When it's a high carbohydrate diet! Preferably a high sucrose diet. The simple way to prove a high fat diet is bad for you is to use a rat or mouse, and feed it sugar, then describe this as a high fat diet in your abstract without specifying exact composition. "Normal" rat food in the USA for toxicology testing used to be NIH-07, at around 5% fat. There were so many problems with renal failure and heart failure that the diet was reformulated in the mid 1990s to the NTP-2000 diet, with just under 10% fat. The spectacular benefits were:
"The NTP-2000 diet prevented nephrocalcinosis and decreased the severity of nephropathy and cardiomyopathy, the common lesions of F344 rats in 13-week studies"
Notice it only took 13 weeks to get the problems and they certainly didn't go away on NTP-2000. Makes you wonder how they can assess drug toxicity in rats this sick from their diet! The other problem on the NIH-07 diet was cancer by 13 weeks, but that didn't seem to improve on the NTP-2000 diet at all. Still looks like chronic carbohydrate poisoning to me.
But the botttom line is that any diet with > 10% fat can theoretically be described as high fat for a lab rat. Up at 45% fat most junk science is happy to use the label "High Fat".
The comments about cardiomyopathy in the above paper are interesting as I came across this paper, only in abstract for unfortunately, but it gives the idea.
They compared the effects of 10% fat "normal" rat food with 45% fat diet, accurately described as "Western" diet, and also with a diet containing 60% of calories as fat. For a rat I would suggest 80% fat might be a better suggestion, but they got the goods at 60% anyway, so good for them. Here's the best section from the abstract
"Oleate oxidation in heart muscle ex vivo increased with high fat diet at all time points investigated. In contrast, cardiac oleate oxidation increased with western diet in the acute, short and intermediate term, but not in the long term. Consistent with fatty acid oxidation maladaptation, cardiac power decreased with long term western diet only"
When you use a genuine high fat diet there are no adverse effects on heart muscle function.
I like their introduction sentence too:
"Obesity and diabetes are associated with increased fatty acid availability in excess of muscle fatty acid oxidation capacity. This mismatch is implicated in the pathogenesis of cardiac contractile dysfunction and also in the development of skeletal muscle insulin resistance".
If you read my post on insulin resistance you'll know why.
Just checked on who did this excellent research and it looks like Taegtmeyer, the group leader, is USA based. Very unusual for me to cite an american paper, unless from one of the few islands of common sense such as Volek et al, but here it is and it's good.