I think that before getting into specific claims, it's important to note that both hypotheses need to be proven in order to be believed. The burden of proof rests on all parties, whether they're proving that dietary fat causes heart disease, or that it doesn't, or whatever other claim they may be making.

Fat Isn't Fattening

The most straightforward research is this: " Dietary fat is not a major determinant of body fat ^[1]." This has been proven over and over: fat that you eat is just another form of calories. It doesn't make you fat and is perfectly healthy.

Low-Fat Diets Not Proven

Maybe you don't think dietary fat makes one fat, but you still think fat is unhealthy. That's where I recommend reading Taubes ^[2]: "the low-fat-is-good-health hypothesis has now effectively failed the test of time."

He goes on:

Until the late 70's, the accepted wisdom was that fat and protein protected against overeating by making you sated, and that carbohydrates made you fat. In ''The Physiology of Taste,'' for instance, an 1825 discourse considered among the most famous books ever written about food, the French gastronome Jean Anthelme Brillat-Savarin says that he could easily identify the causes of obesity after 30 years of listening to one ''stout party'' after another proclaiming the joys of bread, rice and (from a ''particularly stout party'') potatoes. Brillat-Savarin described the roots of obesity as a natural predisposition conjuncted with the ''floury and feculent substances which man makes the prime ingredients of his daily nourishment.'' He added that the effects of this fecula -- i.e., ''potatoes, grain or any kind of flour'' -- were seen sooner when sugar was added to the diet.

I am not an expert in the Omega-3/Omega-6 ratio issue, but it suffices to say that A) it's important ^[3], and B) a modern, processed, grain-heavy, diet is terrible for maintaining a good Omega-3/Omega-6 ratio.

But fat qua fat being a problem is a modern idea, and doesn't seem to have much evidence behind it.

"How Much is Too Much?"

If we are asking "how much fat is OK?" then there are some references. I don't necessarily agree with the science behind this, but one recommendation from the UK's Faculty of Public Health ^[4] is:

Fats should provide no more than 35% of an individual’s food energy intake (ie. excluding alcohol) – predominantly from poly- and monounsaturated fats. Saturated fats should be limited to no more than 11% of food energy intake. Current average intake of saturated fats is 13.4% for adults and 14.3% for children. WHO recommends reducing trans fats to less than 1% of food energy intake, and ultimately phasing them out altogether.

Yes, certain fats are required for human growth and development. This is a known fact for >40 years.

I'd refer you to Barry Bogin's book Patterns of Human Growth ^[1]. In chapter 6 Environmental factors influencing growth he covers the research of the 50 essential nutrients as well as the impact of growth and development of different diets from a large sampling of cross-cultural studies.

Barry isn't trying to sell diet books, he's a hardcore researcher so the book is a bit dense, but worth a read to anyone with serious interest in diet and human development.

For what it's worth Jefromi, I recommend:

Chapter 6 of Life Without Bread Lutz and Allan.

The entirety of Section 1 ("The Fat Cholesterol Hypothesis") of Good calories, bad calories by Gary Taubes.

The entire book The great cholesterol con Malcolm Kendrick (must-read).

And if you prefer Chapters 3 and 4 of the popular The protein power lifeplan by Eades and Eades.

I also strongly recommend the fascinating historic book Eat fat and grow slim by Richard Mackarness.

I actually don't know what you mean, specifically, by "high-confidence" and/or broader proofs or studies, but all of these books (particularly Taubes and Kendrick) are packed with detailed examination of all the relevant major studies, etc.

Essentially, you are probably aware there is currently a major "religious" conflict in medicine/science/government between the "fat is bad for you" camp" and the "don't be ridiculous, fat cannot be bad for you" camp.

I guess, nobody can settle the argument "here and now" but those titles for starters will offer, as I say, particularly the Taubes book, and indeed Kendrick, detailed line-by-line picking-apart of the relevant major studies, and so on. I hope this is relevant to your question.

(A cool soundbite is that your HEART actually runs mainly, perhaps only, on lipides.)

The Eat Like a Predator ^[1] blog post also suggests favoring meat from grass-fed animals (ruminants), like beef, lamb, venison, goat... -- "Ruminants are far better at converting plants into essential fats, complete protein, and bioavailable nutrients than humans are." There's also a link to this site ^[2] which summarizes studies on the benefits of eating grass-fed beef. That site lists many, many studies and I don't feel like summarizing all of them; check them out if you are interested. In short, it shows that Omega-3 fats are healthy and that grass-fed animals have more of them, as well as more of other beneficial fats and vitamins.

Bonus question: You will die if you have no fat in your diet: while carbohydrates can be produced by breaking down proteins, the body has no way of producing fat and it has to be a part of your diet. In fact, Eat Like a Predator ^[3] says:

Frankly, you could stop here, as many native cultures did: as long as you eat organ meats and marrow, fatty, grass-fed ruminant meat provides 100% of your nutritional needs. But most of us enjoy more variety in our diets—and some vegetables and fruits offer tangible health benefits, even if they don’t provide meaningful calories.

Saturated fat is different from unsaturated fat, and fat found in butter and animal fat is saturated.

(This is somewhat simplified) Fat is simply Carbon, Hydrogen and Oxygen elements combined with glycerol. A molecule of fat contains one molecule of glycerol and three molecules of fatty acids. The fatty acid is a chain of CH groups, and when these are combined with single bonds the fat is said to be saturated.

Here's butyric acid found in butter:

CH3-CH3-CH3-COOH

This only has single bonds, no doubles.

A double bond is mono-unsaturated (commonly Omega 9) fat, and more than one double bond is polyunsaturated. Depending on where the first double bonds lies, these can be Omega 3 or Omega 6 fats.

Below is linoleic acid which you find in a whole host of oils such as sunflower oil, walnut oil:

CH3-CH2-CH2-CH2-CH2-CH=CH-CH2-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH

The "=" is the double bond. In this case it's an Omega 6 as the double bond lies on the 6th CH group.

The evidence supporting the idea that saturated fats lead to high cholesterol and consequently heart disease is huge: here's a small sample of reputable data:

There are strong, consistent, and graded relationships between saturated fat intake, blood cholesterol levels, and the mass occurrence of cardiovascular disease ^{(1) [1]}

.

Cardiovascular diseases are associated with lower intakes of green vegetables and higher concentrations of apo-B (a form of so-called bad blood cholesterol) which is associated with increasing intakes of animal protein and decreasing intakes of plant protein.

Western-type diseases, in the aggregate, are highly significantly correlated with increasing concentrations of plasma cholesterol, which are associated in turn with increasing intakes of animal-based foods. ^{(2) [2]}

This is in excess (where you consume over 10% of your daily in calories in saturated fat), and animal fats aren't just one type of fatty acid, they contain several. So relating this back to the claim about steaks - yes steak fat does contains some 'good fats'. The fat found in beef contains Linoleic acid and Oleic acid ^[3], but in very small amounts.

Linoleic is an essential fatty acid which means the body cannot synthesise it. Having said, beef fat is 3% Linoleic acid, so using this as your primary source would be a stupid way of getting the fat.

^{(1) [4] Wikipedia
(2) [5] China Study Summary by Roger Segelken, via a comment link from Dave Liepmann}