A Balanced Diet

A balanced diet provides an adequate intake of energy and nutrients for the 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 fiber 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 the growth and repair of many body tissues. These energy-providing compounds are needed in large quantities in our diet and 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 synthesized into fat for storage, proteins are synthesized 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 the 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 substrate, i.e. to be oxidized 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 mobilized 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 carbohydrates 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 synthesized in the cell in any way. These are therefore known as Essential Fatty Acids. Fatty acids are categorized 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 synthesized 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 is 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 thromboxane which have important roles in the renal, immune, and circulatory systems as signaling 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 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 the growth and repair of body tissues although can be used as an energy source as a last resort. Proteins fulfill 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 synthesized into proteins that can be structural, i.e. collagen in bone, keratin in hair, myosin, and actin in muscle; metabolic enzymes, hemoglobin, 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 synthesized 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 for protein synthesis which either come from dietary protein digested and absorbed in the gut and the breakdown of body protein such as muscle. However, unlike fats and carbohydrates, there is no store of amino acids for cells to draw on, any amino acid above immediate bodily requirements is broken down into urea and excreted. It is therefore important to maintain the dietary intake of protein every day. If the body lacks protein, muscle wasting occurs as muscle is broken down.

If protein is lacking in a diet a person develops kwashiorkor which is caused when high levels of carbohydrates are eaten to overcome the lack of protein in the diet. One symptom of kwashiorkor is the abnormal collection of fluid around the abdomen due to the lack of protein in the blood. The body cannot retain water by osmosis and fluid accumulates in tissues causing them to become waterlogged.

Vitamins

Vitamins cannot be synthesized by the body so must be supplied by the diet. Vitamins have no common structure or function but are essential in small amounts for the body to be able to utilize other dietary components efficiently.

Vitamins fall into two categories, fat-soluble vitamins such as vitamin A, D, E, and K which are ingested with fatty foods, and water-soluble vitamins such as the B group vitamins and vitamin C. Vitamins are known as micronutrients because only small quantities are required for a healthy diet fat-soluble vitamins can be toxic in high concentrations, for example, the body stores vitamin A, or retinol, in the liver as it is toxic if kept in high concentrations in the bloodstream, a dose of more than 3300mg of vitamin A can be considered toxic. Water-soluble vitamins such as vitamin C and B groups vitamins can be excreted in the urine if in excess in the diet.

Vitamins carry out a wide range of functions and prevent specific deficiency diseases. A diet that lacks a certain vitamin is not balanced, vitamins have vital roles in the maintenance of a healthy body.