Toxic metals vs. toxicology: Why dose matters

It’s hard to open social media or check the news these days without seeing a scary health story or alarming meme about heavy metals in foods. The media landscape is awash in seemingly nonstop heavy metal fear-mongering.

Arsenic in Girl Scout cookiesOpens in a new tab. Lead in protein powdersOpens in a new tab. Lead and cadmium in chocolateOpens in a new tab.

These headlines tap into a primal fear: What if the food we're eating is poisoning us?

But here's what's missing from that panic: the difference between detecting a substance and that substance being dangerous. Between "this food contains lead" and "this food will harm you" lies an entire science that most headlines miss.

A toxicant is any substance that can cause harm to living organisms at the right (or wrong) dose.

Toxicology is the science of understanding how those substances actually affect living organisms. It's about dose (that’s what this article is about!), timing, route of exposure, and how our bodies process what we consume (we dig into that in “Bioavailability explained: What our bodies actually absorbOpens in a new tab”.) It's the difference between "this exists" and "this will hurt you."

Heavy metals exist naturally in trace amounts everywhere — in soil, water, and air. That means they're in our food. Not from contamination, but from the earth itself. Humans have been exposed to trace amounts of these substances for thousands of yearsOpens in a new tab.

What toxicology tells us is that the mere presence of heavy metals doesn't make food unsafe. What matters is how much we consume and whether that amount actually causes harm.

Understanding this distinction is what separates rational food choices from fear-based ones.

The 5 Principles of Toxicology (and What They Mean For Your Food)

To understand the actual risks that heavy metals do — and don’t — pose in our diets, let’s examine the five key principles of toxicology: 

• The dose makes the poison. 
• The route of exposure matters.
• Bioavailability is key.
• Our bodies have natural detox systems.
• Every body is different.

Why dose makes the poison

Perhaps no adage is more important to toxicology than “the dose makes the poison.” 

It traces back to the 16th century physician ParacelsusOpens in a new tab, whose actual words were a bit more long-winded: “In all things there is a poison, and there is nothing without a poison. It depends only upon the dose whether a poison is poison or not.”

It helps to consider some specific examples: Formaldehyde is a chemical that, in large enough doses, can cause cancerOpens in a new tab. But a medium-large pear weighing 200 g contains nearly 8 to 12 mgOpens in a new tab of formaldehyde, and no dietitian would tell you to swear off pears for life. Even if you did, many other fruits and vegetablesOpens in a new tab contain formaldehyde too, but the amounts are so small that they don’t qualify as a “poison” to our bodies.

On the flip side, water is so essential to our health that humans can survive only about three days without it. But too much of anything can be harmful, including H2O. Drinking six liters in three hours has killed at least one personOpens in a new tab from water intoxication. 

The Ames Test illustrates this well. Dr. Bruce Ames developed a testOpens in a new tab so sensitive it could detect mutagenic activity — mutations that could potentially lead to cancer — in everyday substances like coffee. It was a major scientific advancement. But when the media and public saw those results, many people interpreted them as "coffee causes cancer."

They lacked the context to understand that the world is full of substances that can cause mutations in cells at high doses, but in the tiny amounts we typically encounter them, they are unlikely to cause harm.

Ames spent much of his later career emphasizing this exact point: Detecting something potentially dangerous does not mean detecting actual danger.

The bottom line: Whether a substance is dangerous does not depend only on what it is. It also depends on how much you consume.

Why route of exposure matters

There are multiple ways substances can enter our bodies: ingesting them through our mouths, inhaling them through our noses, absorbing them through skin contact, or injecting them directly into our bodies. 

And the way our bodies interact with these substances can depend on how they enter our body. 

We can consume some substances in food that might be dangerous if we encountered them another way. But if we eat them, our digestive system, with its strong acids and highly efficient ability to pull nutrients from our food, can limit absorption of them (depending on the exact substance and the does) without them causing too much harm. 

For example, coffee. It’s a great way to get a morning dose of caffeine, but hooking yourself up to an IV of coffee could be lethalOpens in a new tab. Route of exposure matters.

