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KEY POINTS

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  • Nutrients can broadly be defined as chemical substances found in food that are necessary for proper growth, development, reproduction, and repair.

  • Energy in the body is derived from three main nutrient classes: carbohydrates, protein, and fat, which in turn are made up of sugars, amino acids, and free fatty acids, respectively.

  • Hormonal messages generated by the pancreas, adipose tissue, and GI tract orchestrate multiple responses associated with caloric intake and utilization.

  • The “set-point” hypothesis proposes that food intake and energy expenditure are coordinately regulated in the central nervous system to maintain a relatively constant level of energy reserve and body weight.

  • Dieting is defined as the use of a healthy, balanced diet that meets the daily nutritional needs of the body and that reduces caloric intake with increased moderate exercise.

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BIOLOGY OF EATING AND DIGESTION

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All biotic organisms derive energy from food to sustain life. This energy “drives” various cellular functions, including digestion, metabolism, pumping blood, and muscle contractions. Nutrients can broadly be defined as chemical substances found in food that are necessary for proper growth and development, reproduction, and repair following injury.

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Because most bacteria and higher organisms cannot carry out photosynthesis, they derive their energy by metabolism of preformed organic molecules, such as carbohydrates. In general, bacteria utilize simpler organic molecules and animals and humans require more complex macronutrients (proteins, fats, and carbohydrates) to meet their needs.

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Digestion of Foods

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The process of digestion is a remarkable orchestration of many complex biochemical and physiologic events. Breakdown of food begins in the mouth via the actions of enzymes in saliva. In the stomach, food is acted upon by gastric juices, which contains high amounts of hydrochloric acid. Numerous enzymes supplied by the pancreas, liver, and gall bladder aid digestion in the small intestine.

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The latter parts of the small intestine, the jejunum and ileum, are primary sites of nutrient absorption. The surface area of the intestinal mucosa available for absorption is greatly increased due to a combination of folds called valvulae conniventes (folds of Kerckring) and finger-like projections (villi) that are lined with enterocytes. Digestion of proteins begins in the stomach and continues in the lumen of the small intestine. The jejunum is the site of absorption of amino acids, dipeptides, and tripeptides by amino acid and peptide carriers in the enterocyte brush border. Lipids are hydrolyzed by pancreatic and intestinal lipases. Bile salts, along with phospholipids, facilitate the absorption of lipids. Macronutrient molecules (proteins, sugars, and fatty acids) that end up in the circulation undergo metabolism in various tissues to be either oxidized to extract energy or stored for future utilization.

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Integrated Fuel Metabolism

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Energy in the body is derived from three main nutrient classes: carbohydrates, protein, and fat, which in turn are made up of sugars, amino acids, and free fatty ...

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