Mold, Mycotoxins, and Food Safety

Nancy: When you look at ancient religious texts, there are passages that make us all realize that they didn't know what a mycotoxin was, but they knew about moldy food. So it's been around for a while, and every now and then there would be deaths due to these mycotoxins.

Brent: Welcome to The Life Lab by Death Clock. I'm your host, Brent Franson. The mission of Death Clock is to help 100 million people live ten years longer. Today, we speak with Dr. Nancy Keller about mold and mycotoxins. Dr. Keller is a professor of medical microbiology, immunology, and plant pathology at the University of Wisconsin-Madison. She's one of the world's top experts on fungi and mycotoxins, especially how molds like Aspergillus produce toxins in food crops.

Brent: So this conversation is really dedicated to understanding how we should think about mold in the context of our own health. She's an expert in mold related to food and how it shows up in food production and food systems, but we also talk about how we should think about mold in the home. This conversation really changed the way that I think about mold in my own life.

Brent: She's a wonderful guest and about as good as you can get on this topic. I hope you enjoy.

Brent: Dr. Nancy Keller, welcome to the show.

Nancy: Thank you for inviting me.

Brent: Today we're going to talk about mold and mycotoxins. I'm going to be way out of my depth in this conversation, so I'll probably be asking some basic questions, but we'll all learn along the way. Before we do that, can you give us a sense of your background and your day job?

Nancy: Sure. I'm currently a professor at the University of Wisconsin in two departments: medical microbiology and plant pathology. It's really the plant pathology wing of my research that deals with mycotoxins and molds. I got my PhD in plant pathology at Cornell University.

Nancy: I was a Peace Corps volunteer in a little country called Lesotho in southern Africa. I have to talk about that stint of my life, because that's really where I became very much aware of moldy and spoiled food. I was a science teacher at a high school there, and my students received free food from either Europe or the United States, brought in big sacks.

Nancy: It would be maize grain, powdered milk, or powdered eggs. Not infrequently, the food would go bad. I went with the students to inspect it at times, and to be truthful, there were rats all over the place. You could see the mold growing where the rats had chewed into the bags.

Nancy: I got really interested in this. When I entered graduate school, I was predisposed to be interested in food safety. What I found out then is that fungi, or mold as you call it, are the key players that produce all of these mycotoxins—"myco" coming from the Greek word for fungus, and "toxin" meaning, obviously, a harmful chemical.

Nancy: So I became interested in seeing if I could help solve or understand the mycotoxin problem, largely inspired by my experience in the Peace Corps. I deliberately chose positions after graduate school that would allow me to study fungi and the toxins they make.

Brent: Let's start there, because when I think of mold—and I know that's a casual or imprecise term, but it feels colloquially correct—I think about it in two contexts regarding my health: moldy food, and then avoiding mold in my home, though I don't know much beyond that.

Brent: I know if I've got a damp home or a leak, I'm at an increased risk. Let's start with food, since that connects to your origin story in Africa. How should we think about mold and food?

Brent: When I think about moldy food, I assume it's obvious—there's furry growth on it, so I throw it in the trash and don't eat it. It seems fairly easy to avoid. In cases where food is sent to Africa and rats get to it, it's visibly moldy.

Brent: Is that always true? Are there dangerous kinds of mold in food that wouldn't be obvious to us prior to consumption, alongside the obvious ones? How should we think about that?

Nancy: That's a really good question. If you see mold, it has reached a stage where you definitely want to dispose of it. Humans have an innate disgust response to visibly moldy items, and animals avoid them too. However, fungus can grow in food and feed without being visible to the naked eye, while still secreting harmful compounds.

Nancy: That is why regulatory bodies like the FDA test foods to check for these hidden compounds before the fungus is visible.

Brent: What are the health consequences of consuming mold? We know eating something spoiled might cause an upset stomach or short-term food poisoning, but what are the broader health impacts?

Brent: You know what we would think of as food poisoning? I'm sure that that that's happening. But it seems there's a broader array of consequences, health consequences from consuming mold. And so what are those?

Nancy: It isn't the mold itself that causes harm, but the biochemicals produced by the fungus. Consuming a tiny amount accidentally is generally harmless and happens to everyone. Adverse reactions occur when a person or animal consumes a high concentration of these chemicals.

Nancy: We see this more frequently in animals. For example, maize (or corn) is very susceptible to fungi that produce mycotoxins. One toxin, fumonisin, is particularly harmful to horses.

