An intriguing new study shows the effect of a fast food-filled diet. The effect, it seems, is much faster than you might think.

Researchers from the University of Pittsburgh asked 20 Americans to do a diet switch with 20 Africans for two weeks.

Over the limit: The George Institute analysed the kilojoule count of popular meals sold in fast food restaurants.

The Americans were given a high-fibre, low-fat African-style diet with plenty of pulses and beans, while the rural Africans were fed a high-fat, low-fibre Western-style diet, liberal in burgers and fries.

It is not hard to guess which diet is better for us, but the surprise was in the speed our bodies respond to diet.