A funny thing happened…

…this weekend. I spent all day Saturday wrapping up a 20-page draft of a paper on “Conjugated Linoelic Acid, Grass-fed Beef and Human Health” for my Nutrition & Metabolism class (we’re covering lipids this quarter). Sunday morning, I read a post on an urban farming message board I belong from someone who was having an ongoing argument with a family member who insisted that grass-fed beef from a local farm is no different than the beef you buy at the grocery store…and that all cows are fed grass. 
The poster was asking for substantial scientific information to trot out next time the argument arose…so naturally I responded. And when I was done, I thought “Hey, my response would make a good blog post.” So here you go…a slightly edited version.
The science is absolutely conclusive that beef from grass-fed cattle has a different, and more beneficial, fatty acid profile than beef from grain-fed cattle. It all starts with the food source: the conventional grain rations are rich in linoleic acid (LA), which is an omega-6 fatty acid; pasture grass, forage, and cut grass silage (to a lesser extent) have a fatty acid content that’s about 50-75% alpha-linoleic acid (ALA), which is an omega-3 fatty acid.
As LA and ALA are broken down inside the bovine rumen, they follow different biochemical pathways due to their different chemical structures. One of the intermediates for LA is arachadonic acid, which is the precursor for a lot of body chemicals (eicosanoids) that are responsible for chronic low-grade inflammation (which contributes to cardiovascular disease). ALA, being omega-3, continues to get broken down into other types of omega-3s, including EPA and DHA, which are the fats everyone tries to get from fish and fish oil pills.
LA and ALA are both essential fatty acids, because we can’t synthesize them ourselves, but we get too many omega-6s in our diet. An ideal omega-6:omega-3 ratio is between 1:1 and 4:1…in the typical U.S.-style diet it’s more like 10:1…or more. I spent more than a month pouring through the scientific literature, including articles from animal science journals, nutrition journals, biochemistry journals, public health journals (all peer-reviewed), and the science is conclusive that beef from grass fed cows has more omega-3 fatty acids than grain-fed beef but the same amount of omega-6s…that means that the ratio shifts to one that is closer to optimal (where there is disagreement is on exactly how far it shifts…but that partially depends on where the cattle are raised, age and breed of cattle, etc.). They also show a dose-dependent response (aka, the LONGER a steer is on pasture, the better).
Also interesting, is that grass-fed cows produce higher amounts of conjugated linoleic acid (CLA). This is because a grass diet produces a higher pH in the rumen, which allows ruminal bacteria to biohydrogenate LA or ALA into stearic acid (an 18:0 saturated fatty acid). One of the “intermediates” in that biochemical pathway is CLA. Another is vaccenic acid, which can be converted to CLA in the tissues (if we consume animal products with unconverted VA, our bodies can actually do the conversion). There is lack of consensus about whether CLA manifests into direct human health benefits, but what is fascinating is that CLA can compete with arachadonic acid (mentioned above) as an eicosanoid precursor, essentially creating different versions of eicosanoids that are not pro-inflammatory (EPA and DHA can do the same thing…it’s the whole omega-6 vs. omega-3 issue).
Also interesting is that most of the research shows that grass-fed beef has more saturated fat and less monounsaturated fat than grain-fed beef. Say what? I know! However, that extra saturated fat is almost exclusively stearic acid, which is a “neutral” fat that does NOT impact blood cholesterol levels. The reason why is complicated, but, basically, one of the effects of having a nice high-pH rumen with lots of good bacterial action going on is that the expression of a particular gene that codes for a particular enzyme is turned way down. The end results are two: One, the steer produces more saturated stearic acid and less monounsaturated oleic acid; two, the steer stores less fat in fat cells, which is why grass fed beef is conclusively found to be leaner overall, with less intramuscular fat (marbling) specifically. Of course, less marbling is seen as a negative by some!
I did my research via PubMed, and many of the articles I have access to aren’t publicly accessible, but The Union of Concerned Scientists has a good, meaty document (pun intended), “Greener Pastures.” Also, Center for Science in the Public Interest has a good chapter “The Fatted Steer” that’s part of a larger e-book.