My post last August about the weak claim of grass-fed beef to healthful quantities of omega-3 fatty acids has drawn a skeptical response from Robert Buxbaum, who found a contrary view in Michael Pollan's important new book, The Omnivore's Dilemma. I'd like to dissect the discussion a bit, because I think it's a cautionary example of how easily speculations about food and nutrition can stray from the facts, become accepted as fact themselves, and end up being unintentionally misleading.
I have some comments and a question about your August 25 report on omega-3 fatty acids in grass-fed beef. Are you comparing farm raised salmon or wild salmon when comparing grass-fed beef to salmon? As I posted on [Michael] Ruhlman's blog: Pollan, in "The Omnivore's Dilemma," mentioned that our impression of salmon as superior to beef as a source of omega 3 fatty acids was based on measurements taken at the time in which we first became conscious of omega 3s. That was when most available beef was corn fed and most salmon was wild. Pollan further claims that grass fed beef is a better source omega 3s than farmed salmon, which is what most consumers eat now. It seems as if how the salmon, as well as the cattle, are raised is significant in each case.
What Michael Pollan does say on pp. 268-269 of his book is that, like industrial cattle, farmed salmon are fed on grain, and they therefore contain less omega-3s than wild salmon, which eat small creatures that have accumulated omega-3s from the oceanic equivalent of grass, the tiny phytoplankton. He then speculates (my italics) that "if the steer is fattened on grass and the salmon on grain, we might actually be better off eating the beef," and that "the species of animal you eat may matter less than what the animal you're eating has itself eaten."
In fact, farmed salmon are primarily raised not on grain but on fish meal, a feed which is problematic in its own way, but which is plenty rich in omega-3s. A thorough survey published in 2005 found that salmon farmed in various regions throughout the world have consistently higher omega-3 levels than wild salmon, mainly because they are consistently fattier. And beef? The long-chain omega-3s in grassfed beef are present at around 20 milligrams per 100 grams (about a quarter-pound) of beef. The levels in farmed salmon are around 3 grams per 100 grams of fish: more than a hundredfold higher. Even salmon raised experimentally on vegetable oil for three-quarters of their life (to begin to address the issue of sustainability) have 1 gram of omega-3s per 100 grams fish: 50 times more than grass-fed beef. These are huge differences!
So species matters a lot. Ocean-going creatures live in a cold environment and need highly unsaturated fats that won't congeal at temperatures that can approach the freezing point; mammals are warm-blooded and need saturated fats that won't be too fluid at body temperature. Even grass-fed beef still comes from a warm-blooded steer, and farmed salmon is still a cold-blooded fish. And when it comes to the highly unsaturated omega-3s, we're far better off eating salmon.
Of course this is just one small piece of a large and complicated picture. There is plenty to be said in favor of grass-fed beef, plenty of problems with salmon aquaculture, and there's more to a healthy diet than omega-3s, which we can also get from other fish and shellfish. But it's good to have each piece of the picture in the right place, right-side up, however small it is.
Hamilton, M.C. et al. Lipid Composition and Contaminants in Farmed and Wild Salmon. Environ. Sci. Technol. 2005, 39, 8622-8629.
Torstensen, B.E. et al. 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; 53:10166-78.