Many non-organic soy products may contain traces of the neurotoxin hexane, according to a report by a research group that supports organic and sustainable agriculture. How worried should soy foodies be?
Maybe a tiny bit, but only because of the lack of data. The FDA does not currently impose a ceiling on hexane residue in soy foods, but it does limit how much of the chemical can be left in fish protein isolate (5 parts per million), as well as in hop extract and spice resins (25 ppm). The study (PDF) that's now in the news found 21 ppm of hexane in soy meal—the defatted soy flakes that are used in products like energy bars and veggie burgers. The authors of the study told the Explainer that more recent testing has found concentrations as high as 50 ppm. But that's no reason to throw your meatless breakfast links down the disposal. The hexane limits are precautionary. No study has ever tested how much hexane a person can safely eat over the course of a lifetime, but rodent studies suggest that your Thanksgiving tofurkey isn't going to kill you.
Soy processors use hexane to divide the beans into fat and protein (PDF). The beans are cracked, heated, and rolled into flakes, which are soaked in a hexane bath to extract the oil. Then the oil and defatted flakes are each steamed to evaporate the hexane. The crude soy oil is shipped to vegetable oil manufacturers—most vegetable oils are made from soy—to have the remaining proteins and free fatty acids removed. (The Cornucopia study found 10 ppm of hexane in vegetable oils.) The defatted soy flakes can be further processed and included in a wide variety of foods, veggie burgers included.
The government has no evidence that this process is unsafe for consumers. The FDA and EPA typically rely on rats and powers of 10 to set limits on chemicals in the food chain. Scientists establish how much of a chemical they can give to rats over the course of a lifetime without any symptoms. They then take this quantity, called the "no observable adverse effect level" (expressed in milligrams of chemical per kilogram of body weight), and divide it by 10 once to account for humans' higher sensitivity to toxins, then once more to account for the fact that some individuals may be more sensitive than the average. The agencies sometimes divide by 10 a third time if the chemical is typically consumed by vulnerable groups like children or pregnant women. The result is called the reference dose and serves as the basis for the legal limits, after accounting for the average person's daily consumption habits.
There is no reference dose for ingested hexane. What research exists on chronic hexane toxicity (PDF) focuses almost completely on inhalation, because the volatile solvent is floating around at higher-than-normal concentrations in vegetable-oil-processing, fuel-refining, and glue-making factories. (OSHA limits the amount of hexane workers can huff during an eight-hour shift.) Air is a more efficient vehicle than a soy Chick'n Nugget to get hexane into your blood, because the inhaled chemical bypasses your liver.
In the absence of good data, the FDA uses "current good manufacturing practices"—the level producers should be able to stay beneath if they're making an effort—to establish maximum concentration of a food additive. That's where the hexane limits come from.
The few rodent studies that have been done suggest it would be difficult to get enough hexane from your grocer's natural foods section to harm yourself—at least in the short term. Scientists fed about 500 mg of hexane to lab rats for 90 days and found no adverse effects. Even if veggie burgers were to lose none of the volatile hexane during cooking, it would take more than 353,000 veggie burgers a day to reach that level, adjusted for human body weight. At 2,000 mg of the solvent over the 90-day period, the rats started to show some neurological problems. That's about 1.4 million veggie burgers per person, per day. The reports do not consider whether the experimental rats might be more susceptible to long-term problems.
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Explainer thanks David Ailor of the National Oilseed Processors Association, Charlotte Vallaeys of the Cornucopia Institute, Carl K. Winter of University of California–Davis, and Andrew Zajac of the FDA.