Nancy: If a horse eats enough maize containing fumonisin, it can develop leukoencephalomalacia, a fatal brain disease. Valuable racehorses in Kentucky have died from this in the past, so handlers now carefully limit the amount of maize fed to horses.

Nancy: For humans in the US, strict food testing minimizes risk. For instance, aflatoxin—the most potent naturally occurring carcinogen known—is produced by a fungus that grows on peanuts. Peanuts are regularly tested in the US, making exposure very unlikely here.

Nancy: In other parts of the world, aflatoxin exposure is a serious problem. It causes liver cancer and acts synergistically with Hepatitis B, increasing the risk of liver cancer by roughly 60% in infected individuals. This is a major public health issue in parts of Africa, though not in the US.

Nancy: Besides being a carcinogen, aflatoxin can be acutely toxic at high levels. About ten years ago in Kenya, severe contamination of corn crops led to acute toxicity deaths, though such extreme outbreaks remain rare.

Brent: Fungi are one of the main biological kingdoms. Within fungi, a mushroom and a mold are very different organisms, even though both are fungi.

Brent: So the organisms producing these toxins are molds, which fall under the fungal kingdom.

Nancy: It's always good because a mushroom, you'll never call a mushroom a mold.

Brent: But a mushroom is still a fungus.

Nancy: It is a fungus. Correct.

Brent: Fungi produce mycotoxins, which are non-living chemical compounds. The living fungus secretes the chemical, and the chemical itself causes harm to animals and humans. Is that correct?

Nancy: Correct.

Brent: In the case of acute exposure, people usually think of short-term illness like temporary stomach upset, which isn't long-term fatal.

Brent: However, continuous low-level exposure to compounds like aflatoxin in peanuts can increase the risk of liver cancer over time, shortening lifespan through chronic illness rather than acute poisoning.

Brent: And so if you're consuming that over time, you know, liver cancer will reduce will shorten your life. But it's not going to be a bad 2 hours or 12 hours. It's going to be a long drawn out sickness.

Nancy: You've got. Exactly right. Yes.

Brent: Is that also the case for horses? Do they die quickly, or does it shorten their lifespan over time?

Nancy: It progresses over several months. It's a severe disease. Another mycotoxin affects pigs, but if you remove the toxin from their feed, they can recover.

Brent: For humans, is sustained exposure the primary concern, similar to cigarette smoking where risk accumulates with duration and dose?

Brent: The amount you're consuming and the length of time over which you're consuming it is directly proportional to your risk.

Nancy: It depends on the specific mycotoxin. If it's a direct toxin, cumulative exposure causes harm over time. With aflatoxin, because it is a carcinogen, higher exposure increases the probability of developing mutations and cancer.

Nancy: As with any carcinogen, individual timing varies. A single exposure could theoretically trigger a mutation, but statistically, higher cumulative exposure raises the overall risk.

Brent: Average risk increases with exposure. Has regulatory oversight in developed countries largely solved this issue, leaving it mainly a problem in developing nations, or is there still active risk in the US?

Brent: Or is there still some risk of this in, in the US and in first world countries?

Nancy: It remains a difficult issue to eliminate completely. Food producers and brands take extensive steps to monitor high-risk foods.

Brent: So regulatory frameworks and internal quality control at companies play a critical role in preventing contamination before food reaches consumers.

Nancy: Major food companies test their supplies regularly. Consumers can even call manufacturers to ask about their aflatoxin screening procedures for peanuts and peanut butter.

Nancy: Fungi grow readily on peanuts, so commercial processing requires rigorous testing to ensure safety.

Brent: Do you think there is significant aflatoxin exposure from peanut consumption in the US?

Nancy: No, commercial peanut butter in the US is very safe. When harvesting raw nuts locally, most people naturally discard any nuts with visible fungal growth.

Brent: Because peanuts grow in warmer, more humid southern climates.

Nancy: Yes, peanuts require a warm climate to grow, like in Georgia or Texas.

Brent: Your research focuses on detecting and limiting these toxins in the food supply, particularly in developing regions.

Nancy: Early in my career, I focused on identifying the fungal genes encoding the enzymes that produce these toxins. Mycotoxins are just one category of fungal chemicals.

Nancy: Fungi also produce many beneficial compounds like penicillin, cyclosporine, citric acid, and lovastatin. Mycotoxins happen to be the subset of fungal chemicals that are harmful to vertebrates, humans, livestock, and pets.

Nancy: Fungi don't produce these compounds to target humans. Most mycotoxins act as antimicrobials to eliminate competing microbes, allowing the fungus to secure nutrient sources for itself.

