Foods and drinks that are high in sugars are the cause of tooth decay. When you eat these foods the bacteria break down the sugars to make acid. Acid combined with bacteria, food particles and saliva forms a sticky substance called plaque which covers your teeth.
Over time the plaque will weaken the surface your tooth making it more prone to further attack. Eventually the tooth will become so weak that it crumbles.
It is therefore extremely important to limit the amount of sugary or starchy foods or drinks in your diet. Also brushing at least twice a day with Flouride toothpaste should help prevent tooth decay