Rats On Atkins
Via segamartinez, via @DrEades, here's Dr. Robert Lustig ("Sugar: The Bitter Truth") talking to KQED:

How do you get diet induced obesity in a rat? People say put them on a high fat diet. Garbage.

You can't get a rat to eat a high fat diet...unless you add 20% sucrose to the diet.

Basically you're giving them cookie dough.

But if you give them lard they won't eat it. They actually lose weight on that because that's the Atkins diet for them. It's not palatable. They don't like it and they actually lose weight and their metabolic parameters improve.

The only way to get an animal eat monkey or any other animal for that matter to eat a high fat diet is to lace the fat with sucrose.

So the question is: which is doing the damage, the fat or the sucrose in that case? And the answer is both.

As segamartinez writes:

We shouldn't let researchers, or journalists, or professors, or anyone for that matter, off the hook when they implicate a "high-fat" diet in causing, or even curing, an ailment, without them making the distinction that a "high-fat" diet is a "high-fat-high-sugar" diet in this context.

It could be likely that the increase in sugar is the driver of disease, while the "high-fat" content is along for the ride. The results of the 90% fat ketogenic diets certainly seem to make a case for it.