Aflatoxins: Sources, Toxicity, and Health Impacts

Aflatoxins are a family of toxic compounds produced by certain molds that grow on staple crops, and they rank among the most potent naturally occurring carcinogens known to science. First identified in the early 1960s after roughly 100,000 turkey poults died from contaminated groundnut meal in England, aflatoxins have since been linked to liver cancer, stunted growth in children, acute organ failure, and a growing list of effects on the gut and immune system.¹ The problem is global, concentrated in warm and humid regions but increasingly relevant to higher latitudes as the climate shifts.

Where Aflatoxins Come From

Aflatoxins are produced mainly by two species of mold: Aspergillus flavus and Aspergillus parasiticus. These fungi thrive on crops like maize, peanuts, tree nuts, cottonseed, and certain spices, colonizing them in the field before harvest and continuing to grow during storage if conditions allow. The toxins are not visible to the naked eye, and contaminated grain often looks and smells perfectly normal.

Environmental stress plays a huge role in how much aflatoxin these molds produce. Drought and heat stress on the host plant create conditions that favor fungal colonization and ramp up toxin output. Researchers have proposed that aflatoxin production may actually function as a form of antioxidant protection for the fungus itself, which would explain why stressed environments trigger heavier contamination.² Laboratory studies have mapped out how combinations of temperature and water activity switch on the cluster of roughly 30 genes responsible for aflatoxin biosynthesis in A. flavus, showing that specific ranges of warmth and moisture are optimal for toxin production.³

Among the different types, aflatoxin B1 (AFB1) is the most common and the most dangerous. It is classified as a Group 1 carcinogen by the International Agency for Research on Cancer. Other members of the family, including B2, G1, and G2, also appear in contaminated food but are generally less potent. A metabolite called aflatoxin M1 shows up in milk and dairy products when livestock eat contaminated feed, creating a secondary route of human exposure.

How Aflatoxin B1 Damages DNA

When you eat food contaminated with AFB1, your liver enzymes try to process it. One enzyme in particular, known as CYP3A4, converts AFB1 into a highly reactive molecule called the 8,9-exo-epoxide. This is the step that makes aflatoxin so dangerous: the epoxide binds directly to DNA, forming chemical attachments called adducts. CYP3A4 produces only the cancer-causing form of the epoxide, and the enzyme is abundant in human liver tissue, which explains why the liver bears the brunt of the damage.⁴

The DNA adducts formed by this reactive epoxide have been measured directly in laboratory mice given AFB1, at levels ranging from about 1.5 to 45 adducts per million DNA bases depending on the specific adduct type. These adducts cause a characteristic mutation pattern where a guanine base gets swapped for a thymine, and the ring-opened form of the adduct is more mutation-prone than the initial form.⁵ When this mutation lands in a tumor-suppressor gene, it can set a cell on the path toward cancer.

The Liver Cancer Connection

Aflatoxin’s link to hepatocellular carcinoma (HCC), the most common type of primary liver cancer, is one of the strongest environment-to-cancer connections in toxicology. The hallmark is a specific mutation in the TP53 tumor-suppressor gene at a position called R249S. This mutation has been detected in liver tumors from populations with high aflatoxin exposure around the world. In one study of Hispanic patients with HCC in South Texas, the TP53 R249S mutation was found in about 7% of tumors, consistent with aflatoxin playing a role in a subset of liver cancer cases even in the United States.⁶

The risk climbs dramatically when aflatoxin exposure coincides with chronic hepatitis B virus (HBV) infection. Cohort studies in China and Taiwan have provided strong evidence that the interaction between these two risk factors is multiplicative, meaning the combined risk is far greater than what you would expect by simply adding the individual risks together.⁷ This synergy is a major public health concern because the regions with the highest aflatoxin contamination in food, particularly sub-Saharan Africa, Southeast Asia, and China, are also places where chronic HBV infection is widespread.⁸ The overlap means that interventions targeting either exposure, such as HBV vaccination or aflatoxin reduction in food, can have an outsized effect on cancer rates.⁹

Acute Aflatoxin Poisoning

While the cancer risk stems from long-term low-level exposure, high doses of aflatoxin over a short period can cause acute poisoning, sometimes called aflatoxicosis. This is rare in countries with strong food-safety systems but has caused deadly outbreaks in parts of Africa and Asia, typically after communities consume home-stored maize or groundnuts that became heavily contaminated.

