How Do We Really Know if Chemicals Cause Cancer?
There is a growing debate about ultra-processed foods (UPFs) —products designed for shelf life and taste, typically high in added sugars, refined grains, unhealthy fats, and sodium, while low in fiber and essential nutrients.
Today, UPFs make up about two-thirds of the average diet and are linked to chronic illnesses such as heart disease, diabetes, and kidney disease.
But here’s the key question: are UPFs truly causing these diseases, or are they simply associated with them? To find out, researchers rely on two main types of studies—each with its own strengths and flaws, especially when it comes to proving causation.
Epidemiology
These are studies done with people, not lab animals—and that makes them tricky. We can’t put humans in cages and control every bite they eat, so epidemiology is based on what we see, either asking people to remember what they ate and how much in the past days or months—or trying to prospectively follow them and ask them to write down what they are eating.
Because we can’t have a controlled test, we can’t necessarily make them eat exactly what we want. In some cases, they don’t want to tell us, or they can’t remember. We also don’t necessarily know what else they were doing that could cause disease. Usually, the best epidemiology can do is to tell us what “might” be true about causation.
Animal Studies
These are more controlled and therefore more precise as the animals (usually rats or mice) are divided into a test (given the possible offending food) and the control (not given the offending food).
But how much do they really tell us about what’s making us ill?
Here are some of the problems with Animal studies:
· Animal tests use unrealistically high doses. When substances are tested in animals, they’re often given doses far beyond what humans would normally consume. For example, when researchers tested the sweetener saccharine, rats were given the equivalent of about 700 cans of soda a day.
· High-dose cancers don’t reflect real world risks. Even when lab animals develop cancer, the internal mechanisms that cause cancer at those high doses rarely corresponds to what happens at low doses. Take saccharin: Canadian studies once linked it to bladder cancer in rats, but later research showed the culprit was crystal formation from massive doses irritating the bladder lining—not saccharin itself. Because of findings like this, saccharin is no longer considered a likely carcinogen.
· Testing advances make trace risks unavoidable. In 1958, Congress passed a law (the Delaney Amendment) which banned any additive shown to cause cancer in man or animals (in those animal tests). Back then, labs could detect substances at parts per million—roughly one drop of food coloring in a bathtub. Today’s tools are far more sensitive, picking up parts per quadrillion (one drop of water in twenty Olympic sized swimming pools). With this level of precision, trace amounts of potential carcinogens can be detected just about everywhere.
By the 1970s, advances in testing created a problem: under a strict reading of the Delaney Amendment, even trace contaminants or impurity could be “found” to be a carcinogen even when the food additive, as a whole, was not carcinogenic.
As Alan Rulis of FDA’s Office of Food Additives explained, something as small as vinyl chloride residue from a lab’s chemist’s shoes could show up in the test and trigger a false cancer concern. To address this, FDA adopted the Constituents Policy, which clarified that a harmless additive wouldn’t be considered carcinogenic just because of trace contaminants. The Court agreed with FDA’s interpretation in a case involving D&C Green No. 6, and the policy still stands. As Dr. Rulis stated, without it, “the hairs on the end of the tail were wagging the tail that was wagging the dog.”
· Life evolved to handle chemical assaults. Our bodies are assaulted by millions of chemicals each day through air, food, water, and other environmental exposures. When life began four billion years ago, we were exposed to eight times more radiation than we are today. Toxic amounts of chemicals like hydrogen sulfide, cyanide, methane, and heavy metals assaulted life. Life formed and evolved to resist these assaults such that we can quickly repair most low dose assaults. In short, small amounts are not the issue and, in many cases, these tiny assaults provide benefits for us.
In reality, most epidemiological and animal studies can only suggest what might cause of chronic disease.
Just because an animal study finds something to be a carcinogen at high dose, doesn’t mean it poses the same risk at the tiny amounts humans actually consume. Epidemiological studies usually point to associations—e.g., people who eat more ultra processed foods often have higher rates of chronic disease.
But an association isn’t the same as proof. While the link between smoking and lung cancer is clear and direct, the evidence on UPFs is still far from settled.