How much can you eat before that stomach-straining, nausea-inducing feeling strikes? You know the one. It invariably presents sometime after Christmas lunch. Or when you meet family obligations and back up a three-course lunch with a four-course dinner with the in-laws.

Suddenly you're feeling as stuffed as the turducken - for the uninitiated that's a deboned chicken stuffed into a deboned duck, stuffed into a deboned turkey.

The science website Nova, associated with the Australian Academy of Science, points out that the capacity of the human stomach is not as great as you might think.

An adult stomach can comfortably hold about one litre of food and drink. Any more than that and things start to get uncomfortable.

However the human body is nothing if not flexible. Even when put to the test at Christmas time.

Nutritionist and childhood obesity researcher Brooke Harcourt from the Murdoch Childrens Research Institute said a 'resting' stomach cavity is about the size of a fist - though it can comfortably stretch to hold up to four litres of food and liquid.

That uncomfortable feeling of having overeaten is caused by the stomach pressing into surrounding organs - including the liver, diaphragm and lungs.

"In a bad case it can inhibit their function a little bit, so it gets harder to breathe which is why people loosen that top button," Dr Harcourt said.

Located just below the rib cage, there is little room for the stomach to expand - other than outwards.

"As it starts to expand we get what we sometimes refer to as a 'food baby'," Dr Harcourt said.

But be warned. The food baby can only grow so big.

Any more than four litres of food and liquid in the stomach can prompt a gag reflex. This means the digestive system goes into reverse and before you know it, lunch has been regurgitated and landed at your feet. Not a good look.

Dr Harcourt said the gag reflex was triggered not just because the stomach was at capacity - but also because acidity levels become elevated as the hydrochloric acid floods in to aid digestion.

Heartburn can also be triggered by overeating as acid creeps up into the oesophagus - not to mention burping as the body tries to rid itself of the unwanted air ingested while eating.

To avoid overeating Dr Harcourt said the trick was to listen to your body.

"You can also have a glass of water before the meal, so you are already feeling a little full," she said. "The water also gets the digestion process going."

Dr Harcourt said people should be aware that in 80 per cent of cases, when people think they're hungry, they're actually thirsty.

"We just have to try listen to our bodies, which is very hard. It's much easier said than done," she said.