These potent toxins originate from a mold species growing on nuts and are a major threat to human health.  

Delicious, right? Eating the wrong ones can literally kill you without you taking notice. Source

Intro

Today, it should be common knowledge that moldy food is unhealthy because of the chemical compounds produced by those fungi – the so-called “mycotoxins”. But when it comes to toxicity, one subgroup of those beats all the others by lengths: the aflatoxins.

Aflatoxins are produced by some species of Aspergillus molds, which grow predominantly on grains or nuts in in regions with hot climate, but thanks to globalized agriculture, they have become a relevant health issue for inhabitants of temperate zones as well.

Their main representative, aflatoxin B1 (AFB1), is extremely genotoxic (=it damages the DNA) and gained fame as the strongest natural carcinogen (=cancer-promoting/causing compound). 

Reason enough to write another tox-blog. Let’s come together and take a look on where to find those toxins and what they might be doing to you.

Where to find aflatoxins

Aspergillus flavus, the most important aflatoxin-producing organism, lives in the soil in tropical and sub-tropical climate zones. As all molds, it needs a certain humidity to grow. Thus, it mainly gets into the food chain if agricultural products haven’t been stored properly – in particular not dry enough. Then, Aspergillus flavus is able to contaminate grains and nuts.

The former has high relevance for sub-Saharan African countries, where in some areas farming conditions are very bad. Often, barns for storing crops are not well enough protected against moisture, which leads to high concentrations of mycotoxins in local cereals. Especially children have been found to be at high risk, with some local studies (in particular from Nigeria) even suggesting a connection of aflatoxin intake with the higher prevalence of stunted growth in those countries. 

The contamination of nuts is more of relevance to Western countries, as they are the prime importers of those agricultural products, with an increasing demand for exotic (non-native) nuts.

A beautiful Aspergillus "flower" under the microscope. Source

Aflatoxin Toxicity

As written in the intro, AFB1 is extremely genotoxic. But how does it work? 

To answer that question, let’s look back at the article I wrote a few days ago about the xenobiotic metabolism, our intrinsic defense system against many poisons. Somewhere near the end, I told you that occasionally the system backfires and creates a toxin out of a less toxic compound. This is exactly what happens when you take up AFB1.

It is metabolized by a phase I enzyme called CYP3A4, which adds a very reactive functional group (for chemists: it’s an epoxy-group!) to the molecule. With this group, the formed metabolite is able to bind to guanine, one of the building blocks of our DNA, resulting in a so-called “bulky DNA adduct”.

To visualize it, imagine a brick solidly attached to a gear-wheel that is supposed to work in a complicated machine, just 100,000-times smaller. Of course, the machine will malfunction: nuclear enzymes are not able to read the DNA correctly any more at positions where those adducts occur, which leads to errors with translating the gene code to proteins or even with copying the DNA – so-called “mutations”. Mutations in genes that are involved in cell growth or the programmed death of non-functional cells ultimately lead to the development of cancer.

By this mechanism, AFB1 is acutely toxic to both liver and kidneys (the two prime organs for the metabolism), but is also extremely mutagenic and carcinogenic, as demonstrated in several studies. It exerts its toxic effects at very low concentrations (below 10 µg/kg bodyweight), that can be reached by eating just one (!) contaminated nut.

Why monitoring aflatoxins is so difficult & concluding remarks

And that is the main problem here: monitoring every single nut during food quality control is impossible. All you can do is analyze samples. A few nuts here and there, and the occasional processed food. But as the majority of nuts is not contaminated, and one contaminated nut is all you need to ruin the safety of your food, it’s like looking for the famous needle in a haystack.

All we can try to do is prevention, prevention, prevention. To educate farmers – especially in underdeveloped regions - to store their products dry, and to educate consumers to be careful (job right now^^):

If one of your delicious para nuts or cashews doesn’t look right, don’t eat it! If it tastes wrong, spit it out! Liver cancer is one of the last things you would like to experience.

Additional sources: 

Article
Review
Review
Book "Toxikologie für Naturwissenschaftler und Mediziner." Eisenbrandt, Metzler, Menneke. Wiley-VCH
Lectures "Toxicology" and "Food Toxicology" by Prof. Marko, University of Vienna

Disclaimer: In my blog, I'm stating my honest opinion as a  researcher, not less and not more. Sometimes I make errors. Discuss and  disagree with me - if you are bringing the better arguments, I might  rethink.