A systematic review of acute aflatoxicosis incidents found mortality rates ranging from about 16% to as high as 77%, with children under 15 and adults over 40 hit hardest. Vomiting occurred in the vast majority of cases, along with jaundice and abdominal pain.¹⁰ The liver takes the most severe damage, and in extreme cases the poisoning progresses to fulminant hepatic failure. One documented case involved a 28-year-old man who developed liver failure and multi-organ dysfunction after consuming canned food containing AFB1 at a level of about 20 parts per billion. He survived after 45 days of intensive care.¹¹ That concentration, 20 parts per billion, is at or near the regulatory limit in many countries, which underscores how little room for error exists when someone’s total diet comes from a single contaminated source.

Effects on Children’s Growth

Beyond cancer and acute poisoning, chronic aflatoxin exposure in early life has been linked to stunted growth. This is a particular concern in low-income countries where weaning foods are often made from maize or groundnut, exactly the crops most prone to contamination.

A prospective study in rural Malawi measured aflatoxin biomarkers in children and followed their growth over time. Infants with higher aflatoxin exposure at six months showed reduced head circumference that persisted through at least 30 months of age, implying possible effects on brain development. Exposure measured at 18 months was associated with persistent deficits in length, a marker of chronic undernutrition.¹² The damage does not appear to start only when a child begins eating solid food. A study in The Gambia found that aflatoxin levels in maternal blood during pregnancy predicted lower weight and height gain in infants during the first year of life. The researchers estimated that reducing a mother’s aflatoxin exposure from high to low levels could translate to nearly a kilogram of additional weight and 2 centimeters of extra height in her baby within the first year.¹³

Observational evidence from Guatemala and other settings has reinforced this pattern, prompting ongoing studies into the relationship between aflatoxin, the gut microbiome, and childhood growth failure.¹⁴ The emerging picture is that aflatoxin exposure may be one underappreciated contributor to the global burden of childhood stunting, layered on top of poor nutrition and infectious disease.

Damage Beyond the Liver

The liver is aflatoxin’s primary target, but the toxin’s effects reach further. AFB1 disrupts the lining of the intestine by damaging the tight junctions between gut cells. Animal studies have shown that AFB1 exposure reduces levels of key junction proteins, increases cell death in gut tissue, and thins the intestinal wall, effectively making the gut barrier leakier.¹⁵ A leaky gut can allow bacteria and toxins to enter the bloodstream, potentially triggering inflammation throughout the body.

Research is also building around aflatoxin’s effects on the immune system and the brain. AFB1 appears to alter gut microbiota composition and trigger neuroinflammation through what researchers describe as the gut-brain axis. While the evidence for neurotoxic effects in humans is still early, the immunosuppressive properties of aflatoxins are better established and may help explain why aflatoxin-exposed populations in developing countries face higher rates of infection.¹⁶

From Feed to Milk

Aflatoxin contamination is not limited to plant-based foods. When dairy cows eat feed contaminated with AFB1, the toxin is metabolized in their bodies and a portion appears in their milk as aflatoxin M1 (AFM1). The carryover rate averages about 1 to 2% of the ingested AFB1, though it can climb to around 6% in high-producing cows.¹⁷ AFM1 starts appearing in milk as soon as the first milking after the cow consumes contaminated feed, and levels rise over roughly the first week to 12 days of exposure before plateauing.¹⁸

While AFM1 is less potent than AFB1, it still carries carcinogenic risk, and milk is widely consumed by children. Many countries set separate regulatory limits for AFM1 in milk, typically much stricter than the limits for AFB1 in grain. In practice, controlling AFM1 in dairy depends on keeping the feed supply clean, since there is no practical way to remove the toxin from milk after it has been secreted.

Measuring Who Has Been Exposed

One of the challenges with aflatoxin is that exposure is invisible. You cannot taste it, smell it, or see it. Scientists have developed biomarkers that can detect aflatoxin exposure from blood or urine samples, which has been crucial for epidemiological research and intervention studies.

The two most widely used biomarkers serve different time windows. Urinary aflatoxin M1 reflects short-term exposure over the past day or two, while the aflatoxin-albumin adduct measured in blood captures cumulative exposure over several weeks to months, because the adduct persists for the lifespan of the albumin protein. Studies in Tanzanian children demonstrated this distinction clearly: urinary AFM1 levels fluctuated with recent diet, while blood adduct levels changed more gradually.¹⁹ A third biomarker, urinary AFB1-N7-guanine, measures the actual DNA damage product shed into urine and serves as an indicator of the biologically effective dose that has reached and reacted with DNA.²⁰

Surveys using these biomarkers across six African countries found that exposure is essentially ubiquitous in some populations. Average blood adduct levels ranged enormously, from about 10 pg/mg in Ugandan children to nearly 580 pg/mg in Kenyan adolescents during an outbreak year.²¹ The breadth of the difference hints at how much contamination varies with geography, season, and food storage practices.

