Should you avoid eating burnt food?

Do you scrape the burnt bits off a piece of toast? Recent research suggests that might not be a bad idea…

It’s more than likely you still have some of the habits around eating and cooking that you learned from adults when you were young, maybe without even realising. Perhaps you never lick food off your knife, or you always throw salt over your shoulder to ward off evil spirits.

Many of these quirks are probably nothing more than superstition, but one in particular may have been unknowingly prescient a few decades ago, and grounded in a scientific discovery that was yet to happen.

In 2002, scientists at the University of Stockholm discovered that it might actually be wise to scrape the burnt bits off your toast. They found that a substance called acrylamide forms when we apply heat over 120C (248F) to certain foods – including potato, bread, biscuits, cereal, and coffee – and its sugar content reacts with the amino acid asparagine.

This process is called the Maillard reaction, and it causes food to brown and gives it that distinctive flavour. But scientists have found that doses of acrylamide is carcinogenic in animals, but only in doses much highter than those in human food.

Acrylamide could also increase the risk of humans developing cancer, especially children, according to the European Food Safety Authority. But researchers looking into the effects on humans have not yet been able to come to a definite conclusion.

“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,” says Fatima Saleh, associate professor of medical laboratory sciences at Beirut Arab University in Lebanon.

Scientists are sure, however, that acrylamide is neurotoxic to humans, which means it can affect the nervous system. The exact cause for this are still not fully understood, but among the theories are that acrylamide attacks structural proteins within nerve cells or may inhibit anti-inflammatory systems that protect nerve cells from damage.

The toxic effects of acrylamide have been shown to be cumulative, which means that consuming a small amount of acrylamide over a long period of time could increase the risk of it affecting organs in the longer term.

More specifically, evidence from animal studies suggests that long-term exposure to dietary acrylamide could also increase the risk of neurodegenerative disease, such as dementia, and may be associated with neurodevelopmental disorders in children, says Federica Laguzzi, assistant professor of cardiovascular and nutritional epidemiology at the Institute of Environmental Medicine at Karolinska Institutet in Sweden.

“Acrylamide passes through all tissue, including the placenta, because it has a low molecular weight and is soluble in water,” says Laguzzi, who has found a link between higher acrylamide intake in pregnant people and the lower birth weight, head circumference and length of their newborn babies.

The potential mechanism behind acrylamide’s role in increasing the risk of cancer in humans isn’t yet known. Leo Schouten, an associate professor of epidemiology at Maastricht University in the Netherlands, has a theory why it might happen.

After the 2002 discovery of the presence of acrylamide in our food by Swedish researchers, the Dutch Food Authority contacted investigators of the Netherlands Cohort Study on Diet and Cancer, including Schouten, to investigate whether dietary acrylamide was a risk for humans. Schouten and colleagues tried to capture an estimate of how much acrylamide people were consuming based on a questionnaire.

They discovered that the variation between people with low and high exposure in an elderly Dutch population could be explained mainly by one product popular in the Netherlands called ontbijtkoek, roughly translated as “breakfast cake”, which was extremely high in acrylamide due to the use of baking soda in the production.  They investigated the link between non-smokers’ acrylamide intake (as smoking also contains the substance) and all cancers, and found a higher risk of endometrial and ovarian cancers in women with high exposure to acrylamide. They have also found, in further studies, a slight link between acrylamide intake and kidney cancer.

However, these findings are yet to be confirmed by any other researchers. The closest is a US population study, which published findings in 2012 suggesting an increased risk of ovarian and endometrial cancer among non-smoking post-menopausal women who consumed high amounts of acrylamide. Of course, there could be other reasons for this – people who eat high levels of acrylamide might also follow other lifestyle choices that put them at a higher risk.

Other studies haven’t found an association, or saw weaker associations. But it’s unclear whether the association Schouten and his team found was incorrect, or if other studies weren’t able to measure acrylamide intake accurately.

The mechanism behind acrylamide’s potential cancer-causing effect could be related to hormones, Schouten says, because certain hormones have been associated with an increased risk of cancer, especially female genital cancers like endometrial and ovarian cancer.

“Acrylamide may affect oestrogen or progesterone, which would explain the female cancers, but this hasn’t been proven,” says Schouten.

Laboratory studies involving rats have also found links between acrylamide intake and cancer in mammary glands, thyroid gland, testes and the uterus, which also suggest a hormonal pathway, but this does not automatically mean a similar risk to humans.

In 2010, the Joint Food and Agriculture Organization/World Health Organization Expert Committee on Food Additives suggested that more long-term studies are needed to further understand the link between acrylamide and cancer. It did, however, support efforts to reduce acrylamide levels in food.

But one of the biggest challenges is accurately measuring how much acrylamide we consume.

“It’s well established that acrylamide is genotoxic and can cause cancer in animals, but the association between acrylamide and cancer in humans is still unclear,” says Laguzzi. “Most epidemiological studies are performed with acrylamide intake measured through dietary questionnaires that rely on people’s reporting, which can bias the results.” – BBC