ATP is an important molecule for cells to have in sufficient supply at all times. The breakdown of sugars illustrates how a single molecule of glucose can store enough energy to make a great deal of ATP, 36 to 38 molecules. This is a catabolic pathway. Catabolic pathways involve the degradation (or breakdown) of complex molecules into simpler ones. Molecular energy stored in the bonds of complex molecules is released in catabolic pathways and harvested in such a way that it can be used to produce ATP. Other energy-storing molecules, such as fats, are also broken down through similar catabolic reactions to release energy and make ATP (Figure 1).
There are, however, some foods where the caloric expenditure to process them is a little higher than the calories they provide the system. The clear example is water, especially ice-cold water. The body needs to warm it up before absorbing it, leading to a small caloric debt. Foods with very high water content, such as celery, also have this tiny catabolic effect. But the nutritional value of water and celery are not high enough to properly sustain an organism, so relying solely on these foods to lose weight can lead to serious health complications.