Is it ‘toxic?’ Depends on the dose.

By Dr. John M. Rost, Chairman of the North American Packaging Alliance, Inc.

Would you give a “toxic” substance to your child? No. But hold on. Does “toxic” mean what we think it means?

After all, most parents quite regularly give their children a very toxic substance whenever they get sick or run a fever: acetaminophen. That’s Tylenol to you and me. In the wrong amount, it is a highly toxic substance that can cause complete liver shutdown and death. Does this mean a parent is “poisoning” his or her child by giving them pain relief? Of course not, because parents know that if they follow the directions on the bottle and stick to the recommended dose, the pill is safe.

When it comes to toxins, it turns out that dosage is everything. Parents who fret about long-term exposure to very low doses of toxins assume that all substances are either toxic or not. But that’s not how it works. It’s hard for them to accept that in low doses, acetaminophen and many other substances aren’t toxic at all. Most parents don’t know about pharmacokinetics.

The term pharmacokinetics means what happens in the body when a substance is ingested, injected or introduced into the body by some other route. Pharmacokinetics is extremely important to the pharmaceutical researchers. They need to know if a drug will actually reach a patient’s target organ and maintain the molecular form it needs to have the proper medical effect.

What pharmacokinetics tells us is that many toxic substances are quickly metabolized once they enter the body, converted by enzymes into harmless molecules. They’re then excreted. Take acetaminophen, for example. Three different enzymes act upon the pill’s main active ingredient. Two of them convert it to a metabolite that is completely harmless and quickly leaves the body. A third converts acetaminophen into a harmful substance. Luckily, that harmful metabolite is then acted upon by a fourth enzyme and converted to yet another substance—one that is harmless.

Bottom line: Tylenol isn’t toxic in small amounts. That fourth enzyme protects us—unless, that is, we overdose. That’s when our bodies run out of the critical fourth enzyme, leaving the otherwise helpful drug to damage to our liver.

Hence the saying among toxicologists: “The dose makes the poison.”

For several years now the media has been hyping stories about the possible dangers of bisphenol a (BPA). Many consumers will recognize BPA from reusable water bottles that state they are “BPA-Free.” What most consumers don’t know is that BPA is in some ways strikingly similar to acetaminophen. BPA is metabolized by some of the same enzymes that act upon the painkiller. The enzyme that produces the toxic metabolite of acetaminophen does not, however, have the same effect on BPA. Once ingested, BPA is quickly metabolized into an entirely biologically inactive form and excreted in urine.

This process has been confirmed in studies performed by the U.S. Food and Drug Administration and the Pacific Northwest National Laboratory in both mice and humans. Both have concluded that when ingested, BPA does not exist in the body in a form that can cause harm. But those studies stand in contrast to many, more widely publicized studies that either bypassed the route that humans are exposed to BPA and directly injected it into the body or dosed it so high that the enzymes that react with BPA were overwhelmed. The resulting dosages proved poisonous. But neither of those situations represented anything reasonably approaching a normal person’s exposure to BPA.

BPA’s safety from ordinary exposure has been reviewed numerous times by agencies such as the European Food Safety Authority, Health Canada, Food Safety Standards of Australia/New Zealand, and even, ironically, the FDA. All of these agencies concluded that current uses and exposures to BPA are safe.

One of BPA’s main uses is to protect canned foods. BPA is a starting material to make the protective coating used on canned foods—an enormously valuable one. With the use of BPA as a starting substance, there has not been an incident of food poisoning from the failure of a can for nearly 40 years. We are talking about hundreds of millions of real people eating from trillions of cans of food.

BPA and acetaminophen. Both can be toxic at a high enough dose, but don’t forget this crucial detail: With acetaminophen, a toxic dose is just eight to 10 tablets, which unfortunately is a plausible overdose. With BPA in canned foods, getting a toxic dose would require a person to eat from 400 to 500 cans in a single day—impossible to achieve. Yet in the world of internet news, that critical fact never quite fits in the headlines.