Nancy: It's just unfortunate that those defense mechanisms adversely affect us.

Brent: So the fungus uses mycotoxins as a defense strategy to fend off microbial competitors and claim energy sources, and those defensive chemicals happen to be harmful to humans as well.

Brent: And and we're trying to consume that energy in the same way a microbe is.

Nancy: Exactly. When research first began, people wondered why fungi expended energy producing these compounds, but it clear that it serves as a survival mechanism against competing organisms.

Nancy: Our current research focuses on how these compounds keep other fungi or bacteria away. Similarly, fungi produce penicillin to ward off bacteria, which humans harvested as a life-saving antibiotic.

Nancy: Penicillin targets bacterial infection without harming human cells, whereas mycotoxins happen to harm vertebrate cells too.

Brent: Is the end goal of identifying these genes and enzymes to inhibit toxin production through gene editing?

Nancy: That's one approach. Another avenue is finding naturally occurring enzymes that degrade mycotoxins. Competing organisms often produce enzymes to neutralize fungal toxins.

Nancy: We can utilize those degradation enzymes to treat contaminated food or prevent mycotoxin formation during crop production.

Brent: Is contamination primarily environmental, or does it pass through livestock into animal products?

Nancy: Mycotoxins do not accumulate in meat, so eating animal meat is safe. Contamination begins in the field during crop growth and expands post-harvest during storage.

Nancy: Improper storage is where most mycotoxin accumulation occurs. For instance, grinding moldy peanuts into peanut paste conceals the visual mold while retaining high mycotoxin concentrations. A unique exception is aflatoxin in dairy cattle.

Nancy: When dairy cows ingest dietary aflatoxin, it metabolizes into aflatoxin M1 and enters the milk supply. When an outbreak occurred in the US, regulatory screening identified the issue and contaminated milk was destroyed.

Nancy: However, maternal transfer via breast milk remains a concern in developing regions.

Brent: So the primary driver is improper food storage.

Nancy: Always improper food storage.

Brent: How does your research translate into practical applications if storage quality varies widely across regions?

Brent: And so how does how does your work if it manifest itself out in the real world? What's it doing?

Nancy: We are studying a mycotoxin called patulin that affects apples. When patulin contaminated apple juice in the US, regulatory recalls removed it. We identified a bacterial enzyme that degrades patulin.

Nancy: We are developing enzyme formulations to spray on stored apples post-harvest to degrade the toxin or deploy biocontrol agents.

Nancy: We discovered food-safe yeast strains that prevent mycotoxin-producing fungi from infecting apples.

Brent: How should we evaluate household mold exposure and airborne fungi in residential settings?

Nancy: Excessive home mold degrades building materials and causes unpleasant odors or severe allergic reactions.

Nancy: However, data on whether common household mold produces dangerous indoor toxin levels remains controversial. Commercial testing and remediation claims often lack rigorous verification.

Nancy: Having mold in a house is undesirable, but it does not automatically mean dangerous mycotoxins are present.

Brent: So the debate revolves around health risks from indoor mold and how to accurately diagnose exposure.

Nancy: Decades ago, reports attributed pediatric illnesses to inhaled airborne mycotoxins in moldy homes, but follow-up studies disputed those findings.

Nancy: Commercial home mold tests exist, but their clinical utility and accuracy are frequently debated among academic researchers.

Nancy: I've never done it myself.

Brent: It seems part of the confusion stems from confirmation bias—people feeling unwell and attributing symptoms to visible mold—alongside financial incentives around litigation and remediation.

Brent: Does the scientific skepticism originate from those factors?

Nancy: Yes, those factors contribute significantly to the public confusion.

Brent: Is there consensus among top mycotoxin researchers regarding household mold risks, or is opinion divided within the scientific community?

Nancy: Most active researchers in my discipline share a similar cautious perspective.

Brent: So public concern is often driven by media, lawsuits, and industry claims, while academic researchers remain skeptical of broad health claims.

Nancy: Yes. Media reports feature people attributing severe illnesses to household mold. Attributing non-specific symptoms directly to fungal volatiles is clinically difficult to verify.

Nancy: I mean.

Brent: So there isn't a strong scientific consensus that ordinary household mold poses a major toxic health hazard?

Nancy: It depends on scale. Minor basement mold is common, whereas extreme flooding events—like Hurricane Katrina—create major uninhabitable structural hazards.

Nancy: Everyone agrees extreme mold growth is unsafe. The disagreement lies in assessing risk from minor, low-level mold. Distressed individuals often reach out to researchers seeking clear answers that medicine cannot easily confirm.