And let’s recall our pear example. Pears contain formaldehyde, a known carcinogenOpens in a new tab. But the amount of it in pears is far too small to be a concern. Formaldehyd’s health effects depend on both how we encounter it and how much — formaldehyde is most concerning when inhaled in large amounts, not eaten in trace amounts.

Similarly, some shampoos contain formaldehydeOpens in a new tab that can cause irritation for some people with sensitive skin, but it certainly does not increase their risk of cancer.  

Why bioavailability is key

Just because you consume something doesn’t mean your body absorbs it. That depends on a substance’s bioavailability, the proportion of a substance that actually enters our body’s blood circulation. 

Route of exposure can affect bioavailability too. Some medications, such as insulin for people with diabetes or pitocin to induce labor during pregnancy, are given through an injection or IV, for instance, because the body cannot access enough of the drug’s active ingredients if the person takes it orally. 

For any substance to affect our health — positively or negatively — the body usually has to absorb it or inhale it directly into your lungsOpens in a new tab. Sometimes even seemingly small differences in a chemical’s make-up can affect its half-life (aka, the time it takes for half of it to be eliminated from the body). 

Two different kinds of mercury compounds serve as good examples of this: 

• Ethylmercury, which is used to prevent bacterial and fungal contamination in vaccines, has a half-life range of 3-7 daysOpens in a new tab.
• Methylmercury, the kind found in fish, especially larger predator fish like tuna and sharksOpens in a new tab, has a half-life range from about 50Opens in a new tab-80Opens in a new tab days.

That means ethylmercury clears from our bodies much faster than methylmercury — though their differing toxicity reflects their distinct chemical forms too, not just how fast each clears. 

Bioavailability also depends on how substances we consume interact with other chemicals. One reason people need vitamin D is because the vitamin helps the body absorb enough calciumOpens in a new tab. Without enough vitamin D, you only absorb 10%-15% of calcium, but with enough vitamin D on board, you’ll absorb 30%-40%.

The flipside is also true: Some substances can prevent absorption. We noted above that mercury from fish has a pretty long half-life, potentially sticking around in our bodies for months. But you can reduce how much mercury your body absorbs by getting enough seleniumOpens in a new tab, as it binds to mercuryOpens in a new tab and creates a compound that your body filters out as waste without absorbing it. 

Fish can be a key part of a healthy diet, offering essential nutrients like omega-3s that reduce inflammationOpens in a new tab and support heart and brain healthOpens in a new tab. Avoiding fish entirely because of mercury concerns means losing those benefits, and that’s a poor trade-off for your health. 

The smart approach: Eat fish with a high selenium contentOpens in a new tab, such as yellowfin tuna, sardines, shrimp, and salmon. Selenium may help offset some of the mercuryOpens in a new tab these fish contain, so you can get the omega-3s while lowering your risk. 

Understanding the actual risks versus the proven benefits gives you the knowledge to make decisions accordingly. 

How your body naturally detoxes heavy metals 

Finally, our bodies are not defenseless. Our digestive systems are remarkably sophisticated machines, designed to pull nutrients from our diet while filtering out toxins and waste. 

The liver and kidneys partially eliminate what we don’t need — including a sizable portion of the trace amounts of heavy metals we naturally consume every day. Read more about how our bodies detox heavy metalsOpens in a new tab.

Why individual sensitivity matters

Everyone’s body handles heavy metals differentlyOpens in a new tab. For example, the half-life of methylmercury can vary by weeksOpens in a new tab or more between individuals. 

And some people are also more vulnerable to risks from heavy metals. Age, genetics, sex, nutrition, health status, and exposure history can all change how our bodies react. Children’s digestive systems absorb more leadOpens in a new tab, and their bodies aren’t developed enough to block lead from reaching the brain or to detox efficiently.

How to Make Food Decisions Without Fear

If we allow fear of heavy metals and other toxic substances in foods to drive our dietary decisions, then we run the risk of missing out on key nutrients that support long-term metabolic health. 

The alternative to fear-based decision-making is looking at the bigger picture, weighing the actual risks against the proven benefits, and making decisions that maximize our health. 

