Wednesday, November 2, 2011

Can The Oceans Support Recommended Fish and Fish Oil Intakes?

Like many nutritionists, physicians, and 'public health" organizations, I have previously recommended increased intake of fish and fish oils to raise intake of omega-3 fatty acids, supposed to prevent or remedy cardiovascular and neurological diseases and possibly reduce cancer risk. 

Although the American Heart Association and similar organizations endorse this idea, in 2009 Jenkins et al questioned its wisdom on two bases:  the weakness of evidence for benefits of fish and fish oil intake, and the strength of evidence that global fisheries are collapsing and unable to support current, let alone increased, use of fish and fish oils.
"The main problem with this advice is that, even at current levels of fish consumption, fisheries globally have reached a state of severe crisis (Figure 1).5-8 Already, the demand from affluent and developing economies, particularly newly affluent China, cannot be met by the world's fisheries.6 Moreover, declining catches are increasingly diverted toward affluent markets rather than local ones, with dire consequences for the food security of poorer nations, islands and coastal communities.9"

Uncertain health benefits of fish and fish oil

Jenkins et al  point to a number of problems with the evidence supporting use of fish and fish oils:

1.  Healthy subject effect:  " eaters generally have healthier lifestyles than the rest of the population. They exercise more, smoke less and have better diets.1113"  This makes it very hard to determine whether the better health found among people eating more fish is due to their fish consumption, or to these other habits.

2.  Inconsistent results:  Some interventional studies show benefits from increased intake of fish and fish oils, while others do not, and some, such as DART-2, showed harm, with an increased risk of cardiac death among men who took fish oils.

3.  Vegetarians appear to have a lower risk of cardiovascular disease and death therefrom, despite avoiding fish and fish oils, suggesting that non-fish dietary factors play a larger role.

4.  We have little evidence supporting the use of fish and fish oils for metabolic syndrome, diabetes, or neurological or autoimmune diseases, or even for neurological development. 

Regarding the neurological argument, although Jenkins et al do not mention this, Sanders reviewed the available literature and reported that "There is no evidence of adverse effects on health or cognitive function with lower DHA intake in vegetarians." In addition, Beezhold  et al found a lower incidence of depression among vegetarians than among omnivores, despite lower intake of omega-3 fatty acids among the former.  They reported that "participants with low intakes of EPA, DHA, and AA and high intakes of ALA and LA had better mood," contradicting the hypothesis that depression results from insufficient intake of pre-formed long-chain omega-3 fatty acids. 

Declining Fish Stocks

Jenkins et al report:
"In contrast to the uncertainty over the value of omega-3 fish oils in the scientific literature, there is little doubt about the gravity of the fisheries crisis and the prospect of ongoing collapses of fish stocks. There is scientific consensus about the rapid worldwide decline of fish stocks. Notably, and despite increasing fishing effort, global catches have been in decline since the late 1980s (Figure 1A),5 and the number of collapsed stocks has been increasing exponentially since 1950 (Figure 1B).8,47,48 There are also over 100 confirmed cases of extinctions of marine populations in the world's oceans.49"
"When projected forward, these trends imply the collapse of all commercially exploited stocks by midcentury.7,8 Yet the dire status of fisheries resources is largely unrecognized by the public, who are both encouraged to eat more fish and are misled into believing that we still sail in the sea of plenty.50 Indeed, the species that Westerners are supposed to eat in increasing amounts have stocks that are already under tremendous pressure (e.g., yellowfin tuna, the basis of the much recommended North American “tuna-fish sandwich”51) or that have collapsed, sometimes spectacularly, such as cod off the coast of northeastern Canada.52"

According to an article in the National Geographic, large fish stocks have declined 90 percent since 1950.

On November 2,  2006, Richard Black of the BBC reported on a study published in Science: "There will be virtually nothing left to fish from the seas by the middle of the century if current trends continue."  Black reported:

Steve Palumbi, from Stanford University in California, one of the other scientists on the project, added: "Unless we fundamentally change the way we manage all the ocean species together, as working ecosystems, then this century is the last century of wild seafood."

In short, the drive to eat more fish and fish oils will only accelerate the rate of destruction of marine ecosystems. 

