No. Not all fats are equal. However, going back milk vs lamb vs dressings is technically the wrong way to go about it, unless you know why milk vs lamb vs dressing are different.

"Fat" is actually an umbrella term for a couple dozen different types of fatty acids. These fatty acids are what give different fat sources different properties (when not bound by other structures). Here are the primary fatty acids:

Saturated Fats:
Lauric acid
Palmetic acid
Stearic acid
Myristic acid
Butyric acid

Monounsaturated Fats:
Oleic acid

Polyunsaturated Fats:
Linoleic acid (Omega-6)
Alpha-Linolenic acid (Omega-3, ALA)
Eicosapentaenoic acid (Omega-3, EPA)
Docosahexaenoic acid (Omega-3, DHA)

Fats contain hyrdogen and carbon bonds, and the differences in these bonds determined the saturation of the fat.

In saturated fats, every carbon atom contains a hydrogen atom on each side of it (two hyrdogen atoms per carbon atom). This makes straight, sturdy chains.

In unsaturated fats, some of the carbon atoms only have one hydrogen atom. This creates a "bend" in the chain, decreasing its stability. The number of "bends" in the chain is indicated by the categorization of the fat (mono = 1, poly = many).



Trans fats are their own thing, and generally speaking, are present most primarily in hydrogenated vegetable oils. Hydrogenation is the process of forcing hyrdogen atoms into unsaturated chains, making them saturated, but often resulting in improperly saturating them (this is especially a problem in partial hydrogenation).

Unsaturated fats will naturally have two carbon atoms next to each other with only one hyrdogen atom connected to each of them. These hyrdogen atoms are on the same side as each other, which is what creates the bend in the chain. These are called cis fats. Trans fats, however, have those hydrogen atoms on opposite sides, which results in a straight-ish chain.



Enough organic chemistry. How's that relate to food?

Well...guess what -- that palm oil that's supposed to be so much better than animal fats? That's the same fatty acid composition as lard and tallow (more or less). As far as the fatty acids are concerned, there is no difference. In fact, palm oil actually contains more saturated fat than that "evil" lard!

How is that? Because it's less about where the fat comes from, and more about what blend of fatty acids are in that source. "Plant fats" and "animal fats" are actually pretty much the same thing, except that plant fats tend to have more polyunsaturated fatty acids, while animal fats generally lean toward a balance of saturated and unsaturated fats (there are exceptions to both generalities, of course, but we're talking gross generalities).

Coconut oil is so great, because it's one of the richest sources of Lauric acid, which makes up roughly 90% of the fat in coconut oil.

Butter is semi-solid at room temperature and solid in the fridge, because the Oleic acid content is liquid at room temperature, but the Palmetic acid content has a higher melting point, keeping it from completely melting at room temperature.

So, which is better -- lard or palm oil? That's a trick question, because they're effectively the same. The better question is -- what role do saturated fats (SFAs), monounsaturated fats (MUFAs), and polyunsaturated fats (PUFAs) play, and in what proportions should we consume them?

I personally, turn to the composition of human fat as my guide. After all, regardless of what we're eating, if we burn calories from stored fat, that composition is going to be what the body uses. I trust that the body creates such stores in a way that is at least neutral, if not beneficial or optimal, for the body. The fact that the overwhelming majority of the fat in our body consists of the more stable SFAs and MUFAs is no coincidence, in my opinion.

Human body fat is composed of roughly 35% saturated fatty acids (with the bulk of that being palmetic acid), 55% monounsaturated fatty acids (the bulk of which being oleic acid), and 10% polyunsaturated fatty acids, +/- roughly 10% for all three (though PUFAs don't go down to 0). (Source: http://www.jlr.org/content/5/3/343.full.pdf)

This means we should aim to make SFAs and MUFAs the bulk of our fatty acid intake, and not worry so much about PUFAs, except to make sure we're not losing out on Omega-3s. The amount of MUFA vs SFA is roughly split, so the exact amount of each isn't a huge deal. Thankfully, most sources either are sources of both, and it generally works out to about right. This makes it easy to tweak either way by including sources like olive oil (to tip that balance toward MUFA) or coconut oil (to tip it toward SFA).

If you eat meat/fish or consume dairy and avoid the standard vegetable and seed oils, then the PUFAs will generally take care of themselves. High-PUFA vegetable/seed oils will generally consist primarily of Omega-6 PUFAs, which compete with Omega-3 channels and have been found to cause inflammation in the body (which, in turn, causes a number of health issues). Likewise, due to the fragile nature of PUFAs, it's best to get them from their whole food sources -- fish, nuts, seeds, etc. -- as opposed to oils.

Okay...so what's their role?

Fats in general play quite a vital role, including regulation of hormones, creation and transport of fat-soluble nutrients, cell messaging, immune function, cell membrane building, and more. Exactly which fats play what role is a bit unclear (in no small part thanks to all of the politics around fats in general), though what I've found specifically names saturated fats in hormone regulation, cell messaging, and immune function, while monounsaturated fats tend to help "clean up" arterial plaque, and other various "repair" work. And, of course, we require a layer of fat to protect our organs (this is known as "essential body fat").