A Balanced Diet

A balanced diet is one that provides an adequate intake of energy and nutrients for maintenance of the body and therefore good health. A diet can easily be adequate for normal bodily functioning, yet may not be a balanced diet. An ideal human diet contains fat, protein, carbohydrates, vitamins, minerals, water and fibre all in correct proportions. These proportions vary for each individual because everyone has different metabolic rates and levels of activity.

Malnutrition results from an unbalanced diet, this can be due to an excess of some dietary components and lack of other components, not just a complete lack of food. Too much of one component can be as much harm to the body as too little. Deficiency diseases occur when there is a lack of a specific nutrient, although some diet related disorders are a result of eating an excess.

An adequate diet provides sufficient energy for the performance of metabolic work, although the energy food is in an unspecified form. A balanced diet provides all dietary requirements in the correct proportions. Ideally this would be 1/7 fat, 1/7 protein and 5/7 carbohydrate.

Energy is provided by carbohydrates, fats and proteins. Proteins are a provider of energy in an emergency, but are primarily used as building blocks for growth and repair of many body tissues. These energy providing compounds are needed in large quantities in our diet so are described as macronutrients.

We also need much smaller amounts of other nutrients, such as vitamins and minerals. Because much smaller quantities are needed for a balanced diet these are known as micronutrients. Despite the small quantities needed these are essential to provide a healthy diet as they have specific roles in metabolic reactions and as structural components.

Within the cells of our body, the nutrients ingested are converted to other compounds which are then used for metabolism and other cellular reactions. Starch, a major carbohydrate is converted to glucose which can be then synthesised into fat for storage, proteins are synthesised from amino acids, and phospholipids are made from glycerol and fatty acids. However there are some organic compounds which despite being essential for a healthy diet cannot be made by cells so must be provided by diet. These are essential amino acids, essential fatty acids and vitamins.

Carbohydrates

Carbohydrates are a rapid source of energy, they are the body's fuel. The bulk of a balanced diet should be made from carbohydrates. If eaten in an excess of the dietary requirements carbohydrates are easily stored as fats in the cells, although carbohydrate is the first source of energy in the body.

An average adult requires about 12,000kJ of energy a day, most of this is supplied by the respiration of carbohydrates in the cells.

Carbohydrates are used principally as a respiratory substrates, i.e. to be oxidised to release energy for active transport, macromolecule synthesis, cell division and muscle contraction. Carbohydrates are digested in the duodenum and ileum and absorbed as glucose into cells.

Sources of carbohydrates such as starch are rice, potatoes, wheat and other cereals. Sugars are also carbohydrates, sources of sugars are refined sugar - sucrose, which is a food sweetener and preservative and fruit sugars - fructose.

If the diet lacks carbohydrate stores of fat are mobilised and used as an energy source.

Lipids

Lipids are a rich source of energy in the diet, they can be greatly reduced in metabolic reactions and therefore release much energy. They are easily stored in the body and can form a layer beneath the skin of adipose tissue. As lipids are such a rich source of energy they are often not needed for respiration if there are adequate quantities of carbohydrate for the energy output of the body.

Meat and animal products are rich in saturated fats and cholesterol, plant oils are rich in unsaturated fats.

As lipids are digested in the intestine into fatty acids and glycerol, some fatty acids are only available in the diet and cannot therefore be synthesised in the cell in any way. These are therefore known as Essential Fatty Acids. Fatty acids are categorised according to the number of double bonds they have in their carbon chain. Saturated fatty acids have none, monounsaturated fatty acids have one, polyunsaturated fatty acids have more than one. Essential polyunsaturated fatty acids cannot be synthesised in the body from anything else as the correct enzymes to add double bonds after the ninth carbon to the carbon chain are not present. Two essential fatty acids are linoleic and linolenic acid which are found in vegetable oils such as soya, sunflower and maize.

Fatty acids are needed for the formation of cell membrane phospholipids and also for the production of steroid hormones such as prostaglandins and thromboxin which have important roles in the renal, immune and circulatory systems as signalling chemicals.

Deficiencies of essential fatty acids result in limited growth in children, poor healing of wounds, scaly skin and hair loss.

Obesity is a result of a high fat intake in the diet and lack of exercise. Obesity is in fact a form of malnutrition as the diet is not balanced. The risk of developing diseases such as diabetes, hypertension, CHD, arthritis (due to extra pressure on joints), stroke and some cancers are increased dramatically with obesity.

Proteins

Protein is not a direct source of energy in the body, it is used primarily for growth and repair of body tissues although can be used as an energy source as a last resort. Proteins fulfil a wide variety of roles in the body, they are broken down in the stomach and intestines to amino acids which are then absorbed. The body can only form 8 amino acids to build proteins from, the diet must provide Essential Amino Acids (EAAs) which are synthesised into proteins which can be structural, i.e. collagen in bone, keratin in hair, myosin and actin in muscle; metabolic enzymes, haemoglobin, protective antibodies and communicative hormones.

Sources of protein include meat, fish, eggs and pulses. The diet needs to provide 8 EAAs as the body is unable to synthesis proteins without these molecules. 2 other amino acids are synthesised from EAAs so if the diet lacks the original EAAs these other two will not be present either. Phenylalanine is converted to tyrosine and methionine is converted to cysteine. Cells draw upon a pool of amino acids