This news comes under the category, “questions we're glad someone asked.”
Last year, researchers at The Norwegian School of Veterinary Science in Oslo set out to determine what impact the typical farmed-salmon feed regimen—which is high in grains and plant oils—might have on cardiovascular risk markers in consumers (Seierstad SL et al. 2005).
Why did the Norwegian vets seek an answer to this cardiac health question? As they said, “Because of the shortness of marine resources, vegetable oils are increasingly used in fish farming.”
Fortunately for the few who may hear about it, they thought it a good idea to find out what effects the unnatural (for salmon), plant-heavy diet fed farmed fish actually exerts in humans.
The results would have given farmed salmon consumers an unpleasant surprise, if they'd heard about them. But, as far as we can tell, the results of this obscure veterinary science investigation escaped public notice.
We only uncovered it in the course of an unrelated literature search, and we were surprised that The Norwegian School of Veterinary Science—which has close ties to that country's large farmed salmon industry—even chose to examine this subject.
However, they did probe the cardiovascular implications of the vegetable-oil-rich diet fed most farmed salmon. The results, albeit pretty predictable, were not pretty.
How the study worked
The Norwegian vets enrolled 60 patients with coronary heart disease (CHD), and assigned them, randomly, to one of three groups, each of which consumed about 700 gm (25 oz) of farmed Atlantic salmon per week for six weeks (Seierstad SL et al. 2005).
Each of the three groups ate salmon that had been raised on a standard farmed fish diet consisting largely of grains and oils, but each group ate salmon given a different feed, distinguished only by its unique oil profile:
“FO” salmon feed featured 100 percent fish oil, which is very high in long-chain “marine” omega-3s
“CO” salmon feed featured 100 percent canola oil, which is high in omega-6s and low in the short-chain, less beneficial type of omega-3s
“FO/CO” salmon feed, featuring a 50/50 mix of fish and canola oil
As expected, these three distinct feeding regimens produced farmed salmon fillets containing widely varying levels of marine omega-3s:
“FO” salmon feed yielded fillets high in “marine” omega-3s
“CO” salmon feed yielded fillets low in “marine” omega-3s
“FO/CO” salmon feed yielded fillets with intermediate omega-3 levels
To determine the effect of these differing salmon-feed regimens on salmon-consumers' risk of cardiovascular disease (CVD), the researchers tested all of the participants' blood for three key indicators of cardiovascular risk—fatty acid profile, cholesterol profile, and markers of vascular inflammation—before and after the six-week study period.
What the fat-detectives discovered
The before-and-after tests of participants' blood showed significant differences between the groups, especially when it came to their blood levels of omega-3 fatty acids and the ratio of anti-inflammatory omega-3s to pro-inflammatory omega-6 fatty acids.
Compared with the other two groups, participants who ate the farmed salmon fed only fish oil enjoyed markedly higher blood levels of omega-3 fatty acids and a higher (hence, more desirable) ratio of omega-3s to omega-6 fatty acids.
The fish-oil-only group also displayed significant reductions in blood levels of triglycerides (fats) and two key markers of inflammation: vascular cell adhesion molecule-1 and interleukin-6.
As the Norwegian vets concluded, “Tailor-made Atlantic salmon fillets very high in n-3 PUFAs [omega-3s] of marine origin seem to impose favorable biochemical changes in patients with CHD when compared with ingestion of fillets with intermediate and low levels of marine n-3 PUFAs [omega-3s]….
While this outcome seems inevitable in hindsight, the complexities of human and salmon biochemistry precluded any firm conclusions concerning the actual cardiovascular effects of eating typical farmed salmon, which are raised on an unnatural diet high in grains and plant oils.
Thanks to these findings, it's clear that the diets fed typical farmed salmon put a real dent in the cardio-prevention potential of domesticated fish.
And as the Norwegian vets noted, the scarcity and cost of fishmeal and fish oil mean that this situation isn't likely to change.
Fishy “finishing” diet leaves deficient nutrition profile
The farmed salmon industry likes to point out that when fish are switched over to a diet enriched only with fish oil for the last 20 weeks of life, their fillets contain omega-3 levels about 80 percent as high as the omega-3 levels produced diets enriched only with fish oil.
However, even when their diets contain flax oil and canola oil—which contain short-chain omega-3s—switching to a fish-oil-only diet for the last 20 weeks still produces levels of omega-6 fatty acids a full 50 percent higher than in fish fed only fish oil-enriched diets.
In other words, farmed salmon placed on a “finishing” diet enriched only with fish oil still present consumers a salmon product saddled with an inferior fatty acid profile.
Seierstad SL, Seljeflot I, Johansen O, Hansen R, Haugen M, Rosenlund G, Froyland L, Arnesen H. Dietary intake of differently fed salmon; the influence on markers of human atherosclerosis. Eur J Clin Invest. 2005 Jan;35(1):52-9.
Bell JG, Tocher DR, Henderson RJ, Dick JR, Crampton VO. Altered fatty acid compositions in atlantic salmon (Salmo salar) fed diets containing linseed and rapeseed oils can be partially restored by a subsequent fish oil finishing diet. J Nutr. 2003 Sep;133(9):2793-801.
Torstensen BE, Bell JG, Rosenlund G, Henderson RJ, Graff IE, Tocher DR, Lie O, Sargent JR. Tailoring of Atlantic salmon (Salmo salar L.) flesh lipid composition and sensory quality by replacing fish oil with a vegetable oil blend. J Agric Food Chem. 2005 Dec 28;53(26):10166-78.
Bell JG, Henderson RJ, Tocher DR, Sargent JR. Replacement of dietary fish oil with increasing levels of linseed oil: modification of flesh fatty acid compositions in Atlantic salmon (Salmo salar) using a fish oil finishing diet. Lipids. 2004 Mar;39(3):223-32.
Bourre JM. Where to find omega-3 fatty acids and how feeding animals with diet enriched in omega-3 fatty acids to increase nutritional value of derived products for human: what is actually useful ? J Nutr Health Aging. 2005 Jul-Aug;9(4):232-42. Review.