Olive oil
The oil you use cold. What makes it worth buying is the first thing a hot pan destroys.
How to use it
Cold. Over the salad, into a dressing, over vegetables that are already off the heat. A spoonful poured over a finished plate does more than the same spoonful that sat in the pan for half an hour.
The best version
Extra virgin, in a dark bottle, with a harvest date on the label rather than only a best-before date. Taste it neat: an oil that catches at the back of your throat is full of polyphenols, and an oil that tastes of nothing has lost them.
How to prepare it
Nothing to prepare. Storage is the whole job: dark, cool, capped, and finished within a few months. Light and air do what heat does, only slower.
What it does
The polyphenols are the point, and that has been measured with the oil itself. Two hundred healthy men took the same daily amount of olive oil for three weeks at a time, in three versions that differed in phenolic content and nothing else. On the richest oil, 366 mg per kg, HDL rose by 0.045 mmol/L; on the poorest, 2.7 mg per kg, by 0.025. Oxidised LDL fell by 3.21 U/L on the richest and did not move on the poorest.
In the largest diet trial there is, 7447 people at high cardiovascular risk were put on a Mediterranean diet with extra-virgin olive oil, the same diet with nuts, or advice to eat less fat. The olive oil group came out at a hazard ratio of 0.69 for heart attack, stroke or cardiovascular death.
Watch out
Heat costs you the thing you paid for. Extra-virgin olive oil fried for three hours at 180 degrees lost phenolics, and with them 3.7 percent of its monounsaturated and 17.2 percent of its polyunsaturated fatty acids. Frying in olive oil mostly wastes what you paid extra for.
The 2018 trial is a recalculation. The investigators found afterwards that at some sites not everyone had actually been randomised, withdrew their first publication and ran the numbers again. The result held, but this is not a clean experiment and it should be read as one that was repaired.