B is the more correct reason. Once cattle are removed from grass pastures and moved to feedlots to be fattened on grains, they lose access to omega-3s from the grass. Increased access to omega-6s from the grains unhealthily shifts their O6:O3 ratio. Do both have equal levels of O6 as Mark says? I don't know but clearly, the grass fed have higher O3's. I think the levels are less important than the ratios of one to the other.
So maybe I missed something (I'll revisit his post) but it seems that the grains are either
A. restricting the absorption of Omega 3 or
B. the cows get Omega 3's from grasses. (or C. both)
If it's "A" then one can theorize the possibility that grains would inhibit humans Omega 3 absorption. And if so how.
I avoid grain fed beef, not so much for the O6 issue, but more so to avoid the hormones present from manmade sources as well as from the stress the cattle live under while in the crowded, restricted movement environment of the feedlot.
As a source of Omega 3's, grassfed beef has a minimal impact on human needs. Cold water fish and shellfish are the better choices for DHA/EPA Omega 3's.
Stop by to visit at primalways.net
Old Paths ... New Journeys