Fat slows down the speed your stomach empties, so food sits in your tummy for longer when you've eaten a fatty meal. That's why you can feel uncomfortable after eating a large plate of greasy fish and chips. As a guideline, foods that contain 3g of fat or less per 100g are considered to be 'low fat'. Also, try to watch out for saturated fat, as it is considered that a diet high in saturated fat can lead to an increased cholesterol level.