The Purported Benefits of Eating Fish

In the landmark Global Burden of Disease study, researchers compiled the top 20 causes of death and disability. Number one on the list was high blood pressure, two and three was smoke, and the fourth leading cause of loss of life and health was not eating enough fruit. Lack of exercise was number 10, followed by too much sodium, not enough nuts and seeds, not enough whole grains, and then not enough vegetables. Number 18 on the top 20 list was not getting enough long-chain omega-3 fatty acids like DHA and EPA found in seafood, due to their purported protective effect against heart disease. As I discuss in my video Is Fish “Brain Food” for Older Adults?, even years ago when the study was published, researchers were already questioning the benefits of these fish fats, as more and more randomized controlled trials put them to the test and they failed, culminating in the meta-analysis I profiled in my video Is Fish Oil Just Snake Oil? that appeared to put the issue to rest.

Consumption of fish and fish oil wasn’t only hyped for cardiovascular protection, though. Omega-3s have also been touted to treat depression. However, after taking into account all the negative results that went unpublished, there appears to be no benefit for major depression or for preventing suicide, as I explored in my video Fish Consumption and Suicide.

What about omega-3s for the prevention of cognitive decline or dementia? The available randomized controlled trials show no benefit for cognitive function with omega-3 supplementation in studies lasting from 6 to 40 months among healthy older adults.

It may sometimes even make things worse. “Higher current fish consumption predicted worse performance on several cognitive speed constructs. Greater fish consumption in childhood predicted slower perceptual speed and simple/choice reaction time.” This may be due to neurotoxic contaminants, such as mercury, in seafood. We’ve known that the developing brain is particularly sensitive to the damaging effects of mercury, but maybe the aging brain is as well.

This would explain results that have shown higher omega-3 levels to be associated with high levels of cognitive impairment and dementia. More EPA (eicosapentanoic acid) was found in the cognitively impaired, and more DHA (docosahexanoic acid) was found in the demented, presumably because of pollutants like mercury and polychlorinated biphenyls (PCBs) in seafood that have been related to cognitive impairment and Alzheimer’s disease.

The same cognitive “functions disrupted in adults, namely attention, fine-motor function and verbal memory, are similar to some of those previously reported in children with prenatal exposures,” that is, exposed in the womb. And, the adults exposed to mercury through fish consumption didn’t have only subtle EEG brain wave changes, but “observable deficits in neurobehavioral performance measures,” such as poorer performance on tests of fine motor speed and dexterity, as well as concentration, for example. “Some aspects of verbal learning and memory were also disrupted by mercury exposure,” and the greater the mercury levels, the worse they did.

That study, however, was done downstream of a gold mining operation, which uses a process that uses lots of mercury. Other such studies were done on people eating fish next to chemical plants or toxic spills, or eating whale meat. What about a more mainstream population? An “elite group of well-educated participants”—most were corporate executives like CEOs and CFOs––all living in Florida and wealthy enough to afford so much seafood that at least 43 percent exceeded the U.S. Environmental Protection Agency’s safety limit for mercury were studied. Researchers found that excessive seafood intake, which they defined as more than three to four servings per month of large-mouth fish like tuna, snapper, and bass, elevates mercury levels and causes cognitive dysfunction, resulting in about a 5 percent drop in cognitive performance. This may not seem like much, but it’s “a decrement that no one, let alone a health-conscious and achievement-oriented person, is likely to welcome.”

“It is worth noting the irony in the situation; that is, the fact that corporate executives who chose to overconsume seafood for health reasons sustained a drop in their executive functions. Yet, if a 4.8% drop in executive function due to excessive seafood intake occurs in highly functioning, healthy adults with ample cognitive reserve, the major concern for further study is whether similar [mercury] level elevations in individuals already suffering from cognitive decline might result in substantially greater declines,” particularly with cognitive decline, dementia, and seafood consumption on the rise.


Fruit deficiency is the number-one dietary risk factor? For more, see Inhibiting Platelet Aggregation with Berries.

But what about th Inuit? See Omega-3s and the Eskimo Fish Tale.

For more on the shift of the evidence on fish and heart disease, see my Is Fish Oil Just Snake Oil? video.

The greatest danger of mercury exposure may be for children, as I discuss in Mercury vs. Omega-3s for Brain Development and How Long to Detox from Fish Before Pregnancy?.

Other videos on the effect of fish contaminants and health among adults include Fish and Diabetes and Fish Consumption and Suicide.

Mercury is not the only neurotoxic contaminant of seafood, though. See Diet and Amyotrophic Lateral Sclerosis (ALS) and ALS (Lou Gehrig’s Disease): Fishing for Answers.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Concerns About Bone Broth

There are toxicological issues associated with production and processing of meat, such as the presence of various toxic contaminants—from dioxins and PCBs to cooked meat carcinogens. Carcinogenesis, the development of cancer, may be the main concern, but there are a number of other toxic responses connected with the consumption of meat products. Lead, for example, can be toxic to the nerves, gastrointestinal tract, bone marrow, and kidneys.

Where is lead found in the food supply? In general terms, the highest levels of lead, as well as arsenic and mercury, are found in fish. Sardines have the most arsenic, but tuna may have sardines beat when it comes to mercury and lead.

