Science, Art, Litt, Science based Art & Science Communication

Clarification on burnt food and inconclusive scientific research

Q: Is burnt food really bad? I like the smell of burnt food. But one of your answers says it is not good for health. What does scientific research say about it?

What is your advice now regarding burnt food?

Krishna: Yes, I wrote on it some years back.

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I said in my answer that severely burnt food is not good for health.

Now let us consider what other researchers and studies say

Eating burnt toast or slightly charred food is unlikely to cause cancer on its own, and human scientific research does not show a strong or consistent link.
What these studies say
Acrylamide Formation: When starchy foods like bread, toast, rotis, or potatoes are cooked at high temperatures (above 120°C/248°F) until they turn dark brown or black, a chemical called acrylamide forms through a natural reaction between sugars and an amino acid (asparagine).
Animal vs. Human Studies: The International Agency for Research on Cancer (IARC) classifies acrylamide as a "probable human carcinogen". However, this classification is largely based on laboratory studies in mice and rats that were given extremely high doses of acrylamide—thousands of times greater than what humans consume in daily diets.
Human Epidemiological Studies: Large-scale human studies and meta-analyses (such as reviews by Cancer research UK and the European Food Safety Agency) have not found consistent evidence that normal dietary intake of acrylamide increases the risk of common human cancers.
Charred meats vs. Starchy foods: If meat is heavily charred or cooked over an open flame, it produces different compounds called heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs), which have a more plausible link to inflammation and cancer risk with heavy, lifelong consumption. Even then, an occasional charred burger or piece of toast is not a primary driver of cancer.

No need to panic: An occasional piece of burnt toast or a slightly charred roti will not give you cancer.
Aim for golden: Major health and food safety agencies (like the UK Food Standards Agency) suggest cooking starchy foods to a golden-yellow colour rather than dark brown or black just to keep exposure low.
Scrape it off: If a piece of toast or roti has a small burnt edge, you can simply scrape off the charred part to reduce your intake of these compounds.
Focus on the big picture: Real, established cancer risks come from major lifestyle factors like tobacco use, heavy alcohol consumption, chronic obesity, physical inactivity, and diets heavy in ultra-processed or red meats.

The burnt food myth is being dismissed by some as gobbledegook!

However, a systematic review says the above studies are flawed

A systematic review [1] of dietary acrylamide and human cancer, acrylamide exposure and internal dose, exposure assessment methods in the epidemiologic studies, existing data gaps, and future directions says : A majority of the studies reported no statistically significant association between dietary acrylamide intake and various cancers, and few studies reported increased risk for renal, endometrial, and ovarian cancers; however, the exposure assessment has been inadequate leading to potential misclassification or underestimation of exposure.

The review [1] also says: Large variations have been reported in acrylamide content among different brands of similar food items, and such data is not being collected on the FFQ. Although, FFQs are a useful tool for assessing usual dietary pattern they were not designed for capturing chemical exposures. Information such as recipe information, browning of food, length of cooking, micronutrient composition of the raw food, storage of food, and so on is not being collected but is highly important for assessing acrylamide exposures. The measurement errors resulting from above discussed factors have potential for misclassification for ranking individuals for high vs. low acrylamide exposure. The subsequent lack of reproducibility of epidemiological results points toward the utmost need for developing better methods to capture true intake, which would include absorption and metabolism assessments and would be capable of separating dietary exposure from other sources. The studies that estimated acrylamide using FFQ as well as acrylamide adducts, reported a lack of strong correlation between FFQ and HbAA, suggesting that these measurements represent exposures over the different time interval lengths. The HbAA measures the exposure from last 120 days, and even by assuming that the FFQ is adequately capturing long term dietary acrylamide exposures, the correlation could be influenced by seasonality, recent exposures etc. For chronic disease end-points such as cancer, the goal is to estimate long-term exposure as compared to recent ones. So, one could potentially estimate the HbAA pattern by measuring repeated HbAA adducts over a long time will be highly useful (one time point just gives us a snap-shot, not the pattern).

In the absence of food-specific biomarkers, controlled feeding studies among healthy volunteers to evaluate the changes in the HbAA adduct levels by changing the dietary acrylamide intake doses are lacking. Such studies would provide the evidence about the maximal reduction of acrylamide that could be achieved by reducing the food related acrylamide intake and would also provide evidence of the proportion of acrylamide exposure from other environmental sources.