In doing so, we also have to accept the trade-offs. We can’t eat “perfectly,” but we can eat healthfully by making practical nutrition decisions that minimize — not eliminate — trace exposures and ensure we get the essential nutrients we need to live and perform optimally.

Approaching our health decisions with pragmatism instead of perfectionism can help reduce anxiety and the cognitive drain of obsessing about every potential threat (and losing the plot along the way). Constant food fear has real costs: It harms our mental healthOpens in a new tab, limits dietary variety, and can create the very nutritional deficienciesOpens in a new tab we’re trying to avoid. 

5 questions to ask before worrying about food headlines or claims

When you come across a claim about a food containing a certain toxic compound, stop and consider several questions:  

• How much of a dose am I actually consuming? Our article “How to read a toxicology reportOpens in a new tab” teaches you how to calculate this for yourself. 
• Has that dose been shown to cause harm by public health authorities who have evaluated the consensus of research in this area? Check the FDA (for food and drinks), EPA (for environmental contaminants), or WHO (for international standards) to find out what regulatory limits exist and whether your exposure exceeds them.
• Have scientists shown that consuming this substance is actually harmful compared to inhaling or injecting it? 
• How much of this substance will your body actually absorb? How long is its half-life? Are there other compounds you’re eating that increase or decrease how much of it your body absorbs? 
• Is this a substance your body’s natural detox functions are already well-equipped to filter out? 

You may not be able to answer all of these questions, but simply asking them can help slow down the panic that might overwhelm you when you first read an alarming claim about heavy metals or other toxic substances in foods. 

The Bottom Line: Balancing Caution and Context When Making Food Choices

We don’t want to downplay the fact that some foods can contain harmful chemicals that we need scientists and public health authorities to work together to identify and regulate. But current evidence indicates that the levels of heavy metals typically found in foods are low and unlikely to hurt most people. 

It’s still important to read lists of ingredients and pay attention to nutrition labels. But it’s also important to focus on the “big rocks” that will help move the needle on your metabolic health: eating adequate protein, choosing whole foods over ultra-processed ones, staying properly hydrated, getting quality sleep, managing stress. These fundamentals matter far more than obsessing over parts-per-billion measurements of trace metals in otherwise nutritious foods.

Understanding the impact of dose, route of exposure, bioavailabilityOpens in a new tab, and your body’s natural detoxificationOpens in a new tab of the foods you consume can help you make better sense of claims you see about heavy metals in our food. 

To become even more proficient at understanding just how much — or how little — of a substance you’re consuming, read “How to read a toxicology reportOpens in a new tab”. You’ll learn how to put “parts per billion” toxicology results into real world application.

This was the second article in our six-part "Toxicology in Context: Heavy Metals" series. Want to read the other articles? Check them out below: 

• A brief history of heavy metalsOpens in a new tab
• Understanding heavy metal regulations like Prop 65Opens in a new tab
• How to read a toxicology report: What does parts per billion mean?Opens in a new tab
• Does your body absorb heavy metals? Bioavailability explainedOpens in a new tab
• Mechanisms of detoxification: How the body clears heavy metalsOpens in a new tab

FAQ 

Q: Does "dose makes the poison" apply to heavy metals in food?

A: Yes. Heavy metals can be harmful in large amounts but are typically harmless at the trace levels found in food. Your body has natural systems that help process and eliminate many low-level exposures, although this varies depending on the specific metal and the amount consumed.

Q: Are heavy metals in vegetables dangerous?

A: Trace heavy metals in vegetables are generally not dangerous. Vegetables naturally absorb minerals from soil, including tiny amounts of metals, but the nutritional benefits far outweigh these minimal exposures. The caveat is rare instances in which veggies are grown in soil contaminated with industrial levels of heavy metals.

Q: Why do healthy foods contain heavy metals?

A: Heavy metals exist naturally in soil and water. Plants absorb nutrients and water from these sources, picking up trace amounts of metals in the process. People have had safe exposure to these trace amounts throughout all of human history.

Q: Should I avoid foods that have any amount of heavy metals?

A: No. Avoiding all foods with trace heavy metals would eliminate virtually all fruits, vegetables, grains, and seafood—the foundation of healthy eating.

Originally published May 2026; updated July 2026