At the very least, our hunger for fish and fish oils will leave future generations with a world wherein they will be unable to eat cod liver oil or fatty fish even if they needed to, because there just won't be any left in the oceans.

Seafood Poisoning

Perhaps people eating increased amounts of seafood will suffer the same fate as ocean fish.  Seafoods commonly contain chemical contaminants that appear to impair fertility, including mercury, PCBs, PEs, and others. 

Rozati et al found, among Indian men:

"PCBs were detected in the seminal plasma of infertile patients but absent in fertile controls (Table 1)."

"...a comparison between fish-eaters and non fish-eaters, irrespective of the dwelling revealed higher PCB concentrations and significantly lower total motile sperm counts in fisheaters than in non fisheaters (Tables 2,3)."
"Fish-eating urban dwellers had the highest PCB concentrations, followed in order by fish-eating rural dwellers, non fish-eating urban dwellers with an exclusively vegetarian diet and non fish-eating rural dwellers with an exclusively vegetarian diet. The total motile sperm counts in these men were inversely related to their PCB concentrations, being the least in fish-eating urban dwellers followed by fish-eating rural dwellers, non fish-eating urban dwellers with an exclusively vegetarian diet and non fish-eating rural dwellers with an exclusively vegetarian diet (Tables 2)."

Another study reported:

“The lowest levels of p,p'-DDT+p,p'-DDE and PCBs were found in milk from lacto-vegetarians and the highest levels in milk from mothers who regularly consumed fatty fish from the Baltic.”
Norén K. Levels of organochlorine contaminants in human milk in relation to the dietary habits of the mothers. Acta Paediatr Scand. 1983 Nov;72(6):811-6.

Does that make fish look like good brain food?


Anonymous said...

If you scratch fish off the list then where are you going to get any kind of significant amount of EPA and DHA? Have our requirements been over blown?

Assuming an extra low omega-6 diet, is our ability convert omega-3 sufficient? I really don't think so.

A fish-less strict paleo diet might get around 10g of Omega 6 and 5 or 6g of Omega 3.

Sanjeev said...

an under-reported, under-recognized issue, and the powers that be just do NOT seem want to change the status quo.

Indeed, they'll spend tons of money on more expensive boats to catch what's left than spend a dime to figure out other, less damaging ways of doing things.

Let's hope things like the sustainable fishing movements that encourage consumers to support stores that sell sustainably-caught catches

Asclepius said...

Doesn't point 1 (Healthy subject effect: " eaters generally have healthier lifestyles than the rest of the population.) also apply to point 3 (Vegetarians appear to have a lower risk of cardiovascular disease and death therefrom, despite avoiding fish and fish oils, suggesting that non-fish dietary factors play a larger role.)?

Don said...


I think that our omega-3 requirements are small and easily met by alpha-linolenic acid in plant foods (especially flax seeds).

Studies do show that if we eliminate fluid oils rich in omega-6, our conversion is sufficient.

Plus there is a lack of evidence that higher amounts of n-3, EPA, or DHA produces any significant health benefit.

Pregnant women are actually more efficient at the conversion than other people.

I think this whole n-3 hoopla came from marketers of fish and especially fish oils.

Don said...


The fact that vegetarians with low omega-3 intakes, low fish intakes, and higher n-6:n-3 ratios have low risk of cardiovascular disease shows that fish and fish oils are not the key factors preventing cardiovascular diseases. It suggests that fish-eaters and vegetarians share other dietary or behavioral factors (e.g. not smoking, eating more vegetables) that are actually responsible for the reduced risk.

WoLong said...

If we look at Cochrane Reviews on this issue, we would find that the evidence for the supposed health benefits of fish oil/omega-3 fats is rather weak, in most cases, non existent. So there we go.

Jeannie said...

Nice post and a long overdue critical look at impact of dietary recommendations on the environment.
Where do fish get their omega-3 fatty acids from? My understanding is that they get them from consuming algae (or being upward on the food chain from other creatures who consume algae). There are some omega-3 supplements that are algae-derived, so that is one way to get "fish-equivalent" omega-3, without the attendant environmental destruction and ingestion of concentrated toxics like PCBs.