Compiled by: Julius Mwangi
Food and Beverage Operations professionals in Kenya play a crucial role in ensuring that consumers receive meals that not only satisfy hunger but also meet nutritional needs. Understanding nutritional deficiencies and disorders is essential for menu planning, food preparation, and advising clients on healthy eating habits. This chapter delves into the fundamental processes of digestion, absorption, and metabolism of foods and how these relate to nutritional health. Mastery of these concepts enables professionals to contribute meaningfully to public health through informed food service practices.
Digestion is the process by which the body breaks food down into small, simple substances that can pass into the blood and be used for energy, growth and repair. It is both mechanical, as food is chewed and churned, and chemical, as enzymes split large molecules into small ones. Food and beverage professionals need this knowledge to plan meals that are easy to digest for children, the elderly and patients, to understand why some foods cause discomfort and to explain the link between food preparation and the nutrients the body actually receives. Example: the kitchen of a nursing school hostel in Mandera serves soft, low-fat meals to students recovering from stomach upsets on the advice of the school clinic.
The terms below describe the stages through which food passes from the moment it enters the mouth to the moment waste leaves the body.
Digestion is the mechanical and chemical breakdown of food into simple, soluble substances that the body can absorb. Starch is broken down to glucose, proteins to amino acids and fats to fatty acids and glycerol. It begins in the mouth, continues in the stomach and is completed in the small intestine.
Mastication is the chewing of food by the teeth, helped by the tongue and cheeks, which breaks food into small pieces and mixes it with saliva. It increases the surface area of food for enzymes to act on, softens it for swallowing and starts starch digestion by salivary amylase. Food served to people with few teeth, such as the elderly, should be soft or minced so that it does not need much mastication.
Deglutition is the act of swallowing. The tongue rolls the chewed food into a soft ball called a bolus and pushes it to the back of the mouth; the soft palate closes the nose and the epiglottis covers the windpipe so that food goes into the oesophagus and not into the airway. Patients with swallowing difficulty (dysphagia), for example after a stroke, need thickened fluids and pureed food.
Indigestion (dyspepsia) is discomfort or pain in the upper abdomen during or after eating, often with bloating, heartburn, belching or nausea. It may be caused by overeating, eating too fast, fatty, spicy or fried food, alcohol, caffeine, stress or stomach ulcers. Caterers reduce it by offering moderate portions, grilled or boiled options and calm, unhurried mealtimes.
Egestion is the removal of undigested food and other wastes from the body through the anus as faeces. The waste consists mainly of fibre, dead cells from the gut lining, bacteria and water. Regular egestion depends on enough fibre and water in the diet; too little causes constipation.
Absorption is the passage of the simple end products of digestion through the lining of the gut into the blood or lymph. Most absorption takes place in the small intestine, whose lining has finger-like villi that give it a very large surface. Water and some minerals are absorbed in the large intestine, and alcohol partly in the stomach.
Peristalsis is the wave-like contraction and relaxation of the muscles in the walls of the oesophagus, stomach and intestines that pushes food along the gut. Circular muscles behind the food contract while those in front relax, so the food moves forward even when a person is lying down. Fibre gives the muscles something to push against and so keeps peristalsis active.
The human digestive system is the alimentary canal, a muscular tube about 9 metres long running from the mouth to the anus, together with accessory organs (salivary glands, liver, gall bladder and pancreas) that pour digestive juices into it. Each part has a particular job, and food passes through them in order.
The teeth chew the food and the salivary glands release saliva, which moistens food and contains salivary amylase that starts breaking starch into maltose. Saliva is slightly alkaline and also contains mucus that helps swallowing.
The oesophagus (gullet) is a muscular tube about 25 cm long that carries the bolus from the throat to the stomach by peristalsis. No digestion takes place here.
The stomach is a muscular bag that stores food for 2 to 4 hours and churns it with gastric juice. Gastric juice contains hydrochloric acid, which kills most bacteria and provides the acid conditions for pepsin, the enzyme that starts protein digestion; in infants, rennin curdles milk. The food leaves the stomach as a thick liquid called chyme. Fatty meals stay longer in the stomach, which is why they feel heavy.
The small intestine (duodenum, jejunum and ileum) is about 6 metres long and is the main site of digestion and absorption. In the duodenum, bile from the liver emulsifies fat into small droplets, and pancreatic juice supplies amylase, trypsin and lipase. Enzymes on the intestinal lining complete digestion: maltase, sucrase and lactase produce simple sugars and peptidases produce amino acids. People who lack lactase cannot digest milk sugar well (lactose intolerance).
The large intestine (caecum, colon and rectum) absorbs water and salts from the remaining material, forming faeces. Its bacteria ferment some fibre and make vitamin K and some B vitamins. Faeces are stored in the rectum until egestion.
| Organ | Juice | Enzyme | Acts on | Product |
|---|---|---|---|---|
| Mouth | Saliva | Salivary amylase | Starch | Maltose |
| Stomach | Gastric juice | Pepsin | Proteins | Peptides |
| Duodenum | Pancreatic juice | Amylase, trypsin, lipase | Starch, proteins, fats | Maltose, peptides, fatty acids and glycerol |
| Duodenum | Bile (no enzyme) | None | Fats | Emulsified fat droplets |
| Ileum | Intestinal enzymes | Maltase, sucrase, lactase, peptidases | Sugars, peptides | Glucose, fructose, galactose, amino acids |
After digestion, the end products are absorbed and then used by body cells through metabolism. How well this happens decides how much benefit a person gets from the food served. Example: a community health programme in Taveta advises families to give children oranges with their bean meals because vitamin C improves the absorption of iron from beans.
Metabolism is all the chemical reactions in the body's cells that use absorbed nutrients. Catabolism breaks large molecules down to release energy, for example the respiration of glucose to carbon dioxide and water. Anabolism builds new substances, for example proteins for muscle from amino acids and glycogen from glucose. The basal metabolic rate (BMR) is the energy used at complete rest to keep the body alive, and it is the largest part of daily energy needs.
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Create a free accountThis chapter explored the fundamental concepts of food digestion and nutrition, beginning with key definitions related to digestion, absorption, and metabolism. It detailed the structure and function of the human digestive system, explaining how various organs contribute to breaking down food into usable nutrients. The process of food absorption and subsequent metabolism was examined, highlighting how nutrients are transported and utilized within the body. The chapter then addressed nutritional deficiency diseases, focusing on common disorders that arise from inadequate intake or absorption of essential nutrients. It emphasized the physiological impact of these deficiencies and the importance of early identification and management. Assessment methods for detecting nutritional deficiencies were also discussed, providing insight into how healthcare professionals evaluate nutritional status. Overall, the chapter established a comprehensive understanding of how digestion and nutrition interplay to influence health and the consequences when this balance is disrupted.