Nancy: Determining whether systemic symptoms stem from fungal exposure or another cause is extremely challenging for physicians.

Brent: Evaluating research credibility is essential when evaluating health claims, especially in an era of online misinformation. Your high h-index (112) reflects extensive academic citation and research reproducibility.

Brent: Information retrieval systems like Google's PageRank originated from citation analysis concepts like the h-index to evaluate domain authority.

Brent: An h-index above 100 places a researcher in the top 1% of cited scholars, serving as a helpful filter for identifying credible experts.

Brent: Checking researcher credentials helps distinguish validated scientific research from unfounded claims online.

Brent: How do you view metric indicators like the h-index when evaluating online health claims?

Brent: I agree that academic authority matters when evaluating claims.

Nancy: A high h-index indicates that research results have been replicated independently by peers. Much online content about toxic home syndrome comes from sources without verified scientific track records.

Nancy: If you want to say that.

Brent: It's difficult to artificially inflate an h-index over a career because it requires broad consensus and repeated citation by other established scientists.

Brent: Among highly cited researchers in your field, what percentage consider ordinary household mold a primary threat to human health?

Nancy: My work focuses on specific biochemical pathways with clear binary outcomes, such as deleting a gene to test whether aflatoxin is produced.

Nancy: Assessing household mold enters a complex area combining environmental biology, personal anxieties, and social factors surrounding home safety.

Nancy: There are also financial incentives for remediation companies selling treatment services, which complicates objective risk communication.

Brent: Returning to food safety, how has your research influenced your personal food habits and choices?

Nancy: It hasn't changed my habits drastically. I inspect food carefully. If hard cheese develops surface mold, cutting an inch away from the mold is sufficient because mycotoxins do not penetrate deeply into hard cheese.

Nancy: Proper food storage and refrigeration prevent most issues naturally.

Brent: So if there's mold on one end of a hard cheese block, you can trim that section and safely eat the rest rather than throwing out the entire block.

Nancy: It's your eyes.

Brent: You remain confident in the commercial supply chain and food safety standards in the US.

Nancy: Yes, I am confident in the US food safety system.

Brent: Which international regions face greater mycotoxin risks?

Nancy: China has faced significant aflatoxin challenges, though they actively manage crop safety now.

Nancy: Regions with limited regulatory infrastructure and warm, humid climates face higher food contamination risks.

Brent: More developed African economies experience lower risk due to better infrastructure.

Nancy: Yeah. No, I wouldn't be. Yeah.

Brent: Is mycotoxin exposure an issue created and resolved by agricultural modernization? Historical subsistence farming consumed crops quickly, whereas large-scale industrial storage creates conditions for fungal growth.

Brent: And it's that storage that causes these issues. And then it's also modernity that kind of solves this with better storage. Is it kind of a middle of the development curve problem?

Nancy: Ancient religious texts, including Leviticus, reference moldy food and outline warnings to discard it. While early societies did not understand the underlying biochemistry, they recognized the hazard.

Nancy: Modern storage methods control moisture and temperature to mitigate mycotoxin formation effectively.

Brent: Is it reasonable for someone living in the US in 2026 to not view household or dietary mold as a daily health concern?

Nancy: Yeah, that's I feel that way. Yeah.

Brent: It isn't a major day-to-day concern under normal living conditions.

Nancy: Correct? It's not.

Brent: Are there potential benefits to low-level exposure, given how the human microbiome interacts with environmental microbes?

Nancy: While unstudied, scientists occasionally ponder if trace exposures stimulate immune function. Many mycotoxins exert antibacterial effects at low concentrations.

Nancy: Maybe a little bit is good. No, I'm not studying that right now. But it's an intriguing thought.

Brent: Looking forward 50 years, what major breakthroughs do you hope to see regarding mycotoxin mitigation?

Nancy: Developing dietary probiotics capable of degrading aflatoxins in the digestive tract before absorption would protect vulnerable populations in high-exposure regions.

Brent: Similar to consuming a probiotic drink to neutralize ingested toxins.

Nancy: Yeah. That's an idea I have. Yeah.

Brent: Dr. Keller, thank you so much for your research and for sharing your expertise with us today.

Nancy: It was my pleasure.

Brent: The Life Lab by Death Clock is recorded in Boulder, Colorado and San Francisco, California. Produced by Patrick Godinho, music by Patrick Lee, and hosted by Brent Franson, founder and CEO of Death Clock.