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The Hidden Dangers Of Eating Burned Toast: What You Need To Know

It really does contain a carcinogen – but is that the whole story?

author

Dr. Katie Spalding

author

Dr. Katie Spalding

Freelance Writer

Katie has a PhD in maths, specializing in the intersection of dynamical systems and number theory.

Freelance Writer

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Two pieces of toast burning in a toaster

Image credit: wimammoth/Shutterstock.com

To whomever it was who first looked at a piece of bread and thought, “hell, let's cook it again, see what happens,” we salute you. Toast is great: you can slather it with butter and jam, cover it in beans, dip it in eggs, or even make it into its own sandwich, all the while feeling smug about the fact that you’re eating something that is somehow a tiny bit healthier than the bread it came from. There's a reason we use the word to mean "a celebratory speech and drink in honor of something good", and it's because – and this is genuinely true – toast is so yummy and great.

Burned toast, on the other hand? Disgusting. Rank. No better than chewing on a piece of charcoal. Smells like a stroke, and tastes so gross, it can actually give you cancer. 

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And that's not just us saying it – it’s science. Or is it?

What is going on when you make toast?

Like all cooking, toasting is a type of chemical reaction. A very specific one, in fact: it’s called the Maillard reaction, and it only occurs at heats of around 120°C (248°F).

It’s a pretty complex process, when you break it down, but the gist of the Maillard reaction is this: the reducing sugars – which for bread are chiefly maltose and glucose (and, in America, a fair dollop of fructose), but in other foods may include, say, lactose or ribose – react with the amino acids present in the bread, producing both a change in color and a whole bunch of interesting flavor and aroma molecules.

Basically, you can thank the Maillard reaction for the tastiness of most cooked dinners. “In the process, hundreds of different flavor compounds are created,” explains Science of Cooking. “These compounds in turn break down to form yet more new flavor compounds, and so on. Each type of food has a very distinctive set of flavor compounds that are formed during the Maillard reaction.”

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So, what’s the problem? Well, it all comes down to those amino acids – in particular, one called asparagine. It’s present in a wide range of your favorite foods, including potatoes, bread, cereals, cookies, and coffee, and that’s a shame, because back in 2002, scientists in Sweden discovered that the Maillard reaction takes asparagine and produces a nasty little substance known as acrylamide.

What is acrylamide?

Acrylamide is an organic compound which – weirdly, considering it’s formed when we brown toast – is actually white. It’s listed on the NIH National Library of Medicine as a “confirmed carcinogen” and by CAMEO Chemicals as “very toxic”; when it’s not making our roasties so tasty, it’s used in sewage and waste treatment, dyes, and adhesives, and if that wasn’t enough for you, it can explode if it’s exposed to too much heat or natural light.

Despite all that, it’s not something to be angry about finding in your food. “It is likely that acrylamide has been present in food since cooking began,” explains the European Food Safety Authority (EFSA)’s factsheet on the compound. “It is practically impossible to eliminate acrylamide entirely from the diet.”

Unfortunately, acrylamide is – to put it mildly – not very good for us. It’s known to be neurotoxic, meaning it messes with our nervous system, and long-term exposure through food may increase the risk of neurodegenerative disorders such as dementia. It can even cause problems in developing fetuses: “Acrylamide passes through all tissue, including the placenta, because it has a low molecular weight and is soluble in water,” Federica Laguzzi, assistant professor of cardiovascular and nutritional epidemiology at the Institute of Environmental Medicine at Karolinska Institutet in Sweden, told BBC Future.

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And then there’s the cancer risks. Research has shown for sure that high levels of acrylamide cause cancer in animals, and epidemiological studies have hinted that the same may be true for humans: the compound has been linked to higher incidences of breast cancer, kidney cancer, endometrial and ovarian cancer, and, when all else fails, just cancer in general.

But – and we realize this may sound ridiculous after all that, but bear with us – does that actually mean eating acrylamide is dangerous?

Is my toast going to give me cancer?

There’s a big problem when it comes to measuring cancer risk in humans.

“To really be able to be to say that this causes cancer you need to do clinical trials,” Rashmi Sinha, a senior investigator at the National Cancer Institute, told Inverse. “But you can’t do clinical trials with things that are possible carcinogens.”

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Yes, lab studies have shown that acrylamide causes cancer – but “the levels… used in these studies were much greater than those found in human food,” notes the FDA factsheet on the substance, and the tests were being done on mice and rats

In humans, though, we simply don’t – and can’t – get that kind of data.

Instead, Sinha explained, “the main studies have been association [or] prospective studies,” relying on self-reporting over a long period of time. “We ask questions about how [healthy participants] cook their foods and then we follow them up for ten, fifteen, twenty-years,” Sinha said. “[Then we] compare people who had cancer with no cancer to see if there's anything associated with the way that they cook the foods.”

The problem here is that firstly, people are dirty, sneaky liars, and secondly, we’re also really bad at remembering things – so any information researchers get is likely to be at least a little skewed.

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What that means, Fatima Saleh, associate professor of medical laboratory sciences at Beirut Arab University in Lebanon, told BBC Future, is that “after almost 30 years of its classification as a ‘probable human carcinogen’, there is still inconsistent evidence of its definite carcinogenicity in humans.” 

“However, if we continue to do further studies on humans, we might have adequate data to change acrylamide’s classification to a human carcinogen,” she added.

Look, do I have to stop eating toast or not?

Ah, the million-dollar question: can we still eat toast? Well, unless you’re routinely burning it to a piece of coal before you spread your peanut butter over it, then honestly, you’re probably fine.

Not only is the level you’re being exposed to likely lower than it would have been, say, 20 years ago – that’s thanks to campaigns embraced by both governments and individual companies – but our bodies may even have protective mechanisms built-in against the compound, Laguzzi said.

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“Also, we don't just eat acrylamide on its own,” she pointed out. “It's in food, where there could also be other components, like antioxidants, that can help prevent the toxic mechanisms.”

More than that, there are some pretty simple ways to reduce the levels of acrylamide in your cooking. In potatoes, for example, acrylamide formation is reduced by close to 90 percent by just soaking them in hot water for 10 minutes.

For toast, the solution is equally neat: just don’t burn it. “Since acrylamide levels are directly related to the browning of these foods, some countries recommend to consumers: ‘Don’t burn it, lightly brown it’,” advises the EFSA. Other countries have gone further: the UK’s Food Standards Agency, for instance, launched a campaign in 2017 telling citizens to “Go for Gold” when they make toast, which if nothing else underlines how important a foodstuff burned bread is in the island nation.

So, is your toast a serious health hazard? Almost certainly not. Just try not to burn it to a crisp, and you’ll be fine – after all, who really wants to eat a loaf of charcoal anyway?

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All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current. 


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