I ran a trial in Greece a few weeks ago. One participant, eight days, zero bottles of sunscreen, and the ethics committee was me.
Early September. 32°C by ten in the morning. Hiking for hours, shirt off, no hat, no shade, no cream, in the exact sun the leaflet has been warning me about since I was in a pram.
The intervention started six years ago, when the last bottle of seed oil left the house.
Since then every membrane in my body has been rebuilt out of butter, tallow and beef. About three full turnovers of the skin's fat, if you want the arithmetic.
And every summer I gave it a little more sun than the last, the way you would train anything else.
Outcome: I got a bit browner.
That is the whole result. No redness. No heat coming off the shoulders at night. No peeling. No sleeping on my front. No aloe vera. No pharmacy.
The Greek sun did what the Greek sun has done to Greeks for 4,000 years. It tanned me.
Meanwhile on the same beach: a woman reapplying factor 50 every 90 minutes with the concentration of a bomb disposal officer, and still going the colour of a tinned tomato by the third afternoon.
Her skin and mine were under the same light.
Hers was built out of the sunflower spread and the rapeseed the packet told her was kind to her heart. Mine was built out of the fat the packet told her would kill her.
Sunburn is a polyunsaturated fat oxidising inside the skin. Ultraviolet is the match, the seed oil is the kindling, and the redness is the fire brigade turning up.
Take the kindling away and the match lands on stone.
I know what one man's story is worth. It is worth exactly one man. But the chemistry predicted this result before I set off, and six years of messages from redheads, Celts and other former human tomatoes describe the same thing, unprompted.
The dermatologist will tell you I was reckless.
The dermatologist has spent fifty years telling you what to put on the outside of your skin and has never once asked what you built the inside of it from.
I put butter on the inside. Then I went and stood in the sun.
Reckless. Brown, though.