Nutrition |

The science of food

Nutrition has been defined as “the science of food and its interaction with an organism to promote and maintain health”. This definition recognises the constant interaction between the internal environment (inside the organism), with its precisely regulated physiological systems, and the external environment, (outside the organism), with its many physical, technological and physiological systems which determine the availability and choice of foods.

In western society food is not viewed merely as a necessity of life, it is as much an expression of our lifestyle as our choice of work, clothing, housing, etc.

Food is an integral part of most celebrations, it is often a symbol of love, companionship and achievement. Entire industries – restaurants, bars, fast food chains etc., have gown from our “food celebrations”, and obesity has become a serious problem.

The dual emphasis on food and “slimness” has created another problem – viz. eating disorders

A large percentage of people who join health clubs do so to “lose weight” it is therefore important that the fitness instructor has some knowledge of the nutritional needs of the body and the utilisation of food.

Digestion, Absorption And Metabolism

Before discussing the actual processes of digestion, absorption and metabolism, one must be aware of what man is made of in biological terms.

Water accounts for about two thirds (60%) of the body weight, and is found in all tissues. Three quarters of this water is found within the body cells i.e. in the intra-cellular compartment and one quarter is found in the blood, lymph and interstitial tissue i.e. in the extra-cellular compartment (outside the cells). All tissues vary in their water content. For example, bone, teeth, and adipose (fat) tissue contain less water than muscle and nervous tissue. Up to 10% of total body water may be lost without serious consequences.

Protein accounts for 18% of the body weight. Most of it is an essential component of all body cells and probably 2kg can be lost without serious results.

Fat makes up another 18% of the body weight. There are considerable variation in the proportion of fat, depending on the amount that has been deposited for storage. Of the 9kg of fat, no more than 1kg is essential, the remainder representing a store of energy. In obese people, this store can form up to 70% of the body weight.

Carbohydrates makes up a small portion of the body weight as only about 300g (0.3kg) is present in the body. Up to 200g of this may be lost without serious consequences. During starvation, the carbohydrate stores are replenished from the fat and protein reserves.

Minerals, vitamins, hormones and enzymes make up the remaining 4% of the body weight. Only calcium and phosphorus make any significant contribution to the body weight as they form the matrix of bones. All the other vitamins and minerals are present in such minute quantities that they have no significant effect on the total body weight.

Digestion

Digestion is the mechanical and chemical process whereby complex food materials are hydrolysed (broken down) into forms that are suitable in size and composition for absorption into the blood stream and for utilisation by the body.

Proteins need to be hydrolysed into amino acids, carbohydrates into glucose and other simple sugars, and fats into fatty acids and monoglycerides. A few substances, such as water, vitamins and some minerals, can be absorbed as such and need no digestion. This transformation of food is accomplished by both mechanical and enzymatic (chemical) digestion. Before discussing each of these separately, one has to be totally familiar with the anatomy of the human digestive system, generally referred to as the GIT (gastro-intestinal tract).

The GIT is a continuous tube 7,5-9m in length in adults. It’s structure is renewed approximately every 24 – 48 hours. Food enters the GIT through the mouth. From there it travels down the oesophagus into the stomach. A valve at the base of the stomach, the pylorus, controls the flow of food into the first part of the small intestine, known as the duodenum. The food then passes through the second third of the small intestine, the jejenum, and into the last third of the intestine, the ileum. Another valve, known as the ileocaecal valve, controls the flow of the GIT contents into the colon which is also divided into three parts, the ascending, transverse and descending colon.

Finally, the digestive residue passes into the rectum from where it is expelled via the anal canal. Although the liver and pancreas are situated apart from this continuous tube, their secretions are very important in the process of digestion, and they are therefore included in the human digestive system.

Mechanical Digestion

The entire GIT is made up of a long, continuous, muscular tube. The rhythmic, co-ordinated muscle contractions of the GIT ensure that the ingested food is :-

Pushed down the entire length of the GIT
reduced to small particles
well mixed with the digestive juices
given maximum exposure to the digestive enzymes
given maximum exposure to the absorptive surfaces of the GIT

Once food is ingested, it follows the following pathway :

In the mouth, food is cut, ground and mixed with saliva.
Within seconds of swallowing, the rhythmic contractions of the oesophagus forces the BOLUS of food into the top of the stomach.
Each addition of food pushes the previous bolus of food into the central part of the stomach where it may remain for an hour or more as there is little motor (movement) or secretery activity here. Gradually the small but regular contractions in this region increase to break up the food further and mix it with gastric juice, until a thin liquid, CHYME, is produced.
From time to time, the pyloric valve opens and allows small amounts of chyme to flow into the duodenum. Here the circular muscles of the duodenum mix the chyme with the digestive enzymes and the longitudinal muscles propel the chyme forward in a wavelike motion. These rhythmic movements of the intestine are known as Peristalsis.
The unabsorbed food residue then passes through the ileocaecal valve into the large intestine and eventually out of the anal canal by means of voluntary control.

NOTE: after voluntary swallowing, all other muscular activity of the GIT is involuntary, except for defecation, which is again controlled.

The rate at which food moves through the GIT depends on the consistency (fluid or solid), composition (carbohydrates, fat or protein), and amount of food eaten. Liquids leave the stomach 15 – 30 minutes after ingestion and hence have a low satiety value. Carbohydrates, such as doughnuts and sweets, when eaten alone, leave the stomach more rapidly than do proteins such as cheese or meat.

Fats check the secretion of gastric juices and decrease peristalsis thus delaying emptying of the stomach. Ingestion to defecation normally takes 20 – 36 hours. After ingestion of a mixed meal, it takes 4-6 hours for the stomach to empty, another 2-9 hours for the food to pass through the small intestine and it may remain in the large intestine for another 18-31 hours.

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