If everyone lost just 4 or 5 kilograms, mortality rates would drop dramatically. At least that’s one lesson from the economic crisis Cuba suffered in the 1990s.

When the Soviet empire began to unravel in 1989, Cuba was hit with serious food and fuel shortages. From 1991 to 1995, people were getting only about 1800 calories a day and had to walk or cycle wherever they needed to go.

The result was an average drop in body mass index of 1.5 units, and a halving of the obesity rate to just 7 per cent. In the years that followed, deaths …