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I've been studying organic chemistry and I think trans fats are actually more stable than cis fats. This is because the steric hindrance (for trans) would be lower and the molecule energy would be lower too (more stable).

Why are unsaturated fats usually cis at room temperature if the steric hindrance is higher?

Is that because the room temperature provides enough energy to break the pi bond in the double covalent bond (for example) making the molecule rotate and have a isomer of higher energy?

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Nature produces cis fats exclusively (someone may prove me wrong with some weird example.) This is why unsaturated fats such as vegetable oil have lower melting point than saturated fats of the same MW (the cis double bonds lead to a more compact molecule which gets less tangled with its neighbours.)

The reason nature produces cis bonds is due to the enzymatic mechanism in which these bonds are produced. Therefore this is down to reaction kinetics, and as often happens the product which is not the most thermodynamically stable of all possible products is produced. It just has to be more stable than the reactants.

When fat is artificially hydrogenated with a metal catalyst (for example nickel) to increase its viscosity/raise its melting point (to turn vegetable oil into margarine) some of the cis double bonds that escape being hydrogenated are able to change from cis to the more thermodynamically stable trans. These trans fats are unnatural and are therefore widely claimed to be unhealthy.

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    $\begingroup$ I don't see any chemical reason why evolution could not have lead to enzymes that produce trans unsaturated fats. I'd suspect that the underlying advantage for cis are the lower melting points - producing cis unsaturated fatty acids allows to have liquid fats/oils (including solving of unsaturated fats) without the need to be very warm-blooded. $\endgroup$
    – cbeleites
    Commented May 17, 2014 at 18:28
  • $\begingroup$ I agree @cbeleites, trans unsaturated fats would probably be solid at room temperature and the mobility of this molecular compounds in a organism would be affected. That's an interesting point of view $\endgroup$
    – bruno
    Commented May 17, 2014 at 20:53
  • $\begingroup$ Milk contains trans fat. ncbi.nlm.nih.gov/pmc/articles/PMC2596709 $\endgroup$
    – DavePhD
    Commented Apr 8, 2015 at 16:40
  • $\begingroup$ Of course there is a reason why nature prefers cis: It's more reactive, the double bond can more easily be transformed to something else later, form a complex with a catalytic metal ion, addition reacion, etc. trans olefins are dead, mostly. $\endgroup$
    – Karl
    Commented Jun 15, 2016 at 3:24
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But in biological systems, polyunsaturated fatty acids are cis, and for an animal to oxidize them, they first have to be converted to trans, which takes energy. So I think it depends on whether you are discussing in vitro or in vivo polyunsaturated fats.

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