Tucker and Mike both gave me the heads-up on this paper recently.
Linoleic acid causes greater weight gain than saturated fat without hypothalamic inflammation in the male mouse
It's nice and simple: feeding 22.5% of calories as linoleic acid (LA) to a mouse makes it obese and insulin resistant. Feeding 15% of calories as LA makes it identically obese but without the insulin resistance. The argument can be made, very convincingly, that LA is converts to 4-hydroxynonenyl (4-HNE) in proportion to the LA content of the diet. 4-HNE is a powerful driver of insulin resistance so the hypoglycaemic response to exogenous insulin is markedly blunted in the 22.5% LA group of mice. As in the top line here:
Here are the weight gains, which need a little consideration:
With the eye of faith you can see that the top line (22.5% LA) starts off gaining weight faster than the next line down (15% LA) but converges from around 7-8 weeks onward. My guess is that this is when insulin resistance from 4-HNE started to over-ride the insulin sensitising effect of LA which caused the weight gain in the first place.
Now the third line down is the interesting one. This diet only contained 1% linoleic acid. OK, these mice are statistically significantly slimmer than the 15% and 22.5% LA fed mice (p less than 0.05) but they are hardly exactly svelte when compared to the crapinabag (CIAB)-fed control mice (black line down at the bottom). And the CIAB food contained 4.22% of calories derived from LA.
That needs some thinking about.
It makes me ask: Why do the vast majority of high fat fed mice/rats become obese? Apart from the fact that they have been selected for this response.
There's probably another post or two needed on that one.
As an aside I would just comment that while I agree with Tucker that 4-HNE and related products of free radical modified PUFA are the best explanation for this study, my feeling is that the 15% LA group with sustained insulin sensitivity allowing sustained weight gain probably explains the situation in the current human population rather better. As sustained adipocyte distension progresses then we eventually get FFAs released in the presence of glucose and insulin, a Bad Thing. The ROS from this combination will eventually generate 4-HNE too but rather further down the obesity road compared with the 22.5% LA situation.