Finally, something I might know!

Trans Fats are not as common in nature as they are in our diet nowadays. They are created when a fat is put under some extreme conditions, like heat, or acidity and they mutate. I'm sure this used to happen in the old days, but not to any extend where anyone got sick from it.

Until we discovered how to make them industrially, that is. In 1901, German chemist first hydrogenated regular fat, creating trans fats and in 1903 he received a patent. From 1911 onwards, the food industry saw it's potential and started using it on a large scale.

Trans Fats have a lot of very attractive properties for the industry. They are easy to create on a large scale, relatively cheap and they are solid at much lower temperatures than regular fat, so it's usage became very widespread very fast. This was further pushed by the World wars and the following recession, where cheap food and butter replacements were in high demand.

From as early as the 1940s onwards, some nutritionists started to have doubts about trans Fats, and around 1950, the link between trans Fats and cardiovascular disease was proven scientifically. But then came the difficult job to inform the public, and do something about it. During the war and the period after that, many people were financially dependant on the cheap trans Fat products, as it could function as butter, frying oil, lard, bread improver and many many more. At the front, it was the source of fat for the soldiers.

Studies were discredited by the (at that time huge) trans fat indistry, claiming that they were good for you. This was comparable with Shell trying to prevent the public from learning about the downsides of fossile fuel climate change in the 80s and 90s. The public didn't really seem to mind any of the fuss, since this was the wild 60s and 70s and every one was chainsmoking cigarettes anyway.

Later, around the 1990 and 2000, attitudes changed and governments started to take action. Since trans fats are mostly made during processing, it is possible to control the amount present in food, so strict rules were made. The former trans fat industry no longer exists, and all products that used to be made from them now have more healthy replacements.

Trans-fats are generally speaking not natural. The key feature of a fat is a long chain of carbon carbon bonds with only hydrogen otherwise attached to the carbon. If that chain of carbon atoms is a complete unbroken line of single carbon-carbon bonds with each carbon also binding two hydrogen atoms (except at the very end where it would be three) then it is said to be "saturated". If, however, some of those carbon-carbon bonds are instead double or triple bonds (reducing the number of hydrogen atoms bonded to the affected carbon atoms) then it is said to be "unsaturated" (either mono-unsaturated with just one carbon-carbon double bond or poly-unsaturated with multiple double bond).

Because of intermolecular atomic forces saturated fats have higher melting points. At room temp poly-unsaturated fats tend to be oils while saturated fats and mono-unsaturated tend to be solids (like ghee or lard). In baking it can make a difference to the texture of a baked good whether you use oil or solid fat in the process, and in general there are lots of baked goods where more solid fats are preferred for the texture they provide. For example, if you want a flaky pie crust or a biscuit you would generally want to use butter or lard not oil. There are other food products where the type of fat used can make a difference in the overall texture of the end product, such as puddings, creams, and so on. Additionally, poly-unsaturated fats can oxidize and go rancid more rapidly, resulting in shorter shelf lives. However, in terms of mass production, animal fats are generally more expensive and less available on an industrial scale. And as the production of food products that would prefer the use of mono-unsaturated fats have a problem in terms of providing sufficient fats at acceptable costs.

And this is where 20th century chemistry comes in, because it makes it possible to process poly-unsaturated oils from cheap and ubiquitous plant production into those mono-unsaturated fats that there is such a demand for in food production. However, the process of doing so is not identical to the biological processes that produce mono-unsaturated fats so you get something that is similar but not quite identical.

Let's go back and take a look at that singular double carbon-carbon bond in a mono-unsaturated fat chain again. Due to the laws of physics that bond is actually planar, it's flat, and each carbon atom in the bond is within a triangle with that atoms its bonded to separated by 120 degrees from each other. You have the carbon-carbon double bond, then you have either side of the rest of the hydrocarbon chain connected to either carbon, then at the other available bonding points for each carbon in the double bond you have a hydrogen atom. Think about the carbon-carbon bond like a bridge running East-West and then other things connected to either end of the bridge as things on the North or South of each side. Imagine the backbone of the hydrocarbon chain like a long road coming into the bridge on one side and continuing on the other, while the hydrogen atoms are just short roads going into a rest area or something. There are only a few possible different arrangements to this situation, you can have "the road" coming from either the North or South side of the bridge on the West side and it can continue on either the North or South side on the East side. Simplifying further, there are two possible configurations, one where the road continues on the same side and one where it continues on the opposite side after the bridge. In our molecule we call those cis (same side) and trans (opposite side) configurations.

As it turns out, biochemically almost all natural mono-unsaturated fats are cis-fats, that's just the way that the enzymes et al manufacture them in cells. Industrially produced mono-unsaturated fats have been made using a different process call fat hydrogenation. You take an oil and you pass it over a heated catalyst (like finely powdered nickel) in the presence of hydrogen gas. This can "add" the hydrogen gas across an existing carbon-carbon double bond, turning it into a single bond and increasing the level of saturation of the fatty acid chain. This can be used to turn cheap plant produced oils into more lucrative solid or semi-solid fats. However, it not only affects the level of saturation of the fats, the process can also cause the existing double bonds to change from being in the cis-configuration to being in the trans-configuration (which is thermodynamically slightly more favorable). The result is that this hydrogenation process also tends to increase the amount of trans-fats in the resulting product.

This process of hydrogenation didn't really take off at large scale until the early 20th century. First with hydrogenated oils as shortening and margarine, with steady increases in production throughout the century. It was not until the mid-20th century before trans-fats from hydrogenated oils started becoming a significant factor in people's diets. And even then it took a while for it to be significant enough and for the differential health effects of trans-fats vs. other factors became possible to distinguish through study. More so, there was a perception that any level of unsaturated fat was more healthy than fully saturated fat in the mid to late 20th century, so it took a significant effort to get over that bias to prove how unhealthy trans-fats were. By 1990 somewhere between about 1/8 and 1/5 of all of the dietary fat consumed by an average American was trans-fat, at those high levels (and the higher levels within some specific groups such as those eating a lot of margarine or crisco based products) it was much easier to spot the health effects of trans-fats in studies. Additionally, cardiovascular disease is not something that develops instantly overnight, it takes years to develop and years to show health effects. That's partly why it took so long to find the effect.