How are trans fats produced during the conversion of vegetable oil into margarine and what specifically are proven health benefits of a diet with high trans fats?

Answer 1

(Warning -- context required, so the answer is long.)

First -- fats and oils are either "saturated" (which means no C=C double bonds) or "unsaturated" (which means there are one or more C=C double bonds present). You will also hear the terms "monounsaturated" (one C=C double bond) or "polyunsaturated" (more than one). These terms are commonly referred to as part of nutritional information.

The C=C double bonds in fats and oils are naturally in the cis configuration, which makes a "kink" in the otherwise straight carbon chain of the fatty acid. That "kink" makes the molecules "fit" together less well, and results in a lower melting point. That means that saturated fats (no "kinks") are usually solid, and mono- and poly-unsaturated fats are either lower melting solids or liquids (oils). Animal fats are often saturated, while vegetable oils are unsaturated.

Margarine is engineered to have a "just perfect" melting point, where it is solid-ish in the fridge, but it's not so solid you can't spread it on your toast. (Try spreading butter -- animal fat, so saturated -- from the fridge on toast, and you'll understand why the "just right" melting point of margarine is desirable.) Once spread on your warm toast, the margarine obligingly melts nicely -- "just perfect."

This is accomplished by "partial hydrogenation" of the vegetable oils. Complete hydrogenation would result in a saturated fat, and too solid. But if you hydrogenate SOME of the C=C double bonds, it lowers the melting point just enough to make the margarine solid-ish in the fridge, but not so solid that it's like spreading lard.

Unfortunately, partial hydrogenation results in the isomerization of some of the cis double bonds to trans double bonds. This is because the two hydrogen atoms are added reversibly, and in two separate steps. The hydrogen gas adsorbs onto the surface of the metal catalyst, and breaks the H-H bond so that individual H atoms are wandering around the surface of the catalyst. After the alkene of the unsaturated vegetable oil also adsorbs to the surface of the catalyst, the H atoms are added, one at a time, to the alkene carbons.

But as it turns out, after the first H atom adds, it can also "un-add" and re-form the C=C pi bond, re-forming the individual H atom on the surface of the catalyst. Because trans double bonds are more stable than cis, when the pi bond is re-formed, it is more likely to be re-formed as a trans double bond than a cis double bond. (A simple C-C single bond rotation after the first -H is added, but before it is un-added, accomplishes the isomerization from cis to trans.)

Most of the C=C bonds that react are fully hydrogenated to alkane, but a small portion (about ~10% if I remember correctly) of the alkenes in partially hydrogenated vegetable oil are now trans.

I'm sure that none of the chemical engineers who developed margarine anticipated this effect -- or, if they did, thought it would represent a health hazard. But it turns out that trans-fats are metabolized differently than naturally occurring cis fats (or saturated fats), and those metabolic differences apparently confer some health hazards. So, to answer your last question, I do not believe there are ANY health benefits of a diet high in trans fats. Quite the contrary! Most foods (like margarine) which have relied on partially hydrogenated vegetable oils have moved away, and found other solutions to achieve the desired effects, as part of an effort to avoid these health hazards.