And each of their activities not only keeps them fit.

And that is a tragedy.

This is the secret economics of thinness.

Across the developed world, the richer people are the thinner they tend to be

This relationship though is driven almost entirely by women.

This chart shows the share of men and women in France that are classified as obese and their income levels.

Obese women tend to earn far less than their thinner peers.

But obese men and thin men tend to earn about the same.

The same is true in America and in South Korea, obese men tend to earn more than their peers.

But in each of these countries, richer women are much more likely to be thin than poorer ones.

In fact, the correlation is so strong that for an obese woman of average height, losing £65 could have the same impact on her wages as gaining a master's degree.

Why is this?

Traditionally, people have believed that poverty which can make it difficult to access healthy foods or find time to exercise can explain why in general, poor people are more likely to be obese.

But those arguments should apply equally to both men and women and they don't.

So perhaps the causality runs the other way.

It is being thin that helps women to become rich.

In many ways, thinness is an economic pursuit.

Think of the time and money that women invest in trying to eat healthily or attending exercise classes.

And while social attitudes towards weight may have shifted in recent decades, the economic reality so far has not.