The healthy liver regulates the use of glucose as it allows certain levels to circulate in the blood for use by the cells of the body. If the carbohydrate intake is higher than immediately needed, the liver will normally convert extra glucose into glycogen, a highly branched polysaccharide, and this glycogen can be stored in the liver or in the muscles. At a later time, if energy is needed when there are no dietary carbohydrates available, the liver will convert glycogen back to glucose and return it to the bloodstream, while the muscles may use the muscle glycogen directly for energy.
If we consume higher levels of carbohydrates than are immediately needed or can be converted to glycogen (that is, if there are already sufficient storage sources), then the liver will convert the excess glucose into fatty acids and then triglycerides that can be stored as body fat, a process termed lipogenesis. If carbohydrates are consumed in high quantities on a regular basis by a person with a sedentary lifestyle, weight gain occurs. Fat is a reserve source of energy. With decreased carbohydrate intake and increased activity levels, fat reserves are converted back to fatty acids for body fuel, a process called lipolysis. This generally produces weight loss.
Even when there is little or no intake of carbohydrates, the body attempts to maintain a steady blood sugar level through many mechanisms. Glucose is used by the liver as a source of energy to help synthesize a variety of essential substances. Insulin, a pancreatic hormone, regulates blood sugar levels by stimulating glucose uptake by the cells. Activity and exercise also can reduce blood sugar by increasing tissue glucose needs. A number of hormones influence the production of glucose when the body, and especially the brain, needs more energy. Epinephrine (adrenaline) stimulates glycogen breakdown and raises blood sugar. Steroids enhance conversion of fats and proteins into glucose, and adrenocorticotrophic hormone (ACTH) can interfere with insulin activity. Glucagon is produced in the pancreas and can raise blood sugar, while thyroid hormone may increase intestinal absorption of glucose as it attempts to stimulate metabolism.