CSPI overblows the cancer risk of caramel coloring in soda

The safety of soda has been in the news a lot lately. The news even seems bad for diet coke, which hits close to home for me given my diet coke addiction. The worst seems to be this correlative study proposing a link between diet sodas and stroke risk:

The study, which followed more than 2,500 New Yorkers for nine or more years, found that people who drank diet soda every day had a 61 percent higher risk of vascular events, including stroke and heart attack, than those who completely eschewed the diet drinks, according to researchers who presented their results today at the American Stroke Association's International Stroke Conference in Los Angeles.

Disturbing news, however, it's still just a correlative finding from the Northern Manhattan studyand until things are studied more rigorously, I probably won't quit my current caffeine source. After all, it can reflect patients who are drinking diet drinks because they are diabetic, a known cardiovascular risk factor not excluded in their analysis.

Now the Center for Science in the Public Interest (CSPI) has entered the soda fray with a report suggesting the caramel coloring has unacceptably high levels of a carcinogen called 4-methylimidazole.

An independent study commissioned by the Center for Science in the Public Interest (CSPI) uncovered 4-methylimidazole, or 4-MI, in Coke, Diet Coke, Pepsi and Diet Pepsi at levels 4.8 times greater than those allowed in beverages in California.

4-MI is a byproduct of the reaction that produces the caramel coloring in brown sodas. The chemical has been found to be carcinogenic in animal studies.

The state of California has banned 4-MI in any amount that could potentially lead to one cancer case in 100,000 people. However the levels found in these 4 leading Cola brands indicated a lifetime risk of 5 cancers out of 100,000, assuming that people drink one soft drink per day. That risk rises to 10 cancers out of 100,000 people who drink only soft drinks containing caramel coloring.

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But what is the evidence this level of 4-MI could pose a health risk?

A quick pubmed search later and one finds toxicity studies in rats and mice were performed that found no difference in mortality between the groups, but at the higher doses, alveolar/bronchiolar adenoma and carcinoma was found at a higher rate than in the lower dose or control groups in mice and in female rats. But what were the doses? Based on their numbers and the average weight of a rat (300-500g) and a mouse (30g) this would mean a female rat eating 120-150mg of 4-MI a day or mice eating 2-4mg might see a higher rate of these cancers. Now the amount in Coca-Cola appears to max out at 150 micrograms based on the CSPI, and the average weight of a human is 70kg for a dose of about 0.002mg/kg from a can of Coke. So for a human to receive a comparable dose to the lower of the two groups with higher cancer incidence they'd have to drink about 60,000 cans of coke a day. Worse for the CSPI argument some other animal data suggests 4-MI may decrease the rates of some other cancers.

Now, how about the risk calculation cited by the CSPI from California of a 5x increase to 5:100,000? I looked at the califormia prop65 document describing the above study and the basis for their risk calculation cited above. You can read their methods in the appendix themselves, but they basically extrapolate based on size between humans and mice, assume the dose-response is linear, and assume consuming 1/100,000th the dose will result in 1/100,000th the risk. Their estimate of the lower bound of risk in humans would then be 16 micrograms/day. But this estimate is based on a lot of assumptions about effects of lower dosages of the chemical over a longer exposure that strongly skew the minimum safe dose to a tiny fraction of what they gave the mice and rats in these studies.

In other words, 16mcg of 4MI a day is an estimate of minimal risk, based on a few assumptions divided by a huge number. The CSPI however, rather than taking this number with a hefty grain of stroke-causing salt, instead treats these numbers as absolute and reliable indicators of risk and frankly come off a bit kooky:

"Coke and Pepsi, with the acquiescence of the FDA, are needlessly exposing millions of Americans to a chemical that causes cancer," said CSPI executive director Michael F. Jacobson.

Really? This is a trace exposure of a chemical that has only been shown to cause cancer at huge doses in a model organism. And it sounds like the FDA isn't buying it either:

FDA spokesman Doug Karas said the findings of the report were not of concern to consumer health.

"A person would have to drink more than a thousand cans of soda in a day to match the doses administered in studies that showed links to cancer in rodents," he told Bloomberg.

A lot more than a thousand a day. I fully admit I'm not a toxicologist and my knowledge of generating these risk estimates is sparse. Nor do I want to discount taking a "better safe than sorry" approach and being very conservative with needless exposure to risk. But this is on the extreme end of silly to me. You can take almost any chemical we're exposed to routinely, give rats or mice enormous doses of it, and see increased risks of cancer. These estimates rely on some big assumptions, and after looking at the literature I am not going to worry about caramel-colored sodas based on this report. Frankly, the CSPI comes across as needlessly hysterical over a largely theoretical risk.