How Burnt Toast And Roast Potatoes Became Linked To Cancer
Advice that overcooked starchy foods can contain acrylamide, a chemical liked to cancer, has caused a furor this week. Is it just another health scare?
In October 1997, something troubling was happened around the Hallandsas ridge in southwestern Sweden. Farmers had found cows paralysed or dead in their fields, lifeless fish were spotted floating in a local river and workers at a construction site began suffering from nausea and prickling sensations in their fingers.
Suspicion fell on a major construction project to drill a railway tunnel through the ridge. The project had been plagued by leaks and the construction company had resorted to injecting 1,400 tons of a sealant called Rhoca-Gil into tunnel walls. Tests confirmed that the sealant had leaked high levels of a toxic chemical into surrounding ground and surface water. The chemical was acrylamide.
What began as an environmental scandal soon morphed into a public health scare. Professor Margareta Tornqvist, an environmental chemist at Stockholm University, was enlisted to carry out tests on the construction workers, which revealed high levels of acrylamide in their bloodstream. Unexpectedly though, significant levels of the chemical were also present in a control group.
We realised that if this background signal really was acrylamide it meant that ordinary people are always exposed to acrylamide, said Tornqvist.
Since the compound is not found in wild animals, processed food was identified as the likely culprit. After discounting beef burgers as the source, Tornqvist and colleagues had discovered that acrylamide is found in highest concentrations in starchy food, like bread and potatoes, when it is cooked at high temperatures.
By 2000, the team had published a study involving rats being fed fried food that concluded that acrylamide consumption is associated with a considerable cancer risk.
The resultant public health advice became embroiled in controversy, however (For me as a scientist it was a bit of a tough period, Tornqvist recalls). And nearly a decade on, there is a lingering question, never satisfactorily resolved, over whether we should be worried about acrylamide in our diet.
This week, the UKs Food Standard Agency became the latest regulator to draw consumers attention to the issue with its Go for Gold campaign that urges the public to avoid singeing their toast or leaving roast potatoes to char in the oven.
As a general rule of thumb, aim for a golden yellow color or lighter when frying, baking, toasting or roasting starchy foods like potatoes, root vegetables and bread, the FSA recommends. At the same time, it acknowledges: there is more to know about the true extent of the acrylamide risk.
Critics of the advice were quick to point out that animal studies linking acrylamide to cancer have used doses so far above the average daily consumption in humans that extrapolating the results is questionable even assuming the effect is comparable across species.
A person would need to eat 75kg (165lbs) of chips every day for two years, for instance, to be getting the cumulative dose of acrylamide (200 mg/kg daily) used in one rat study that showed the compound could cause DNA damage when it latched on to hemoglobin in the blood.
Tornqvist counters this argument with the observation that, although some chemicals have a minimum threshold concentration below which they are not dangerous at all, this is not the case for DNA-damaging chemicals, known as carcinogens.
With chemicals that damage DNA its a linear dose response, so even the smallest dose contributes to the risk, she said. There is no threshold dose for the effect.
It is partly the lack of any technical lower bound for toxicity, though, that has led to literally hundreds of every day chemicals being classed as carcinogens. The American Cancer Society describes acrylamide as a probable carcinogen in a list that also includes hot beverages, working as a hairdresser and insecticides. Short of retreating into a bunker (solar radiation also features), many items do not lend themselves to practical measures to reduce exposure. With acrylamide, at least, agencies can point to the simple step of not burning your toast.
Attempting to establish the existence of the link between a ubiquitous chemical and an incremental cancer risk is a daunting challenge, but there has been no shortage of attempts to do this.
A recent meta-analysis found no evidence of a risk for cancers of the esophagus, stomach, pancreas, breast, lung and prostate. Again, this does not rule out the existence of a risk, but it does put a very low upper bound on the problem.
David Spiegelhalter, professor for the public understanding of risk at the University of Cambridge, said: Nobody can put any numbers on acrylamide, but it cant be that bad. Wed know about it if it was.
By contrast, Cancer Research UK highlights that around 18,000 cases of cancer each year in the UK are caused by being overweight or obese, and it predicts that alcohol will cause around 135,000 cancer deaths over the next 20 years.
Even if the risk is small undetectable in a population, even Tornqvist argues that it is worth acting on.
Why should [food agencies] bother about a lot of other things with much smaller risk if they don’t bother about acrylamide, she said. Shouldn’t we try to protect people from that the best we can?
There are indications that, on advice from the FSA and other agencies, the food industry has taken the message on board. Between 2007 and 2015, there was an an average 30% reduction in acrylamide across all products in the UK, according to FSA figures.
However, Spiegelhalter argues that, while there may be a good case for issuing industry guidelines, there must be a higher bar for public health warnings. The ultimate outcome of continual vague fear-mongering, he said, is not a society that examines its diet with meticulous and scientific rigor. Instead, other, more pressing health messages are simply diluted.
People are fed up with continually being given public health advice about what appear to be rather minor issues, he said. I don’t believe exercises like this increase peoples trust in science. It just gives people an excuse to ignore what they don’t want to hear.