TLDR: Trans fats may not be as bad as they are made out to be.
This paper is comparing a high linoleate diet (using lard) to a soya oil derived (Primex) diet where much of the linoleate has been industrially hydrogenated in to trans fats and fully saturated fats.
A comparison of effects of lard and hydrogenated vegetable shortening on the development of high-fat diet-induced obesity in rats
The thing I like best about it is that, wait for it, they measured the fatty acid composition for their diets! HPLC and all that. Then, they put the results in the paper! On the down side their concepts about energy balance are pure CICO and rats have lingual receptors for fat which link to "hedonistic" centres in the brain. So they understand nothing, but we can forgive them for that.
Here are their results:
I think this might suggest that linoleic acid is obesogenic. Not that I've ever mentioned that before. Now obesity is obesity. What about health? Here are the numbers which matter, obviously insulin is the one we want to look at:
So, clearly, eating 11% of your calories as linoleic acid makes you fat and ill. The fat in the HVF (and the NF control) diet is made of:
"Diet compositions are presented in Supplementary Table 1. Primex pure vegetable shortening, a mixture of partially hydrogenated soybean and palm oil, was used by Dyets Inc. (Bethlehem, PA, USA) to formulate NF and HVF experimental diets".
Compositions panned out as :
Now these are measured, nothing is accepted from USDA food tables etc. So..... If we look at health outcomes (Table 3) in conjunction with diet composition (Table 1) it become pretty evident that, in these rats, trans fatty acid (TFA) feeding at 15% of total calories is positively health generating compared to linoleic acid feeding at 11% of calories. I did not expect this.
Aside: If any diet trial does not have its fatty acid composition measured you have no idea how much linoleic acid it contains. Usually more than you think. Also, joke of the century so far:
Question: If you are in a position of power over innocent folks who are trying to eat healthy food, which fat would you ban?
Answer: The wrong one!
I was on a PubMed search looking for trans fat toxicity. In this current study trans fats not only fail to cause insulin resistance, they render insulin-glucose parameters identical to the NF fed rats, despite the TFA fed rats carrying an extra 100g of adipose tissue.
That is very interesting. You can't answer the whys and wherefores from this paper. The missing piece of information is probably FFAs.
Trans fats are very odd. On acute exposure to TFAs isolated adipocytes release stored FFAs. This is what Cromer et al have to say in their study:
Replacing Cis Octadecenoic [Oleic] Acid with Trans Isomers in Media Containing Rat Adipocytes Stimulates Lipolysis and Inhibits Glucose utilization
"Overall, results of this study clearly show that conversion of octadecenoic acid from the cis isomer [oleic acid] to the trans isomer [elaidic acid] in adipocyte media will substantially increase lipolysis and inhibit glucose oxidation and conversion to cell lipid".
Just to clarify: Acute exposure of freshly isolated adipocytes to trans-oleic acid causes fat release, decreased glucose uptake and decreased glucose incorporation in to lipids. Sounds like a weight loss drug to me.
I was expecting this lipolysis to show up as elevated fasting FFAs and elevated fasting insulin. But there isn't any insulin resistance under fasting conditions visible in Table 3 so I think it is reasonable to assume there is no elevation of FFAs at this time either......
The only explanation I can come up with is that the adipocytes of the TFA fed rats are not as "full" as they should be, due to trans fatty acid induced lipolysis. Giving some "space" within an adipocyte allows the insulin sensitising effect of PUFA oxidation to show, certainly while fasting and lipolysis is the correct state to be in. Hence the fasting insulin levels are normal despite the 100g of extra bodyweight.
Or you could theorise that excess lipolysis from the trans fats is being almost exactly matched by decreased lipolysis from the insulin sensitising effects of linoleic acid. The combination just happens to pan out close to normal, provided the rats carry 100g of excess adipose tissue.
Some degree of post prandial hyperinsulinaemia/insulin signalling seems essential just to maintain those extra 100g of accumulated fat, but it's clearly not visible in the post absorptive phase.
If anyone can come up with a better explanation, I'm all ears.
BTW, this obviously relates to Axen and Axen's work. Their hyper-obesogenic diet was based on something Crisco-ish from the 1990's:
"The hydrogenated vegetable fat contained ∼25% long-chain saturated, ∼44% monounsaturated and ∼28% PUFA, with ∼17% of total fat as trans fatty acids (manufacturer’s communication)".
With fat making up 60% of the calories in the diet, and that fat being 28% PUFA, this is somewhere around 17% of total calories as linoleic acid. That is a LOT of linoleic acid. At the time I thought that the trans fatty acids would be to blame. Nowadays I'm not so sure.
How much of the bad rap that trans fats have received is from the PUFA which travelled with them at the time? Without mentioning the amount of fructose in the biscuits. Modern Primex is much more hydrogenated, so lower in PUFA, than whatever Axen and Axen used. It's far less obesogenic too.
Other odd final thought: People who are obese and insulin sensitive: Are they the folks who eat most trans fats along with their hearthealthypolyunsaturated™ linoleic acid???????