ACS Distance Education UK
The food we eat travels through our digestive system in order to be broken down and, thus enabling the nutrients in food to enter the blood stream and be used by the body as required.
This mechanical breakdown process starts in the mouth as a result of chewing. Saliva containing mucus, which acts as a lubricant to swallow food, and the enzyme amylase is secreted from salivary glands which are found around mouth. The enzyme amylase begins the initial stages of food breakdown – digestion.
Food then reaches the stomach as a result of muscular contractions of the oesophagus pushing it along. Glands lining the stomach secrete hydrochloric acid that dissolves food particles and a digestive enzyme which is specifically designed to break down proteins – pepsin.
The final stages of digestion and almost all of the nutrient absorption occur in the small intestine. It is here that nutrients from food are absorbed by the body into the bloodstream through the intestinal wall. The pancreas (a large gland located behind the stomach) secretes further digestive enzymes and another fluid rich in to neutralise the acid from stomach, into the small intestine. The liver secretes bile into the small intestine. Bile contains ions to neutralise acid from the stomach and bile salts to help break down fats.
Undigested material is passed to the large intestine, where reabsorption of salts and water (and few nutrients) take place. Finally, contractions of rectum, the last part of large intestine, expel the faeces through the anus. Faeces are expelled from the body anything between one to three days after eating the food.
Digestion is affected greatly by the microorganisms that are found in the digestive system. Maintaining appropriate good bacteria in the gut is affected among other things, by ensuring you eat adequate fruit and vegetables.
Some people take "probiotic" capsules regularly. Others eat fermented foods. All of these things can contribute to gut health.
Carbohydrates contain carbon, hydrogen and oxygen. As a cheap source of energy, they are widely consumed, and form the basis of the diet of much of the human population.
Proteins are molecular compounds containing carbon, hydrogen, oxygen and nitrogen atoms; and often also sulphur and some phosphorus. Every cell in the body is partly composed of proteins. Protein is a structural material required for growth and repair, and replacement of damaged tissues. It is also required for the process of living, for glandular secretions, enzymes and hormones. Excess protein can be broken down and converted to variety metabolic products, and can be used to contribute to energy cycles in the body. When broken down efficiently, protein typically accounts for 10-15% of energy generated in cells. Nitrogen which is not used by the body during energy production is excreted (removed) from the body as a waste product in urine. It is for the reasons outlined above, that protein is important to athletes.
Plants manufacture their own protein, but humans cannot do this. People must be provided with ready made protein either in animal or plant foods. Protein is commonly divided up into plant and animal proteins. Foods high in protein include all meats, fish, peas, beans, legumes, egg, milk and cheese.
Proteins consist of a chain of hundreds or more amino acid units – amino acids are the building blocks of protein held together by bonds. There are only approximately 20 different amino acids found in food and body proteins. By being linked together in different combinations and proportions, an almost endless variety of different proteins can be formed.
The body has limited powers of converting one amino acid into another as the need arises. There are, however 8 amino acids which cannot be manufactured by the body in sufficient quantities, and these 8 are known as "essential amino acids". They must be provided in the diet of every adult human. The essential amino acids for adults are: Isoleucine; Leucine; Lysine; Methionine; Phenylalanine; Threonine; Tryptophan; and Valine.
The amount of protein required depends on both your level of exercise and your overall weight.
Fats are also compounds of carbon, hydrogen and oxygen but they contain less oxygen than carbohydrates. Fats are solid at room temperature, whereas oils are liquid. Humans convert excess carbohydrate into fat.
Fats have various roles in the body and are vital in maintaining human health through maintaining body temperature and promoting cell function. With regards to nutrition, fats do serve as an energy source for the body. As stated above, fats converted from carbohydrates or fat molecules can be directly stored in adipose (fat) tissue can be broken down in the body to release glycerol and fatty acids. The glycerol can be converted to glucose by the liver and therefore can be used as a source of energy.
Fatty acids (one of the components of fats), depending on their molecular structure, can be termed saturated, unsaturated, mono-saturated or polyunsaturated.
Fats with a high percentage of unsaturated fatty acids are usually liquid at room temperature. Under controlled conditions unsaturated fatty acids can be converted to saturated fatty acids by a process known as hydrogenation. Hydrogenation is used in the manufacture of margarine, to turn liquid oil into a solid.
Fats are found in foods as visible fats, e.g. in butter, margarine, oils and the fat on meat as well as invisible fats in milk, nuts, lean meat, some fish etc.
Fibre is essential for maintaining the health of the digestive system. Plants and plant products are the sources of fibre in the diet. There are two types of fibre – water soluble and insoluble fibre. Water soluble means the fibre can dissolve in water, whereas as insoluble means the fibre doesn’t dissolve or breakdown in water. Why is this relevant?
Soluble fibre is needed to slow the breakdown of carbohydrates into simple sugars. By doing so, simple sugars are released into the blood stream more slowly and thus blood sugar levels are stabilised. Also, as this type of fibre moves through the digestive tract, it forms a thick gel-like mass on which cholesterol molecules bind. This therefore reduces the level of cholesterol entering the blood.
Insoluble fibre helps keep the digestive tract, specifically the colon, healthy by not being digested or absorbed by the body which means it moves through the colon encouraging regular bowel movements. This is important as a build up of faecal matter in the colon can lead to toxins entering the blood stream. This type of fibre also draws water in, making stools larger and easier to expel from the body.
The benefits of fibre are clear from the information outlined above; however, there is one extremely important reason why we should increase fibre consumption – cancer prevention. The bacteria which live in the colon can breakdown (digest) some fibre, by doing so they create an acidic environment inside the colon. Researchers have found that an acidic environment in the colon decreases the risk of colon (colorectal) cancer developing. This is currently a cancer which commonly leads to death.
The Need for Fluids
Every cell in the human body needs water. Water is vital for cellular functions and much more. It is needed to regulate body temperature and provide a means for nutrients to travel to cells and organs via the blood. Water also transports oxygen to cells, and removes waste from cells. In biological terms it is needed for most digestive, absorptive, circulatory and excretory functions.
There are so many different opinions stating how much water a person should drink each day. Often you may hear people say 6-8 glasses per day, but a glass is not a volumetric measure as all glasses are different sizes! Some other people recommend water intake based on the individuals weight - someone who weighs more needs more water. If we were to say what the recommended water intake should be, we suggest drinking between 2-3 litres every day. If the person is active, they should drink an additional litre for every half hour of vigorous exercise.
Water is lost from the body in many ways – perspiration (sweating), urinating, faeces and through exhalation. If someone does not drink enough water they may experience tiredness, headaches, dry eyes and mouth and experience a difficulty concentrating.