Why is trans fat always italicized on food labels, so that it says trans fat? Is it just due to convention, or is there an actual reason (like for emphasis)?
From my Organic Chemistry days, I remember that the isomers (compounds with the same chemical formula but different structures), for cis and trans, were italicized.
If you look on the Wikipedia page for cis-trans isomerism, it follows this convention, as every cis and every trans is italicized.
Cis is Latin for same side. A cis bond is one where the the substituent groups are both on the same side of the carbon-carbon double bond. Trans is Latin for across, meaning the groups are on opposite sides of the double bond.
I don't know why it started (maybe because it was a foreign word), but it's been done and repeated for so long that it is now conventional to do this. Indeed, it is wrong by IUPAC (International Union of Pure and Applied Chemistry) standards to write it any other way (except in contexts where italics are not available; then underlining is necessary.)
It is italicized based on the mistaken belief that there is a convention.
The ACS Style Guide is considered the authority for writing chemical names in English. On page 145 we see that cis and trans (and all other locants) only need to be italicized when hyphenated in a chemical name, e.g. cis- 4-Chloro-3-buten-2-one. This clarifies their role in the name. When free standing, the text can remain plain.
However, the older IUPAC Blue Book (1979), where all the official international rules for naming organic molecules are described, does italicize cis and trans as free-standing words.
These two books are generally consistent. The fact that they are not for this convention suggests that it doesn't really matter. But when you see trans italicized, the person who did it thought they were following a convention.