Foods that are sources of acrylamide also contain a range of nutrients and most of these foods are high-energy sources, resulting in a high correlation between energy, nutrients, and acrylamide. High-energy intake itself is associated with obesity and the increased the risk of several cancers, as well as exposure to acrylamide. Most of the studies included in this review used several statistical approaches to reduce the effect of energy on the association between acrylamide and cancer.

Although a number of studies have reported disparity in acrylamide intake, with children being at the highest exposure level, none of the studies to date has evaluated childhood exposure to acrylamide as a potential risk factor for cancer later in life. It is unknown if the magnitude of associations would be different for populations who are generally considered vulnerable such as older adults, and populations who are at higher risk for cancer due to weak immune response and other underlying diseases.

The paper says the research that says acrylamide is not associated with cancer is flawed because of these reasons:

Another one says: In general, the results of the dietary AA intake analyses showed no associations between increasing categories of AA intake and EOC and EC risk, except for a possible association between higher AA intake and Type-1 EC in women who were non-smokers and non-users of oral contraceptives (HRQ5vQ1=1.97, 95% CI=1.08-3.62). [2]

The nested case-control study of AA and GA biomarkers and EOC (334 cases) risk also conducted in non-smoking post-menopausal women found some weak evidence for associations between higher biomarker levels and EOC risk, but none of the associations were statistically significant, and there was no evidence for a dose-response relation.

In conclusion, our results in women from western Europe did not provide convincing support for the hypothesis that higher AA and GA exposures are associated with higher risks for endometrial and epithelial ovarian cancers. Additional studies of AA biomarkers and EOC risk should be conducted to rule out any possibility of increased risk with higher AA levels.

The results in women from western Europe did not provide convincing support for the hypothesis that higher AA and GA exposures are associated with higher risks for endometrial and epithelial ovarian cancers. Additional studies of AA biomarkers and EOC risk should be conducted using larger sample sizes to rule out any possibility of an association between AA exposure and EOC risk.

The researchers themselves are accepting that their samples are smaller in size, conducted on 'European women' and the conclusions are vague.

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, say some of my colleagues.

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.

So what the laymen should consider here?

1. Some studies say : There is no "evidence" that eating acrylamide increases the risk of cancer in people.

Right. The phrase "absence of evidence is not the evidence of absence" is a well-known logical and scientific maxim. It means that failing to find proof that something exists does not automatically mean that it definitely does not exist.

Not seeing something (or not having data for it yet) is different from proving it is absent. For example, just because a species of fish has not been found in a deep-sea trench does not mean it is extinct—as seen with the coelacanth, which was thought to be extinct until a live specimen was caught.

However, when a careful and "sound scientific method" looks for something that should show up (like side effects in a heavily tested drug), a lack of findings can validly point toward absence.

Here sound scientific method is very important.

2. Some reviews about burnt food say the studies that say there is no evidence - are flawed! Hmmm!

3. Existing data gaps.

4. Some studies didn’t find any 'strong evidence' between eating acrylamide and risk of some cancers.

So there is weak evidence!

5. "More research is needed on the impact of acrylamide on cancer risk in humans.

More studies on larger samples and people of different genetic variations using sound scientific methods should be conducted to come to "firm conclusions".

And you asked for my advice.

I still don't eat burnt food.

Because burnt food loses most of its nutritional value. Extreme heat and burning destroy essential macronutrients (carbohydrates, proteins, and fats) and break down heat-sensitive vitamins.

Carbonized or completely burnt parts turn into ash and carbon, which have virtually no usable caloric or nutritional value.

What is the use of eating it, then?

When the conclusions are not certain it is better to be on the cautious side.

If you like the taste of burnt food, eat it occasionally, not on a regular basis.

Acrylamide is in many products, it’s hard to completely avoid it. But you can limit the amount you take in. Most of the acrylamide we consume comes from chips and crisps, so eat these less often. If you cook or fry potatoes, do so until they’re golden rather than dark brown or black.

If you want to cut the level of acrylamide in your diet, follow this advice. Aim for a golden yellow colour when baking, toasting or roasting starchy foods such as potatoes, pulses and bread.

I remove the burnt part and eat the rest of it, if somebody offers me such food.

People look at me in a strange way when I do that.

And I read, "Why is she removing the black spots on a chapati? Isn't she crazy?", on all the faces around me.

But I am unfazed as usual!