Why is a potato like a lump of sugar? This advertisement will appear in Mid-July and Early August Popular and Women's Weeklies Because a potato and a lump of sugar are both turned by your digestive system into exactly the same thing —glucose—fuel which your body "burns" to give you energy and warmth. All the starchy and sugary foods are alike in this. If you're short of rice or of jam, for instance, you can get your body fuel from potatoes. But, from the point of view of general health, there is a difference between the various starchy and sugary foods.