Educational information only, not medical advice. Please talk with your physician before changing your diet, taking supplements or ordering lab tests, especially if you are pregnant, take blood thinners or have a medical condition.
Why food first? Oily fish supplies EPA and DHA along with protein and other nutrients. The populations with the highest average index are fish-eating populations [3].
Choosing low-mercury fish. Mercury released into the air, largely from human activity such as coal-burning power plants, settles into water through rain, snow and dust. In the water, microbes convert it to methylmercury, which builds up as it moves up the food chain. Large predator fish end up with the most [5][6]. Small fish that are low on the food chain tend to be lower in mercury.
The FDA and EPA sort fish into three groups [7]:
| Fish | FDA/EPA advice |
|---|---|
| Anchovies | Best Choice |
| Atlantic mackerel | Best Choice |
| Sardines | Best Choice |
| Spanish mackerel | Good Choice |
| King mackerel | Choice to Avoid |
Read the label. "Mackerel" can mean very different fish. Atlantic mackerel is a Best Choice, while king mackerel is on the avoid list [7].
How much fish moves the index? We couldn't find a study that measures how many cans of fish per week raise the index by a given amount, so the honest answer is that the evidence is limited. For supplements there is better data, which is covered in the next section [8].
About fish oil. Fish oil is made by cooking fish or fish by-products, pressing out and separating the oil, and then refining it. Refining typically involves degumming, neutralization, bleaching and deodorization, and some oils are further concentrated [9][10]. Supplements can be a reasonable option for people who don't eat fish. Discuss them with your doctor first.
Why smaller, younger fish are lower. Methylmercury biomagnifies, meaning it becomes more concentrated at each step up the food chain [19]. Small fish that feed low on the food chain and live short lives, such as sardines, anchovies and Atlantic mackerel, have less time and fewer prey layers in which to build it up.
The advice also notes that when children eat the higher amounts recommended by the Dietary Guidelines, they should be fed from the lower-mercury subset of Best Choices. That subset includes anchovies, Atlantic mackerel and sardines [7]. Mercury measurements by species are available in FDA's monitoring data table [20].
Questions? Email omega3@rome.md
References
- Schuchardt JP, et al. Red blood cell fatty acid patterns from 7 countries: Focus on the Omega-3 index. Prostaglandins Leukot Essent Fatty Acids. 2022;179:102418. https://pubmed.ncbi.nlm.nih.gov/35366625/
- USGS. Mercury Contamination of Aquatic Environments. https://www.usgs.gov/water-science-school/science/mercury-contamination-aquatic-environments
- US EPA. Mercury Emissions: The Global Context. https://www.epa.gov/international-cooperation/mercury-emissions-global-context
- FDA/EPA. Advice about Eating Fish. https://www.fda.gov/food/consumers/advice-about-eating-fish
- Walker RE, et al. Predicting the effects of supplemental EPA and DHA on the omega-3 index. Am J Clin Nutr. 2019;110(4):1034-1040. https://pubmed.ncbi.nlm.nih.gov/31396625/
- From Fish Waste to Value: An Overview of the Sustainable Recovery of Omega-3 for Food Supplements. Molecules (PMC). https://pmc.ncbi.nlm.nih.gov/articles/PMC7918619/
- Highly Valuable Fish Oil: Formation Process, Enrichment, Subsequent Utilization, and Storage of EPA Ethyl Esters (PMC). https://pmc.ncbi.nlm.nih.gov/articles/PMC9865908/
- USGS. Mercury 101. https://www.usgs.gov/labs/mercury-research-laboratory/mercury-101
- FDA. 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