What? You want to know why? And what on Earth hydrogenation is? I guess we can delve into this…It all starts in the 1890s in a chemistry lab somewhere in France…
In the Beginning
A short history of trans-fats
In the 1890s, Paul Sabatier (a French chemist) developed the hydrogenation process . Hydrogenation involves bombarding a molecule with hydrogen atoms so that a hydrogen atom might break a bond between atoms making up the molecule. If it is a single bond, then the molecule will break apart at that point and the reaction is called hydrogenolysis (literally the hydrogen is used to lyse, or cut, the molecule). If it is a double or triple bond, then the hydrogen simply breaks that bond and is incorporated in the molecule as a whole .
This process most often takes place in the presence of a metal catalyst, like nickel or platinum [2, 3]. Sabatier would win a Nobel prize for his work with organic molecule synthesis (and discovering that nickel is a catalyst for hydrogenation reactions) in 1912, but before that, hydrogenation* found its way to the food supply [1, 3] .
Ten years prior, in 1902, the chemist Wilhelm Normann used the hydrogenation process on liquid fat to produce a semisolid substance known as trans-fat. He patented this process in 1903 and food production companies really took a liking to this process. Why wouldn’t they? It makes food more stable and it makes food’s taste and texture better .
In 1911, Proctor & Gamble introduced the first hydrogenated vegetable oil product: Crisco. Yeah! Vegetable-lard substitute! But seriously, this stuff is terrible for you (I’ll get to that in the next section). Anyway, following the introduction of Crisco, we get margarine  (vegetable-butter substitute! Fun fact: margarine was once dyed pink by law so that everyone would know it was imitation food).
Okay, now I am about to start editorializing: just as I said in the article on unsaturated fats, consuming vegetable oil is essentially a patriotic duty. subsidies on corn and soy lead to a huge surplus of these products and what on earth can we do with them? Well, we can process it into other things, so-called value-added products. Corn oil, corn-syrup, food additives, livestock feed–all because we have a surplus of corn. Michael Pollan has an interesting article about this called We Are What We Eat. We have to get rid of the corn (and other vegetables) somehow, and margarine and vegetable oils become a great way to do that from the industrial food prospective. Which pretty much means it is a terrible idea to eat it.
Anyway, now that these fats have made their way into our food system, things snowball from here. During World War II, the use of margarine increased during butter rationing. In the 1980s, advocacy groups protested the use of saturated fats in fast food fryers (you know, just the most stable fats for high-temperature frying) and fast food joints moved to using partially hydrogenated oils for those fries and fritters. In the 1990s, a rash of studies came out showing that–wait for it!–trans-fats are bad for you (so much for that whole plant-based foods are great blanket statement). The American Heart Association has a nice little timeline that sums all of this up nicely .
Come to the Dark Side–We Have Oreos
Trans-fats are bad for you. Before anyone asks about an echo, yeah, I am repeating myself. I have written about other fats being okay in the rest of the series (in the right proportions and in moderation!), but these little devils are no good.
A little biology may be in order: trans-fats are unsaturated fats that have been manipulated into a straight shape. Since it is straight, so it packs together well, making it solid or semisolid at room temperature, much like butter. Unlike butter, it is the unholy union of polyunsaturated fat and hydrogen and it must be stopped.
See, trans-fats do something that no other fat (at least in my reasearch) is known to do: increases the level of LDL cholesterol and decreases the level of HDL cholesterol in the blood [5, 6, 7]. Trans-fat intake is an indicator of Type II Diabetes and Coronary Heart Disease (CDH), at least in part because it is positively associated with inflammation (found by studying women, this particular finding may be of special interest to you ladies out there)  and remember all the stuff inflammation can lead to from the article on unsaturated fats?
Trans-fats provide no known nutritional benefit. And they literally can kill you given enough time. Trans-fats are a perfect example of a nutrient that can be cut out of the diet without any negative consequences . Except maybe a little nostalgia. See, Oreos are full of trans-fat. All those other packaged cookies, crackers, and other yummies you used to beg your mom for at the grocery store? Same deal.
I am about to ruin Oreos for you forever (hopefully–I know that sounds awful, but this is for your own good), so stay strong and keep reading: the filling is basically powdered sugar and Crisco. Yep. Oreos are basically a really cheap, industrial attempt at chocolate sandwich cookies with vanilla buttercream frosting in the middle. Many prepackaged foods are made with trans-fats, so it is best to look at the label now that trans-fat is a required listing on the Nutrition Facts label.
So, put down the Oreos, as tempting as they might be. Trans-fats may be terrible for you and the baked goods in those crinkly packages that we all know and love might be several degrees below ideal, but remember one thing: Haagan-Dazs and chocolate are not off-limits**.
*Just to clear some things up: trans-fats are a particular molecule with hydrogens configured in such a way as to produce unsaturated fats with straight carbon tails. The trans-fats are produced by partial-hydrogenation. Full- or complete-hydrogenation does NOT result in trans-fats. This article concerns ONLY the products of partial-hydrogenation. Additionally, some trans-fats are found in the meat and dairy products of some animals, like cows. These so-called ruminant sources of trans-fats are not universally agreed upon as good or bad. However, they make up a relatively small amount of trans-fats sources, so this article will focus on industrially produced trans-fats.
**As sweets! They are not off-limits as sweets! They are, of course, limited by our favorite rule: that of moderation.