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Nutrition
In order to live and carry out all daily activities we need energy. Most of the energy supplied by our food and drink is used to maintain basic body functions, i.e. it keeps the heart beating, the blood circulating, and the lungs and other major organs, including the brain, constantly working. A variable amount of energy is needed in addition to the basal metabolism to account for activity – for most people this adds up to half as much as the basal requirement to the day’s needs.
Energy is measured in kilocalories (kcal) or kilojoules (kJ) (1 kcal is 4.18 kJ). Although scientists use the precise term kcal, most people simply call this unit a ‘calorie’, and adults generally require between 1500 and 3000 kcal or calories per day to maintain body weight, depending on gender, body size, and activity level. Taking in more energy than required (or not compensating for a high energy intake by increasing activity) results in weight gain; insufficient energy intake for the level of activity you do results in weight loss.
Energy is supplied by the major components of our food, i.e. carbohydrate, fats, and protein (sometimes called macronutrients), as well as by alcohol. One gram of pure carbohydrate or protein provides approximately 4 kcal, while fat supplies 9 kcal per gram and alcohol 7 kcal per gram; however, as few foods are composed of just one nutrient, the energy content of the food depends largely on the amount of water in the food and the proportions of protein, fat, and carbohydrate (including dietary fibre) making up the dry weight. For example, foods such as fruits and vegetables generally contain large amounts of water, no fat, little protein, and varying amounts of carbohydrate, but because of the high water content may have energy values as low as 1 kcal per gram, whereas foods with a high fat and low water content, such as nuts, will have energy values in excess of 6kcal per gram.
Dietary fibre, or non-starch polysaccharides (see below), are considered to be part of the carbohydrate component of foods but supply about 2 kcal per gram. A high dietary fibre content therefore acts to dilute the energy content of a food.
Functional Foods
These are foods that appear to have health benefits beyond the provision of nutrients and energy. A recent symposium on the topic gave the following definition ‘a food can be said to be functional if it contains a compound, which may or may not be a nutrient, that affects one or a limited number of functions in the body in a targeted way so as to have positive effects on health’. The health benefits may be physiological or may take the form of a positive psychological effect. Functional foods may be foods that contain the beneficial substance naturally, e.g. fruits and vegetables contain a variety of antioxidant substances that are not strictly nutrients but have beneficial effects: wholegrain cereals contain dietary fibre that may have beneficial effects on gut function and help prevent heart disease; soya beans contain phytoestrogens that may have beneficial effects as described above. However, increasingly food manufacturers are producing foodstuffs with ‘functional’ added ingredients that may be of benefit to health. For example, spreads with plant sterols or plant stanols added may help lower cholesterol levels; addition of specific bacteria, called probiotics, to yoghurts and yoghurt drinks, may have beneficial effects within the gut and beyond; and chewing gum containing phosphatidylcholine is claimed to improve memory.
The following is a table that explains the benefit of every element, and in what food it exists
| Mineral | Dietary sources | Main functions |
|---|---|---|
| Calcium (Ca) | Dairy products: cheese, milk. yoghurt. Bread and breakfast cereals. Fish eaten with bones, e.g. canned sardines, salmon, whitebait. Green, leafy vegetables. Pulses, e.g. baked beans, lentils. |
Building and maintenance of the skeleton (and teeth). |
| Phosphorus (P) | Milk and milk products, eggs, nuts, cereals, meat and meat products, vegetables, potatoes. Carbonated drinks. | Present in all cells of the body – 85% in skeleton. Important in energy transfer in the body. Dietary deficiency unlikely. |
| Magnesium (Mg) | Bread and cereals, beverages, such as beer and coffee, vegetables and potatoes, milk and milk products, meat and meat products. | Linked with Ca in bone development, protein synthesis. Part of many enzyme systems, e.g. in energy transfer. |
| Sodium (Na) | Salt added to food at table or in processing. Concentrated in foods such as ham, bacon, cheese, foods canned in brine, salted nuts, potato crisps or biscuits, yeast extracts, bottled sauces. Bread and breakfast cereals, meat and meat products, and milk also contain significant amounts. | Regulation of fluid balance and blood pressure. Na intakes are usually higher than desirable in developed countries. |
| Potassium (K) | Vegetables and potatoes, fruit, drinks, e.g. coffee (especially instant), milk and milk products, chocolate, cocoa, malted milk, yeast extracts, chutneys and pickles, cereals, meat and meat products. | Regulation of acid–alkali balance and fluid balance. Muscle and nerve function. (95% of body’s K is present in cells, and total body K is used to measure lean body mass.) |
| Iron (Fe) (Also see text) | Meat and meat products. Main source in UK diet is cereals and cereal products because fortified with Fe, followed by meat and then vegetables. | Oxygen carrier in blood and muscle, Enzyme systems for energy transfer. Dietary deficiency possible in women of childbearing age, especially adolescents; infants over 6 months and toddlers; people consuming unbalanced vegetarian diets. |
| Zinc (Zn) | Red meat, fish and shellfish, milk and dairy products, poultry, eggs. Cereals and bread, green leafy vegetables, and pulses are also good sources, butbio availability is lower compared with animal sources. | Important in many enzyme systems in the body and takes part in metabolism of protein, fat and carbohydrate. Component of insulin and growth hormone. Subclinical deficiency may occur when requirements are high but intake is reduced due to poor appetite; e.g. postsurgery or infection. Also possible if no animal products eaten and consumption of phytate-containing cereals is high. |
| Copper (Cu) | Rich sources are shellfish, liver, nuts, and cocoa. Main sources in UK diet are meat and meat products, cereals, vegetables and potatoes, beverages, e.g. tea and coffee. | Component of a variety of enzymes; contributes to elasticity of collagen and elastin, especially in blood vessels. Involved in antioxidant mechanisms in the body and in prevention of infection. |
| Selenium (Se) | Meats, cereals, vegetables, and fats in UK. Depends on Se content of soils, so in USA and Canada cereals will be a better source. Toxic in large amounts. Recommended upper limit from all sources 450μg. | Antioxidant mineral – glutathione peroxidase, an enzyme that protects tissues from oxidative breakdown, contains Se. May also be involved in protein, fat metabolism, and in thyroid function. |
| Iodine (I) | Milk, seafoods, and dried seaweeds Iodized salt. | Essential component of thyroid hormones that regulate metabolism. In fetus and infant, protein synthesis in brain and central nervous system is dependent on iodine. Deficiency is rare in Europe and the USA, but is still a problem in many parts of the world. |
| Fluorine (F) | Water, especially in tea | Bones and teeth. |