The problem is that “fish-consumption advisories related to human health protection do not consider the fish by-products fed to farmed animals,” like farmed fish. If some tilapia are fed tuna by-products, they could bioaccumulate heavy metals and pass them onto us when we eat them. Researchers found the highest levels in frozen sole fillets, averaging above the legal limit for lead.

Lead exposure has been shown to have adverse effects on nearly every organ system in the body. Symptoms of chronic exposure range from memory loss and constipation to impotence and depression. These symptoms present after pretty hefty exposure, though. However, we now know that “[b]lood lead levels in the range currently considered acceptable are associated with increased prevalence of gout and hyperuricemia” (elevated levels of uric acid in the blood). According to the Centers for Disease Control and the World Health Organization, a blood lead level needs to be less than 25 micrograms per deciliter to be “non-elevated.” You’d assume that at values under 25, there’d be no relationship with health outcomes, but even throughout this “acceptable” range, lower lead means lower uric acid levels and lower gout risk. So, even blood lead levels 20 times below the acceptable level can be associated with increased prevalence of gout. “These data suggest that there is no such thing as a ‘safe’ level of exposure to lead.” 

Once lead gets into the body, it tends to stay in the body. It builds up in the bones such that it may take 30 years just to get rid of half. The best strategy? Don’t get exposed in the first place.

If lead builds up in bones, though, what about boiling bones for broth? As I discuss in my video Lead Contamination in Bone Broth, we know bones sequester lead, which can then leach from the bones. So, researchers suggested that “the bones of farmyard animals will sequester lead, some of which will then be released into broth during its preparation.” Who eats bone broth? Bone broth consumption is encouraged by many advocates of the paleo diet. Online, you can learn all about purported “benefits” of bone broth, but what they don’t tend to mention is the theoretical risk of lead contamination—or at least it was theoretical until now. Broth made from chicken bones was to have markedly high lead concentrations, up to a ten-fold increase in lead. Researchers concluded, “In view of the dangers of lead consumption to the human body, we recommend that doctors and nutritionists take the risk of lead contamination into consideration when advising patients about bone broth diets.”

But what if you only use bones from organic, free-range chickens? They did use only bones from organic, free-range chickens.


For more on the paleo diet, see:

Other products contaminated with lead include Ayurvedic supplements, protein powders, wild animals shot with lead ammunition, dairy products, and tea from China:

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

Should Plant-Based Women Supplement with DHA During Pregnancy?

A systematic review of randomized controlled trials of DHA supplementation of pregnant and breastfeeding women failed to find any clear and consistent short- or long-term benefit for psychomotor, mental, visual, or physical development. Perhaps DHA supplementation during pregnancy has no effect because the body wisely protects the growth of the baby’s brain by drawing off of maternal stores of DHA, upregulating maternal DHA synthesis, and preferentially shuttling it to the fetus. But what if moms don’t start out with large maternal stores? In other words, maybe DHA failed to help women who were already getting enough, but perhaps women with very low intakes would benefit from DHA supplementation. My video Should Vegan Women Supplement with DHA During Pregnancy? explores the evidence available to date.

It’s interesting to note that, by 1978, researchers already were suggesting a plant-based diet as the diet of choice in the treatment of our number-one killer, heart disease, but babies breastfed by vegan moms had significantly less DHA in their bloodstreams, presumably because the moms had significantly less DHA in their breast milk.

The question is whether these differences are of any consequence. The growth and development of vegan and vegetarian born children are normal as long as they’re getting their B12, and “[t]here is no evidence that neural or intellectual functions are impaired.” In fact, the two studies we have on kids in vegetarian communities showed they had higher IQs, though that may be because their parents tended to be better educated. However, even though the kids seemed fine, that doesn’t rule out the possibility that there may be some “subtle differences…in visual or neural functioning.”

It would be interesting to compare the function of babies getting vegan breast milk levels versus general population levels. In one of the studies I profile in my video, it shows that vegans hit a level of 14, vegetarians 30, and omnivores 37. Another study compared 0 to 32, 64, and 96, and, though 32 worked better than 0, more than 32 didn’t add anything. This could explain why the general population at 37 doesn’t benefit from additional DHA supplementation. But what about down at 14? Most studies at that level show no advantage over 0, though one study found a benefit supplementing at as low as 5, but that doesn’t help us.

Just because babies breastfed by vegan moms have significantly lower DHA levels in the blood, that doesn’t necessarily mean they have lower levels in their brain, which is where it counts. What we need is a randomized, controlled trial in non-fish-eaters of DHA supplementation. Until then, it’s going to remain uncertain. So, what should pregnant and breastfeeding women who avoid fish do in the meanwhile? Low intake of DHA doesn’t “necessarily equate with fetal DHA inadequacy,” but new data suggest that some infants may not be getting enough and could benefit from their moms supplementing. Given this, I recommend pregnant and breastfeeding women on plant-based diets to follow the consensus guidelines to get about 200mg of preformed DHA from an uncontaminated source, like algae oil, which is probably the best combination for all women given the state of our world to minimize exposure to toxic pollutants such as dioxins, PCBs, and mercury. 


To gain a better understanding of why algae oil is better than purified fish oil, I encourage you to watch my video Should Pregnant and Breastfeeding Women Take DHA.

I’ve discussed concerns about pollutants in seafood during pregnancy in a number of my videos:

Finally, for more on taking long-chain omega-3s to protect your heart, see: 

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: