Fish and Mercury

by on April 1, 2013

April 2013: Fish and Mercury

NOTE from John: As contest season approaches, I know you all will be eating a lot of fish, so I thought it would be helpful to talk about mercury. This is near and dear to me because in 2005 (right before I got sick), I actually had toxic levels of mercury confirmed by bloodwork via Dr. Serrano. I was eating a pound or two of orange roughy a day. I remember being told that Tilapia, was a bottom feeder, and that orange roughy would make me look better and bigger (no I am not making that up). That was the last time I ever ate orange roughy. Now we are not trying to scare you into never eating certain fish such as orange roughy, but just be careful with your selections if fish is going to be a big part of your nutrition plan, that is the message!

Many athletes include fish in their diets and for good reason; like all animal proteins, the protein in fish is loaded with essential amino acids and of the highest biological value. Fish are also an excellent source of particular micronutrients and cold water fish are also loaded with omega 3 fatty acids. There is a downside, however; years of industry/industrial pollution have drastically increased the levels of mercury in the environment, which eventually finds its way into our lakes, rivers and oceans. Small marine life readily absorbs this mercury from the environment. As small fish are eaten by bigger fish, mercury is passed up the food chain, becoming increasingly concentrated. While fish is a great, nutritious source of protein, knowledge about mercury contamination, what types of fish to eat, and how often we should eat them is key to getting the most out of this high-powered protein source.

Fish Nutrition

While fish nutritional composition varies with the species, fish is overall a high quality and nutritious protein source. Pound for pound, fish tend to have higher amounts of high quality, high biological value protein relative to other animal proteins. They are also an excellent source of vitamin D, selenium and iodine (for salt water fish). Certain types of fatty fish are the most potent dietary sources for the omega 3 fatty acids EPA and DHA. Unlike the plant based omega 3 fats which are inefficiently converted to the more bioactive EPA and DHA, fish are a direct and concentrated source of these two essential fatty acids.

The Mercury Issue

Make no mistake about it, while things have recently taken a turn for the positive to the advent of new/cleaner technologies and tighter regulations, most commercially available fish will contain at least some mercury. Across the US, mercury pollution has contaminated 18 million acres of lakes and wetlands (comprising an estimated 43% of the total) and over 1.4 million miles of rivers. The problem is getting worse: From 2006 to 2008, the number of lake acres under mercury advisory increased by 18%, and the number of river miles increased by 52%. Importantly, not all waterways have even been tested, so the problem is potentially much worse. In 2008, every state in the US issued fish consumption advisories, warning citizens to limit how often they eat fish caught in local bodies of water due to high levels of mercury contamination (1).

This begs the question, where does the mercury come from, and how does it get into the environment? In the US, the major sources of mercury contamination are coal-burning power plants, boilers, steel production, incinerators, and cement plants. Coal burning power plants are actually the largest source of contamination, spewing around 33 tons of mercury pollution into the atmosphere annually. Because coal is naturally contaminated with mercury, when burned for electricity it releases mercury into the atmosphere. Gold mining is one of the most significant sources of mercury contamination outside of the US. Miners throughout Asia, Africa, and South America use mercury to extract gold from ore. Mercury is added during the extraction process, which binds to gold. The ore is then burned, releasing mercury into the air and leaving the gold behind. Although effective, this low-tech practice releases a ton of mercury vapor into the atmosphere.

After it is released into the air, mercury accumulates in the atmosphere and returns to the earth when it rains. Eventually all the mercury released into the environment finds its way into oceans and other bodies of water, contaminating fish and seafood.

The first thing that happens to mercury when it contaminates bodies of water is that it is absorbed by naturally occurring bacteria and converted to methyl mercury. The problem with methyl mercury (also referred to as ‘organic’ mercury or ‘MeHg’) is that it is better-absorbed by organisms than free mercury, readily contaminating small fish. Bigger fish tend to eat lots of smaller fish, so the tiny levels of mercury in small fish accumulates in the larger fish that feed on them. Instead of being diluted as it is passed on to higher animals, mercury tends to become more concentrated. The result is an ever- increasing accumulation of mercury as you move up the food chain. For this reason, large, predatory fish that eat lots of contaminated smaller fish can accumulate mercury levels in upwards of 10,000x greater than would naturally occur in the environment (2). Residing at the top of the food chain, humans are at risk for ingesting dangerous levels of mercury when we consume contaminated fish. Mercury in the amounts that it is found in contaminated fish is odorless, colorless, and accumulates in the meat of the fish, so it is difficult to detect or avoid. Contamination can’t simply be trimmed away by avoiding skin or other parts

Health effects of Mercury

Eating fish that are highly contaminated with mercury can have negative health consequences, especially for particular populations. It is especially dangerous for pregnant women and small children. Mercury tends to concentrate in the umbilical cord blood that supplies the fetus, so women who are pregnant or may become pregnant need to take particular care to avoid contamination as much as possible.

During the first several years of like, a young child’s brain is still developing and rapidly absorbing nutrients. Even in low doses that would not affect a normal adult, mercury can impair the development of young children, causing learning disabilities, delaying walking and talking, and shortening attention span. Worse, prenatal exposure to high levels of mercury can cause deafness and blindness, and even mental retardation in the most severe cases. In adults, mercury poisoning can affect fertility and blood pressure regulation as well as cause memory loss, tremors, and vision loss. It is also becoming increasingly evident that mercury exposure may promote heart disease (3, 4).