Reducing Aflatoxin Before It Reaches Your Plate

Strategies for cutting aflatoxin exposure break down into three stages: what happens in the field, what happens during storage, and what happens during food processing.

In the field, one of the most promising approaches uses non-toxin-producing strains of A. flavus as a biological control agent. When applied to a field, these harmless strains outcompete the toxic ones for space on the crop. Field trials have shown dramatic results: aflatoxin levels in treated maize plots dropped by 98 to 99% compared to untreated plots, and the protective effect carried over into storage, with reductions of 69 to 99%.²² Commercial products based on this principle are already registered and in use in several countries.

Storage is where much of the contamination accumulates, especially in the tropics. A randomized intervention study in Kenya found that improved post-harvest practices, including proper drying and storage, reduced aflatoxin contamination in stored maize by about 53%. The share of households with maize exceeding the regulatory limit of 10 parts per billion dropped from 31% to 9%.²³ Hermetic storage devices, which seal grain away from air and moisture, have shown particular promise. Metal silos and sealed plastic bags kept aflatoxin increases below 5% per month regardless of grain moisture level, while conventional bags allowed contamination to rise sharply.²⁴

At the processing stage, traditional food preparation methods can make a real difference. Nixtamalization, the alkaline cooking process used to make tortillas and other maize products in Latin America, reduced AFB1 concentrations by about 81% on average in controlled laboratory tests using contaminated Guatemalan maize. Extended steeping achieved an 86% reduction.²⁵ The alkaline cooking liquid reaches a pH above 10, which is high enough to chemically modify and detoxify aflatoxin molecules.²⁶ This may partly explain why populations in Central America who eat nixtamalized maize as a dietary staple have historically had lower rates of aflatoxin-related disease than some African populations eating un-processed maize.

Interventions After Exposure

For populations already chronically exposed, scientists have explored ways to reduce the amount of aflatoxin the body absorbs. One well-studied approach uses a type of calcium-rich clay called NovaSil (NS). When added to the diet at small concentrations, the clay binds aflatoxin in the digestive tract and carries it out of the body before it can be absorbed. A trial in Ghanaian adults at high risk of aflatoxicosis found that NS clay effectively reduced biomarkers of aflatoxin exposure without interfering with levels of vitamin A, vitamin E, iron, or zinc.²⁷

Researchers are also investigating microbial and enzymatic methods for breaking down aflatoxins in food and feed after contamination has occurred. Certain bacteria, yeasts, and fungi produce enzymes that can transform AFB1 into less toxic compounds. The main enzymatic pathways involve oxidation, reduction, or hydrolysis of the toxin molecule, each targeting different parts of the aflatoxin structure.²⁸ Physical and chemical decontamination methods, including treatments with acids, gases, and adsorbent materials, have also shown effectiveness in reducing aflatoxin levels in feed and food, though scaling these approaches to real-world food systems remains an ongoing challenge.²⁹

Climate Change Is Expanding the Map

Aflatoxin contamination has traditionally been a problem of tropical and subtropical regions, but warming temperatures are pushing the risk zone northward. Modeling studies in Europe predict that as temperatures rise, the area suitable for A. flavus growth and aflatoxin production in maize will expand considerably. Under current conditions, significant aflatoxin risk in European maize falls mainly below about 45° North latitude. In a scenario with 5°C of warming, maize farming could extend as far north as 60° latitude, and the areas projected to see the largest increases in aflatoxin contamination include Eastern Europe, the Balkans, and the Mediterranean.³⁰

This is not a purely theoretical concern. Rising temperatures, drought, and shifting rainfall patterns are already favoring aflatoxigenic fungi and pre-harvest contamination in European crops.³¹ Countries that have not historically needed to worry about aflatoxins may soon face significant food-safety challenges, requiring new monitoring programs and potentially new crop varieties bred for resistance.

Regulatory Gaps and Global Inequality

Aflatoxin regulations vary enormously around the world. Wealthier nations tend to enforce strict maximum limits, often in the range of 4 to 20 parts per billion for total aflatoxins in food, backed by testing infrastructure and import controls. But a systematic review of global aflatoxin legislation found huge disparities in the limits countries set, driven by different methodologies for assessing exposure risk. The countries most affected by aflatoxin contamination, particularly in peanuts and peanut products, often have few or no enforceable standards at all.³²

This creates a paradox. The populations with the highest exposure and the most to gain from regulation are the least protected by it. Strict import standards in Europe or the United States can actually make the problem worse for producing countries by diverting the cleanest crop toward export markets and leaving the most contaminated product for domestic consumption. Addressing aflatoxin effectively requires solutions that work at the community and household level in the countries where the burden is heaviest, not just at national borders where trade happens.