Trans fats are a particular form of fat which is rare in nature but came into use on a global industrial scale during the twentieth century. They are strongly implicated in strokes and heart disease, and the limited research carried out on some other medical conditions has also raised concerns. You can skip to the next section if you are not interested in the technical bit.
A fat is a substance containing one or more fatty acids attached to a bridging piece. Each fatty acid is basically a long chain of carbon atoms with a pair of hydrogen atoms attached to each carbon, rather like candle wax.
If a pair of hydrogen atoms is missing, the chain is said to be "unsaturated". The distance of the first gap from the trailing end of the chain is the omega number of the fat or oil, so I will (rather unscientifically) call them "omega gaps". The missing pair can be taken in either of two arrangements:
The cis and trans forms are said to be isomers. Because it is closer to the original saturated fat in shape, the trans form keeps some properties similar to it, where the cis form differs sharply. For example the kink in the cis form makes it more fluid, melting at a lower temperature, while the trans form behaves much like the trans. In fact, the trans gap is sticky and may even raise the melting point.
A few trans fats seem to occur naturally, for example short-chain trans fats can be present in organic cow's milk and derived dairy products. Whether these fats are healthy or harmful is not known, but they occur at levels well below the UK recommended maximum intake of 5 grams per day.
But most trans fats are created when natural, healthy omega oils are heated or processed. This can happen in either of two ways:
In ordinary frying and grilling, where the oil is heated well above the boiling point of water, a small amount of the oil can transform into a trans fat. This is thought to pose a health risk, although most evidence appears to be indirect. Omega 9, such as olive oil, cannot be transformed and has long been regarded as safer, although a recent (ca. 215/16) study suggests it may bring its own risks (incidentally, this casts doubt on the usual assumptions as to why Omega 3 and 6 oils should not be fried too hot).
The industrial process of hydrogenation is used to fill in the "omega" gaps and turn the oil into a hard saturated fat. It is very widely used to make vegetarian margarine and shortenings for cooking. Partial hydrogenation happens where one or more omega gaps remain. When this happens the gap can flip over to create a trans fat. These are the most harmful fats of all, most implicated in heart disease and strokes. Partial hydrogenation is done deliberately to tailor the melting characteristics of the fat. While partial hydrogenation is becoming rarer it is still legal in most countries. Although it has mostly now been replaced by mixing fully hydrogenated saturated fats with Omega oils, some products are still on the market.
Even supposedly fully hydrogenated fats still have a residue of such trans fats in them. the residues in "fully" hydrogenated fats mean that they will always pose a small health risk.
If you feed an animal on trans fats from partial hydrogenation, these will appear in every food product from that animal, such as bacon, sausages, steaks and cow's milk. This too has been very widely done. The mix of effects, in say frying a harmful trans-contaminated product in an omega oil, or frying an organic product in a trans-contaminated cooking fat, does not seem to have been studied - or even taken into account in studies of related isues.
A few countries have all but banned trans fats, setting low maximum allowed levels for traces in food. However most, including the UK and the US, are pursuing a variety of voluntary approaches to reducing the levels. The truth is, trans fats are so widely used in the food chain that cleaning them out is a major global exercise and is going to take years.
Meanwhile, especially if you are in a high-risk group such as middle-aged and overweight men, you may want to cut down on products which are most likely to contain significant amounts of trans fats. This can be tricky because until recently they were in almost every kind of prepared food. According to New Scientist magazine (22 July 2017, page 24, frying oils containing them remain the most popular with independent fried chicken fast-food restaurants. So in the UK, USA and most other countries, avoid fried chicken outlets unless they say their frying oils are trans-fat free.
Food labelling can be misleading too. Because a trans fat still has an omega gap, even if it is the wrong way round, it is still technically an Omega fat or oil. So when you see say a margarine product labelled "high in omegas" you don't know whether they are the good "cis" omegas or the bad "trans" omegas! And when you see "Free of trans fats" on an expensive brand, what does that say about the cheaper brands?
Following dietary advice to cut down on "bad cholesterol" can also help, as this is how the trans fats get distributed round your body.
Updated 22 July 2017