BPA is absorbed in the mouth; could explain high blood levels
Gayrard, V, MZ Lacroix, SH Collet, C Viguié, A Bosquet-Melou, P-L Toutain and N Picard. 2013. High Bioavailability of Bisphenol A from Sublingual Exposure. Environmental Health Perspectives online 12 June 2013. http://dx.doi.org/10.1289/ehp.1206339
French scientists have discovered a new pathway by which bisphenol A can enter the body: under the tongue. Via that route, it bypasses the metabolic pathways in the liver that convert it to an inactive form before it gets into the blood and on to vulnerable tissues.This discovery provides a possible explanation for why active BPA is being reported in blood at levels that are higher than expected.
BPA is used to make polycarbonate plastic and epoxy resins. It can be found in hard plastic bottles, CDs, eyeglass lenses and a wide array of similar products with shatterproof, glass-like characteristics. The resin form is used to coat the inside of most food cans. It also is used on some thermal paper receipts.
A wide array of health effects in lab animals or humans has been associated with exposure to BPA, including heart disease, diabetes, prostate lesions, mammary tumors and several types of neurological deficits. The National Toxicology Program concluded that it had "some concern" that BPA could affect neurobehavioral development in children.
The authors of the new study say they are the first to demonstrate that exposure to BPA under the tongue, which is called sublingual absorption, is almost completely available for interacting with tissues.
Some scientists, industry representatives and U.S. Food and Drug Administration officials have pointed to the liver's de-activation of BPA as so effective that only infinitesimal amounts of active BPA could be in the blood so it couldn't possibly be having any effect. This assumption was based on experiments with people and rodents that bypassed the mouth by using gelatin capsules to deliver the BPA (in people) or throat tubes to the stomach (rodents).
Sublingual absorption is a common pathway by which pharmaceuticals enter human blood. For example, nitroglycerin is delivered under the tongue in first aid for a heart attack.
This study, conducted by scientists working for the French government and several French universities, used dogs as experimental subjects because the bony structure of a dog's mouth is much more like a human mouth than is a rodent's. Dogs are routinely used as surrogates for people in studies about sublingual absorption of pharmaceuticals.
The scientists conclude that BPA can be "efficiently and very rapidly absorbed" via the sublingual route, and that this pathway can lead to "far higher BPA internal exposures than known for BPA absorption from the gastro-intestinal tract."
These observations may help resolve one of the current hot-button arguments about BPA safety. Over two dozen studies of people have reported detectable amounts of active BPA in blood, some with relatively high amounts. These studies have been challenged, however, because studies of how BPA is rapidly metabolized in the liver indicate that there should be no measurable amount in blood. Some scientists have argued that the studies reporting BPA in blood should be ignored because they are flawed, most likely because equipment used in obtaining the blood contained BPA and contaminated the samples.This discovery provides an alternative explanation.
The study also raises a broader issue about using stomach tubes to deliver chemicals to the stomach of a lab animal. This is a common procedure in toxicology, used because it provides excellent control over how much of a chemical actually gets into the mouse. But if sublingual absorption is a common entry pathway, thousands of experiments costing many millions of dollars could be providing misleading data about chemicals' risk to people.