Metabolic pathways are usually interconnected in ways that enable certain molecules to enter more than one pathway. For example, carbohydrate molecules from foods may enter catabolic pathways and be used to supply energy, or they may enter anabolic pathways and be stored or be converted to nonessential amino acids.
Excess glucose in cells may enter anabolic carbohydrate pathways and be linked into storage forms such as glycogen. Most cells can produce glycogen, but liver and muscle cells store the greatest amounts. Following a meal, when blood glucose concentration is relatively high, liver cells obtain glucose from the blood and synthesize glycogen. Between meals, when blood glucose concentration is lower, the reaction is reversed, and glucose is released into the blood. This mechanism ensures that cells throughout the body have a continual supply of glucose to support cellular respiration.
Glucose can also react to form fat molecules, which are later deposited in adipose tissues. This happens when a person takes in more carbohydrates than can be stored as glycogen or are required for normal activities. Because the body has an almost unlimited capacity to perform this type of anabolism, overeating, even if mostly carbohydrates, can result in becoming obese (overweight).