Suet is a traditional cooking fat in the US, which is a country that loves its cows. It's the fat inside a cow's intestinal cavity, and it can be rendered into tallow. Tallow is an extremely stable fat, due to its high degree of saturation (56%) and low level of polyunsaturated fatty acids (3%). This makes it ideal for deep frying. Until it was pressured to abandon suet in favor of hydrogenated vegetable oil around 1990, in part by the Center for Science in the Public Interest, McDonald's used tallow in its deep fryers. Now, tallow is mostly fed to birds and feedlot cows.
I decided to make pemmican recently, which is a mixture of pulverized jerky and tallow that was traditionally eaten by native Americans of many tribes. I bought pasture-raised suet at my farmer's market. It was remarkably cheap at $2/lb. No one wants it because it's so saturated. The first thing I noticed was a yellowish tinge, which I didn't expect.
I rendered it the same way I make lard. It turned into a clear, golden liquid with a beefy aroma. This got me thinking. The difference between deep yellow butter from grass-fed cows and lily-white butter from industrial grain-fed cows has to do with the carotene content. Carotene is also a marker of other nutrients in butter, such as vitamin K2 MK-4, which can vary 50-fold depending on what the cows are eating. So I thought I'd see if suet contains any K2.
And indeed it does. The NutritionData entry for suet says it contains 3.6 micrograms (4% DV) per 100g. 100g is about a quarter pound of suet, more than you would reasonably eat. Unless you were really hungry. But anyway, that's a small amount of K2 per serving. However, the anonymous cow in question is probably a grain-finished animal. You might expect a grass-fed cow to have much more K2 in its suet, as it does in its milkfat. According to Weston Price, butter fat varies 50-fold in its K2 content. If that were true for suet as well, grass-fed suet could conceivably contain up to 180 micrograms per 100g, making it a good source of K2.
Tallow from pasture-raised cows also contains a small amount of vitamin D, similar to lard. Combined with its low omega-6 content and its balanced n-6/n-3 ratio, that puts it near the top of my list of cooking fats.