (EPIC-Norfolk), UK study, which concluded that higher uBPA concentrations (>4 ng/mL) were associated with incident CAD (OR 1.39 95% CI 1.06 to 1.84, p
0.020). This longitudinal study effectively ruled out reverse causation, strengthening the evidence for causal inference.
A cross-sectional analysis of pooled data from NHANES 2003–2006 showed that uBPA was associated with general and central obesity 
, confirming earlier findings of an association between daily excreted BPA and waist circumference in Italian men 
. However, sensitivity analyses of the 2011 EPIC study excluded those with obesity and adjusted for blood lipid concentrations and levels of physical activity, yet found little effect on the association; neither did adjustment for vitamin C as a marker of diets poor in fruit and vegetables, or adjustment for education, smoking or blood pressure. The uBPA association with CAD incidence in the EPIC sample therefore appears to have been independent of conventional CAD risk factors. This conclusion was also reached in a study of 1016 elderly subjects 
which showed a weak relationship between circulating BPA and the thickness and composition of the intima-media complex in the carotid artery, considered to be a marker of lipid infiltration of the vascular wall 
. The effect remained significant after adjustment for conventional CAD risk factors, suggesting that any vascular effect was occurring independently of these.
Our analysis shows that BPA was similarly elevated in patients with intermediate vessel changes and patients with severe stenoses (combining the individually small groups with 1-, 2- and 3- vessel disease). This is consistent with observations that coronary plaques develop focally, independently, and exhibit morphologic and histological heterogeneity between and within coronary arteries 
. Localization of coronary lesions occurs despite similar exposure to pro-atherosclerotic risk factors and this has been attributed to localized hemodynamic disturbances, especially to endothelial shear stress 
. Endothelial shear stress promotes an athero-protective endothelial cell phenotype, but low endothelial shear stress induces an atherogenic phenotype that reduces nitric oxide bioavailability, promotes low density lipoprotein cholesterol uptake and oxidation, recruits inflammatory cells and promotes smooth muscle cell migration to the intima 
. Localized areas of low endothelial shear stress are found at branch points, bifurcations and areas of large curvature in coronary arteries. These regional differences in hemodynamics appear to prime endothelial cells to respond distinctly to systemic risk factors 
. If BPA plays a direct role in promoting atherosclerosis, our observations of individual differences in plaque burden in the presence of similar BPA exposure probably reflects the heterogenic susceptibility of specific vessels, and specific regions within vessels, to atherogenesis and plaque progression.
The mechanism by which BPA ingestion and metabolism influences vascular function and risk of cardiovascular disease has not been elaborated. A number of environmental contaminants have been associated with vascular endothelial dysfunction 
and accelerated progression of atherosclerosis 
. We recently suggested plausible mechanisms by which BPA might increase the risk of cardiovascular disease 
, including reduced nitric oxide bioavailability, altered vascular reactivity to endothelin-1, oxidative stress and inflammation. Laboratory exposure studies have shown that BPA can induce oxidative cellular damage in a range of experimental contexts 
. For example, oral intake of BPA over a 30 day period led to oxidative stress in rat hepatocytes. Circulation of BPA and redistribution around the body could contribute to oxidative endothelial cell damage, a suggestion supported by a reported positive associations between urinary BPA and oxidative stress markers (malondialdehyde and 8-deoxyguanosine) in a study of 960 adults 
. BPA shows estrogen and anti-androgen activity 
hence BPA may exert estrogenic effects or antagonize endogenous estrogens in cardiovascular tissues by binding to soluble or membrane bound estrogen receptors 
. We recently reported associations between higher uBPA concentrations and higher estrogen receptor beta, (ERβ) expression and estrogen-related receptor alpha (ERRα) expression 
, reinforcing the evidence for estrogenic activity in vivo
BPA is also capable of causing non-genomic effects in vitro. A rapid disruption of Ca2+
homeostasis has been shown in a range of cell types 
suggesting the involvement of cellular transport mechanisms and ion channels. Estrogen and estrogen-receptor modulators can increase the activity of large conductance Ca 2+
1.1) channels 
. Recently, BPA in the micromolar range was shown to activate Maxi-K (KCa
1.1) ion channels in human coronary smooth muscle cells in culture, sufficient to hyperpolarise the membrane potential 
. The effects of BPA on channel activity were rapid (<1 min) and reversible and could theoretically lead to a decrease in excitability in cells that express other voltage dependent ion channels.
One limitation of this study is that the BPA measures are from single spot urine samples, which could be considered as limited measures of long term exposure. Urine is however the recommended matrix of choice for biomonitoring studies of BPA, since BPA is rapidly metabolized and excreted from the body and remains stable in frozen urine on long term storage 
. Single spot samples have been found to be moderately sensitive in predicting an individual’s longer term tertiary BPA categorization 
measured stability of BPA over 2 week intervals in first voided urine samples from 60 women and found a Spearman correlation of 0.5, indicating that around 75% of the variability of BPA was unaccounted for. Exposure of the general population to BPA is considered to be primarily from food and beverage consumption and a recent study has highlighted the inhibitory effect of food on first pass metabolism, which would lead to a longer residence time for BPA in the body 
. There is in addition considerable evidence for non-food exposure to BPA. For example, Zalko and colleagues 
show that viable skin efficiently absorbs and metabolises BPA, which would bypass first pass metabolism. Given that BPA is widely found in receipt papers, from where transfer to skin can occur 
, frequent exposure of the general population to BPA through this non-food source is a potential additional route of exposure.
This could help to explain why despite its rapid metabolism, BPA is present in such a large percentage of the population at any one time. It is likely that the use of single spot samples would, if anything, result in a smaller (diluted) estimate of the strength of association between BPA and CAD.
Another major limitation is our relatively small sample sizes. Small sample size (n
86 for the intermediate group) may explain why the central estimate for the uBPA intermediate vs none vessel disease estimate is larger than that for the severe vessel disease group: the very wide confidence intervals on this intermediate estimate (CI 0.98 to 3.27, ) makes interpretation of this hazardous and argues for a better powered study. Because of the temporal variability in BPA exposure and errors in estimation by urine specimens, larger sample sizes are needed. Although our strongest association was found on comparing three vessel disease to none, the intermediate uBPA associations across increasing severity groups show no obvious dose-response relationship. This may be due to possible mechanisms of effect, but may well also be influenced by the very small sample sizes in each specific severity group. Larger studies are clearly needed to estimate the dose response curve for the detailed groupings. It is noteworthy that Ning et al 
did not find a monotonic association between BPA and the risk of the presence of self-reported diabetes, despite clear statistical differences across quartiles of exposure.
Much remains unknown about the mechanisms involved in the association between BPA and CAD in humans. Future scientific work in humans is, of course, constrained by ethical limits and the practicality of repeated BPA exposure measures and long term follow-up studies. Without these constraints, controlled trials would be needed to prove causation in humans, but such evidence is almost certainly beyond reach.
In our relatively small sample of patients investigated for ischemic heart disease referred for coronary angiography, BPA exposure (evident in urinary BPA concentrations) was higher in those with severe coronary artery stenoses compared to those with no vessel disease. Larger studies are needed to estimate true dose response relationships. The mechanisms underlying the association remain to be established.