Bioaccumulation is a process that explains how fat-soluble pollutants are taken up into fatty tissues of plants and animals exposed to contaminated environments or food. These chemicals are not very soluble in water but are highly soluble in fats and oils, as well as soils and river sediments that contain a lot of organic matter. Levels of these chemicals increase the higher you go in the food web, with animals at the top being the most contaminated. This process occurs in both aquatic and terrestrial food webs.

These are the steps of bioaccumulation:

• Step 1: Plants absorb contaminants from the water they take up, concentrating them into their fatty tissues at higher levels than the water. 
• Step 2: Herbivore animals eat plants containing contaminants and concentrate those into their fatty tissues at higher levels than the plants since they preferentially metabolize tissues like fats but not the contaminants present in those tissues.
• Step 3: Carnivorous animals eat herbivores, as well as smaller carnivorous animals, concentrating contaminants from those sources into their fatty tissues at even higher levels than the creatures they eat in the same way as Step 2.

We humans are also part of the food web, whether we like it or not. If your diet includes a lot of meat, poultry, and fish, as well as eggs and high fat dairy, you, like lions, bears, and sharks, are at the top of the food web when it comes to being exposed to higher levels of bioaccumulated contaminants. In a review of studies on dioxin-like compounds in the U.S. diet,¹ researchers estimated that 42% of the average American’s exposure comes from consumption of meat, 17% from dairy, 8% from fish, 7% from poultry, and 3% from eggs. They estimated that people who consumed less meat, poultry, and fish would have lower intakes of these chemicals than those who ate more. The study did not estimate intake levels for a fully plant-based diet; however, since eating less meat, poultry, and fish would reduce exposure, eating none should result in even lower intake of dioxin-like compounds.

An example of higher contamination as you go up the food web is mercury levels in tuna. Below are results from an FDA survey of mercury contamination in fish.² As tuna species get larger and further up the food chain, levels of mercury increase. Therefore, it’s best to choose light tuna and minimize consumption of the more contaminated ahi to reduce your exposure to neurotoxic mercury. (Mercury in micrograms/3 ounces; *Note – these species are sold as ahi.)

• Light tuna (canned)                11
• Albacore (canned)                  30
• Yellowfin* (fresh/frozen)       30
• Bigeye* (fresh/frozen)            59

Why be concerned about exposure to these chemicals? They are linked to health issues such as cancer, hormone disruption, reproductive impairment, and neurological toxicity.^5-7 And, unlike more water-soluble contaminants like plasticizers that are excreted from your body within a few days, fat-soluble contaminants can take months to decades, even centuries for you to metabolize or excrete, depending on their structure and other factors.^5,8,9 Therefore, avoiding them is the best approach. 

If you do eat fatty animal products, here are a couple of tips to reduce your exposure to bioaccumulated contaminants:

• Remove fat from meat, poultry, and fish before cooking.
• Try to get them from cleaner locations, e.g. not raised downwind of industrial, agricultural, or urban regions.

Ultimately, if use or production of these chemicals decrease, their levels in our food and our bodies will also decrease. Until then, try out more plant-based dishes. In an upcoming post, I’ll discuss resources and cookbooks to help you do that.

References

1. Institute of Medicine (US) Committee on the Implications of Dioxin in the Food Supply (2003). Dioxins and dioxin-like compounds in the food supply: Strategies to decrease exposure. 
2. Mercury levels in commercial fish and shellfish (1990-2012). https://www.fda.gov/food/environmental-contaminants-food/mercury-levels-commercial-fish-and-shellfish-1990-2012
3. Kalantzi, O. I. et al. (2001). The global distribution of PCBs and organochlorine pesticides in butter. Environ. Sci. Technol. 35:1013-1018.
4. Weiss, J. et al. (2005). A Worldwide Survey of Polychlorinated Dibenzo-p-dioxins, Dibenzofurans, and Related Contaminants in Butter. Ambio 34:589-597.
5. ATSDR Case Studies in Environmental Medicine. Polychlorinated Biphenyls (PCBs) Toxicity (2014). U.S. Department of Health and Human Services, Agency for Toxic Substances and Disease Registry.
6. EPA (2023). Learn about Dioxin. https://www.epa.gov/dioxin/learn-about-dioxin#:~:text=Dioxins%20are%20called%20persistent%20organic,and%20can%20interfere%20with%20hormones. 
7. EPA (2023). Health effects of exposures to mercury. https://www.epa.gov/mercury/health-effects-exposures-mercury
8. Idowu, I. G., et al. (2023). Polychlorinated biphenyl (PCB) half-lives in humans: A systematic review. Chemosphere, 345.
9. Milbrath, M. O’G. et al. (2009). Apparent half-lives of dioxins, furans, and polychlorinated biphenyls as a function of age, body fat, smoking status, and breast-feeding. Environ. Health Perspect. 117:417-425.