Why have fruit juice at lunchtime, rather than breakfast? Fructose, the fruit sugar, is rapidly absorbed and increases blood sugar levels quickly, thereby stimulating insulin production. As a result of the insulin the blood-sugar level falls and there is a surge of low-density lipoprotein fats released by the liver. These fats are not artery-friendly. The effect of a heavy dose of fructose is greater at breakfast time than later in the day, for the body's metabolism is not the same throughout the day; hence fruit juices are better taken with lunch than breakfast.