When mercury accumulates to sufficient levels, it acts as a neurotoxin. While it has been known for some time to cause a number of detrimental effects in living organisms, the exact mechanisms of mercury induced toxicity are still being worked out. What is known, based on cellular and rodent studies, is that the mechanism of mercury neurotoxicity is complex and multifaceted. Through a combination of disrupted intracellular calcium signaling, impaired glutamate homeostasis, and increased oxidative stress, neuronal signaling is impaired in a dose-dependent manner (5). What this means is that mercury, when it accumulates to sufficient levels, short-circuits neuron cells that are particularly dependent on calcium and glutamate signaling. While it has not been formally studied, it is also plausible that mercury in higher levels can affect insulin sensitivity and muscle performance, possibly even in a CNS-independent manner. Disruption of calcium signaling has catastrophic effects in muscle cells which are dependent on intracellular calcium flux to regulate the excitation-contraction coupling of actin and myosin filaments to generate muscle contraction. Moreover, increased oxidative stress on a systemic level could contribute to increased inflammation and decreased insulin sensitivity.

Given the potential issues with mercury contamination, the natural reaction of many people is to avoid fish altogether. While this would certainly reduce mercury intake, it would also eliminate a great protein source that is totally healthy and nutritious. The key is in knowing what types of fish to eat, and what types of fish to avoid. Frequency of consumption is also an important variable; because almost all fish will have at least trace amounts of mercury, consuming even medium mercury fish with a high frequency can lead to mercury accumulation over time.

Tuna is a perfect example here. While it is not classified as a ‘high mercury’ fish, tuna is by far the most common source of mercury exposure in the US (6). Many of you reading this article can relate: I don’t know of a bodybuilder that hasn’t implemented canned tuna at some point in their diet plan. Personally, I used it so heavily in the past that currently I can’t even stand to go down the tuna isle in the grocery store. I remember those days distinctly… as a busy undergrad working my way through school, I carried a can opener with me all the time. No time for a meal? No problem; tuna straight out of the can, maybe 3-4 times a day, depending on the phase of my diet or what I had going on that particular day. In retrospect, that was probably a mistake, as my mercury levels at that time were probably a bit out of the healthy range. At the time, I wasn’t knowledgeable about the real issues with mercury in canned tuna, and others can no-doubt relate. For this reason, being informed about the mercury content of fish is key to avoiding any potential health issues. To that end, I’ve listed some common types of commercially available fish and the recommended frequency for their consumption (2).

Which types of fish to eat, and how frequently – this is if you eat fish consistently year round, and is pretty aggressive. If you tend to eat fish only during a certain time of year, you can get away with more. See my comment below about how long it takes the body to clear mercury.

Low mercury fish: (enjoy whenever)

Haddock (Atlantic)
Mackerel (N. Atlantic, Chub)
Perch (Ocean)
Wild caught Salmon (Canned)
Wild caught Salmon (Fresh)
Wild caught Tilapia
Trout (Freshwater)

Medium-mercury fish: (consume less than 6 servings a month)

Bass (Striped, Black)
Cod (Alaskan)
Croaker (White Pacific)
Halibut (Atlantic)
Halibut (Pacific)
Mahi Mahi
Perch (Freshwater)
Tuna (Canned
chunk light)
Tuna (Skipjack)

“Wild Planet” tuna is John’s favorite as the tuna are caught when they are smaller.

High Mercury fish: (eat 3 servings or less a month)

Mackerel (Spanish, Gulf)
Sea Bass (Chilean)
Tuna (Canned Albacore)
Tuna (Yellowfin)
Very high mercury: (avoid altogether or consume very infrequently)
Mackerel (King)
Orange Roughy
Tuna (Bigeye, Ahi)


Fish protein is an exceptionally high quality, muscle building protein, and fattier fish are the some of the most concentrated dietary sources of EPA/DHA and vitamin D out there. Although mercury toxicity is a concern, choosing the right types of fish with an appropriate frequency will allow you to take full advantage of this excellent protein source while avoiding any negative effects.

If you have been consuming high mercury fish (or lots of medium mercury tuna for an extended period of time), the good news is that all is not lost. The human body is readily capable of clearing mercury over time, and mercury levels will start to decrease within six months of reducing mercury intake.

To keep levels from getting out of control to begin with, however, be sure to choose fish from the low mercury list for everyday or more frequent consumption, and limit the consumption of medium and high mercury fish according to the guidelines above.

Reference List

  1. National Resources Defense Council.
  2. Natural Resources Defense Council. Consumer Guide to Mercury in Fish.
  3. Roman HA, Walsh TL, Coull BA, Dewailly E, Guallar E, Hattis D, et al. Evaluation of the cardiovascular effects of methylmercury exposures: current evidence supports development of a dose-response function for regulatory benefits analysis. Environ Health Perspect 2011;119:607-14.
  4. Karagas MR, Choi AL, Oken E, Horvat M, Schoeny R, Kamai E, et al. Evidence on the human health effects of low-level methylmercury exposure. Environ Health Perspect 2012;120:799-806.
  5. Farina M, Rocha JB, Aschner M. Mechanisms of methylmercury-induced neurotoxicity: evidence from experimental studies. Life Sci 2011;89:555-63.
  6. Oken E, Choi AL, Karagas MR, Marien K, Rheinberger CM, Schoeny R, et al. Which fish should I eat? Perspectives influencing fish consumption choices. Environ Health Perspect 2012;120:790-8.

Bill Willis, PhD