Couch potatoes may be born, not self-made. Research from New Zealand suggests that overeating and sluggish behaviour could begin in the womb.

In the US some 30% of the population is clinically obese and obesity is on the rise even in developing countries, with researchers blaming junk food, sweets, alcohol, computer games, television and the car.

But a team from Auckland University report in the American Physiological Society journal that there could be a biological cause, claiming that the unborn baby adapts to clues about the world beyond the womb and adjusts to aid its future survival.

The reasoning is that if the mother is undernourished during pregnancy, the infants prepare themselves for a hungry world by storing up fat.

Children of low birth weight have been shown to develop obesity in adult life. The team tested rats and found that undernourished rodents were more likely to overeat and be lethargic in later life.

The researchers say the theory could explain why public health campaigns for people to exercise more and eat less were often ineffective: "Healthcare funding may better be spent on improving pregnancy care rather than waiting until metabolic and cardiovascular disorders manifest in adults years or decades later."

· As food portions in US restaurants keep growing, the Mayo Clinic has just issued guidelines on food servings. "A medium potato," it says, should be the "size of a computer mouse" while 1oz of cheese should resemble four dice and "a teaspoon of peanut butter "equals the tip of your thumb".