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Carbohydrates are polymers composed of molecules containing atoms of carbon, hydrogen and oxygen. One gram of carbohydrate can provide four kilo-calories of energy, which makes it the most efficient instant energy source.

Starch accounts for approximately 60% of the carbohydrates consumed by humans. Since amylase enzymes cannot function in the stomach's acidic environment, starch can only be digested in the mouth and small intestine. Simple sugars are found naturally in milk and fruits in the form of glucose, lactose, galactose and fructose. These sugars are metabolized via glycolysis and then by the citric acid cycle.

Cellulose is a carbohydrate found in plant cell walls that provides strength and stiffness to the plant. Humans cannot digest it due to the absence of cellulase, the cellulose digesting enzyme. However, these indigestible carbohydrates add fiber. Fiber adds bulk to the food that is consumed, and aids digestion by helping the partially digested food move through the intestines smoothly.

Disorders of carbohydrate metabolism

The normal range of glucose in the blood is 70-99 mg/dL. Any deviation in this level is a sign of poor metabolism and a weak regulatory system. For example, dysfunction of insulin production and secretion, as well as the target cells' responsiveness to insulin, can lead to a condition termed diabetes mellitus. An increasingly common disease, diabetes mellitus has been diagnosed in more than 18 million adults and 200,000 children in the United States. It is estimated that up to 7 million more adults have the condition but have not been diagnosed. In addition, approximately 79 million people in the US are estimated to have pre-diabetes, a condition in which blood glucose levels are abnormally high, but not yet high enough to be classified as diabetes.

There are two main forms of diabetes mellitus. Type 1 diabetes is an autoimmune disease affecting the beta cells of the pancreas. These beta cells do not produce insulin; thus, synthetic insulin must be administered by injection or infusion. This form of diabetes accounts for less than five percent of all diabetes cases.

Type 2 diabetes accounts for approximately 95 percent of all cases. Lifestyle factors such as poor diet, inactivity, and the presence of pre-diabetes greatly increase a person's risk for type 2 diabetes. About 80 to 90 percent of people with type 2 diabetes are overweight or obese. In type 2 diabetes, cells become resistant to the effects of insulin. In response, the pancreas increases its insulin secretion, but over time, the beta cells become exhausted. In many cases, type 2 diabetes can be reversed by moderate weight loss, regular physical activity, and consumption of a healthy diet; however, if blood glucose levels cannot be controlled, the diabetic will eventually require insulin.

This text is adapted from Openstax, Anatomy and Physiology, Section 17.9: The Endocrine Pancreas

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