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INTRODUCTION

KEY POINTS

  • 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.

BIOLOGY OF EATING AND DIGESTION

Biotic organisms derive energy from food to sustain life. This energy “drives” cellular functions, including digestion, metabolism, pumping blood, and muscle contractions. Nutrients are chemical substances (typically found in foods or supplements) that are necessary for proper growth and development, reproduction, cellular function and maintenance, and repair following injury. Because most bacteria and higher organisms cannot carry out photosynthesis, they derive their energy by metabolism of preformed organic molecules, such as carbohydrates.

Digestion of Foods

Digestion is a remarkable orchestration of many complex biochemical and physiological events that occurs throughout the gastrointestinal (GI) tract, involving mechanical and chemical breakdown of food into simpler nutrients that are absorbable. 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 contain high amounts of hydrochloric acid. This along with enzymes such as pepsin and α-amylase act upon proteins and carbohydrates, respectively, to generate polypeptides and simpler sugars (such as dextrins from starch). Digestion in the small intestine is aided by enzymes supplied by the pancreas to the duodenum. Bile made in the liver and stored in the gall bladder until secreted aids in the absorption of dietary fats. Pancreatic juice contains digestive enzymes including carboxypeptidases, lipases, amylases, and nucleases, plus bicarbonate and divalent cations (mostly sodium and potassium) that help neutralize the gastric acid. The small intestine secretes many enzymes including aminopeptidases, lipases, and disaccharidases.

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. The luminal surface of the enterocytes is further lined by microvilli. The luminal and basolateral surfaces of the enterocytes are rich in transporters that mediate nutrient uptake into enterocytes. Commensal gut microbiota also play important roles in breakdown of complex carbohydrates (such as dietary fiber) into substrates that can be utilized by the host. ...

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