Agriculture Reference
In-Depth Information
AbstRAct
Carbohydrates are the major dietary sources of energy for humans. While most
dietary carbohydrates are derived from multiple botanical sources, lactose and tre-
halose are the only animal-derived carbohydrates of relevance for human diet. The
nutritional value of all carbohydrates depends on their digestion into monosaccha-
rides by glucosidases of the gastrointestinal tract. Digestion of starch, the carbohy-
drate most abundantly consumed by humans, depends on the concerted activity of
the six enzyme activities: salivary and pancreatic amylases, sucrase-isomaltase, and
maltase-glucoamylase. Simpler carbohydrates such as sucrose, lactose, or trehalose
only require the activities of sucrase, lactase, and trehalase, respectively, for their
digestion. Glucose, galactose, and fructose are the main monosaccharides produced
by digestion of dietary carbohydrates that are metabolized by humans. Depending on
the requirements of individuals, these monosaccharides may follow metabolic path-
ways of either energy generation or energy storage. The imbalance between energy
expenditure and dietary intake of energy-rich foods has a direct relationship to the
development of diseases involving energy metabolism, such as obesity or diabetes.
These effects have led to attempting the classification of carbohydrates based on
their potential to induce physiologic effects in humans. However, to understand and
manipulate the physiologic responses of the human organism to carbohydrate feed-
ing, detailed studies to identify the multiple enzyme/substrate interactions among
the large variety of available carbohydrates and the eight glucosidic activities of the
human gastrointestinal tract are necessary.
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Carbohydrates constitute the main source of energy supplied by foods and may
account for up to 100% of the total dietary caloric intake. However, in an average
Western diet carbohydrates contribute around 50% of the ingested calories. These
macronutrients can be ingested in a large variety of forms and chemical structures,
including monomers or polymers derived mainly from the monosaccharides glucose,
fructose, and galactose. Starch is the most abundant digestible carbohydrate present
in human foods; however, the consumption of simpler carbohydrates, such as sucrose
or natural and synthetic syrups, has increased substantially with the development of
industrialized foods. In addition, lactose is the characteristic sugar present in milk of
most mammals. Other nondigestible carbohydrates, such as cellulose or inulins, are
also present in plant foods and are classified as dietary fiber.
Without exception, the energetic nutritional value of carbohydrates depends on
the ability of the human gastrointestinal tract to digest them, breaking them down
and transforming them into monomers of glucose, fructose, or galactose. These
monosaccharides are susceptible to absorption by specific transporters located in the
epithelial cell layer lining the small intestine. Once absorbed, the monosaccharides
are deposited into the blood circulation for their subsequent transport to the liver and
other tissues throughout the body. In the liver, most monosaccharides different from
