What it is
Trans fats are unsaturated fats reshaped — mostly by industrial partial hydrogenation — into a form that behaves like the worst of both worlds: they raise LDL cholesterol and lower HDL. That double hit is why they're the rare nutrient with no redeeming quantity.
How much
As little as possible. There's no floor, no target — just a ceiling near zero. The good news: industrial trans fats have been banned or phased out across much of the world, so modern diets carry far less than they used to. Trace amounts occur naturally in dairy and meat from ruminants; those aren't the concern the industrial ones were.
How well it's tracked
Reported on most labels, with a catch worth knowing: labeling rules often allow rounding to zero below a threshold per serving — so a food eaten in multiples can contribute more than its label admits. A day showing 0 g is probably right, not certainly.
Food sources to watch
Where they still appear: some margarines and shortenings, certain fried and packaged snacks, and baked goods in markets with looser rules. "Partially hydrogenated oil" in an ingredient list is the tell.
In the app
Trans fat has its Insights card and contributors view — mostly useful as a tripwire: if the line ever isn't flat at the bottom, the contributors will name the culprit.