Scientists are starting to unravel the mystery of why some snacks seem impossible to eat in small amounts.

Researchers examined rats eating crisps and found it's not just the fat and carbohydrates that send pleasing messages to the brain.

The reward and addiction centres in the rats' brains were triggered by something else - but the German team say more work is needed to find out what those triggers are.

German study

Health 24 reports that a study carried out by Tobias Hoch, Ph.D. sheds light on a condition called "hedonic hyperphagia" - it's a scientific term for 'eating to excess for pleasure, rather than hunger'.

Recreational over-eating is a major cause of obesity, something that's straining health systems worldwide.

Hoch and his team studied two groups of rats to see what effects their diets had on them - they fed one group a diet of crisps, the other was given standard rat food.

Among the reasons why people are attracted to these foods, even on a full stomach, was suspected to be the high ratio of fats and carbohydrates, which send a pleasing message to the brain, according to the team. In the study, while rats also were fed the same mixture of fat and carbohydrates found in the crisps, the animals' brains reacted much more positively to the crisps.

Body shape linked to kidney disease

Meanwhile, another group of scientists say they've found a link between body shape and kidney disease.

Researchers in the Netherlands say people with apple-shaped bodies have an increased risk of harmful high blood pressure in their kidneys.

They say this may be responsible for their increased risk of kidney disease in later life - but it can be successfully treated.