Bisphenol A (BPA) - a chemical used in the manufacture of polycarbonate plastics and some epoxy resins – has been in the news quite a bit lately.
Headline-grabbing news items have been breathlessly reporting on a recent study showing that after consuming a bowl of soup, urine levels of BPA soared by 1200%. But behind those scary-looking figures lies a far more mundane reality – our bodies are actually really good at removing BPA and risks from food are negligible.
Now a new study is purporting to show that consumption of moderate levels of BPA affect breast tissue in monkeys. How relevant is this study to humans and how worried should we be? The answer is – not much and not very.
Let me explain why.
What is BPA anyway?
Commonly used in the manufacture of plastics, BPA is an unlikely environmental toxin. While it does persist in the soil, unlike perflourinated compounds, which can take four years or more to be removed from the body, the half-life of BPA in the body is measured in hours.
BPA enters the environment in a number of ways as part of plastic manufacturing and use. It can enter our foods in small amounts from contact with plastic wrapping, plastic bags and the plastic lining on tins.
At reasonably high levels, BPA is toxic to animals and humans. Of most concern is the fact that BPA can mimic the hormone estrogen, and so might have effects disproportionate to its concentration. Still, it’s wise to remember the toxicologists’ dictum – “it is the dose that makes the poison.”
How much is enough?
Acceptable levels of exposure have been set from exposing animals to various levels of BPA throughout most of their life span for up to three generations. The concentration that had no effect on animals was determined and the acceptable human-exposure level (0.05 mg/kg/day) was set by dividing that concentration by 100 to provide a generous safety margin. So the safe limit of BPA consumption for someone weighing 80 kilograms is 4 milligrams every day.
How, then, does this limit compare with what you could expect to be exposed to from food? If we look at canned soup, which has one of the highest levels of BPA (and take the soup with the highest level of BPA), you would need to drink 100 cans of soup in a day to reach the acceptable exposure level. Remember that this level has already been set at 100 times lower than the levels that have no effect on animals.
Of course, BPA is in more foods than soup, but I’ve deliberately chosen a food with one of the highest concentrations of BPA. Even if you only consumed packaged and canned food, to get to the threshold BPA level would require such prodigious food intake that BPA levels would be the least of your worries!
So keeping in mind the heroic food consumption required to reach harmful levels of BPA, what can we say about the most recent paper?
What the researchers found
Researchers fed pregnant Rhesus monkeys BPA during 100 days of their pregnancy, measured blood levels of BPA and looked at the breast tissue in female offspring at the end of pregnancy. Their idea was that if there’s a significant estrogenic effect from BPA, it should affect the offspring’s breast development.
The study authors used monkeys because they’re more similar to humans than the commonly used lab animals, such as rats and mice. This is a strength of the study.
They were also trying to keep the blood plasma levels of BPA to that seen in humans. This should be a strength, but I think they’ve made a mistake in their concentrations, which means their results are not relevant to human exposures.
Problems in the study
The first problem is the amount of BPA they exposed their animals to. They used 0.4 mg/kg/day, which, if you quickly look up the page a bit, you’ll see is eight times the maximum allowed exposure for humans.
What’s more, they were giving the dose in one shot and measuring the blood levels around four hours later. BPA is removed from the body very rapidly, and by the time the researchers measured the BPA concentrations, they would have fallen substantially. This kind of rapid peak exposure is unlikely to be relevant to the more continuous low-level exposures in humans.
The researchers fed such high concentrations of BPA to the monkeys because they were trying to achieve the blood plasma levels of BPA found in humans. This was a good idea, but to show how it went wrong, I need to digress for a moment.
BPA is rapidly metabolised by the body, the most common resulting metabolite is one where BPA has a sugar attached to it (compare the image to the left with the image of BPA above).
The plasma levels of these metabolites are much higher than unmetabolised BPA. But metabolites cannot act as estrogen mimics, only the unmetabolised (“free”) BPA can do this.
The problem arises when you try and measure BPA. Because it’s so low in human plasma, it’s very tricky to measure, and there are a whole mess of problems that can give you false high values.
The researchers relied on one paper for their levels of free BPA. But there are several papers using sensitive methods that show “free” BPA is well below the levels the researchers used, such as this one, here and this one). The levels used in this study are probably on the order of 50 times higher than normal human levels.
What they found
Even with these very high levels, the researchers didn’t find much. Of the several indices of breast structure that they measured, only one was different (and then not by much). This change isn’t likely to reflect a disease process.
In fact, the study confirms that even at levels of intake much higher than the maximum exposure limit, BPA has no significant adverse effects.
So there’s no need to worry about current human exposure to BPA. Of course, if you want to reduce your intake, the best way is to eat less processed food, which also has numerous other health benefits from reducing salt, sugar and fat intake.
This article has been amended. The original defined BPA as “a solvent added to a synthetic resin to promote plasticity and flexibility and reduce brittleness.”