Food Groups in Human Nutrition

Introduction
Variety in food is very important for the maintenance of good health and prevention of disease. Fats, lipids, carbohydrates, minerals, trace elements, vitamins and water must all be present in adequate amounts in the daily diet. Recommended daily allowances (RDA) represent a reference base for nutrition intake levels of the essential nutrients judged to be adequate for meeting the known nutritional needs of the healthy population groups. They do not indicate individual requirements, which are variable.

A healthy diet should consist of varying amounts of the following food groups: cereals, legumes, vegetables, fruit, meat, fish and sea foods, eggs, milk and milk products, fats and oils, beverages.

Food definition: According the EU legislation (Regulation EC No. 178/2002) "food" (or "foodstuff") means any substance or product, whether processed, partially processed or unprocessed, intended to be, or reasonably expected to be ingested by humans. "Food" includes drink, chewing gum and any substance, including water, intentionally incorporated into the food during its manufacture, preparation or treatment.

Cereals
Cereals are the staple food of most diet, they may provide up to 2/3 of the energy intake in developing countries and about 1/3 in developed countries. The major cereal grains are wheat, rice and corn. Other cereals include barley, sorghum, millet, oat, rye. All cereals have similar content of main nutrients: 7-14% of protein, up to 75% of carbohydrates, and 2-7% of fat. Starch is the principal constituent of the grain. Cereals, especially wholegrain are an important source of potassium, calcium, magnesium, iron, selenium (if the soil they grew upon contains it), most of B vitamins, and fibre. Wholegrain flours are much richer in micronutrients than refined flours. Niacin in maize, is bound in a form that humans cannot utilize and there is only a little of tryptophan - the limiting essential amino acid in maize - from which humans can form niacin. For this reason pellagra may occur in populations relying mainly on maize. Pellagra today occurs only in very poor population groups or in emergency situations and refugees in Africa and Asia. It does not occur among the native people in America - its original home - due to the traditional treatment of maize with lime or wood ashes - mild alkali liberates the bound niacin .

Brown rice is rice from which only the hull has been removed. Milled rice, marketed as polished or white rice, is rice from which the hulls and most of the underlying layer (germ, pericarp, and aleurone) have been removed. Protein, fat, vitamins and minerals are present in greater quantities in the bran removed than in the remaining endosperm, which is composed mostly of starch. In populations relying primarily on polished rice vitamin B1 deficiency (beri beri) may occur. Beri beri, similarly as pellagra today occurs only in very poor population groups or in emergency situations and refugees .

Legumes
Legumes (pulses) include peas, lentils, beans including bean, and ground nuts. Legume seeds are a good source of protein and energy. Approximately 23% of the energy comes from the protein and 68% from carbohydrates. Boiled legumes provide approximately 1300kJ of energy per 100g (except for soya beans: 1600kJ/100g). Dietary fibre composes 10‐20% of dry legumes which may interfere to some extent with bioavailability of minerals. Legumes are a good source of vitamins B1, B2 and niacin, folic acid and iron (although the bioavailability of iron is lower than from animal foods). The fat content, except for soya beans, is low. Legumes do not contain fat-soluble vitamins or vitamin C.

Tubers
The main tuber in western culture is the potato. Potato is a good source of energy (300 kJ/100g) mainly from starch; protein content is low (2 g/100g) but its biological value is quite high, thiamine, iron, and fibre. It contains significant amounts of vitamin C and the amounts of potatoes consumed in the Czech Republic and some other countries make it a major source of this vitamin. Some of the vitamin C is lost during boiling. Potato chips do not contain vitamin C. Fried potatoes absorb fat during frying, therefore add to the fat consumption.

Sweet potato and yam are other important sources of energy widely cultivated across the tropical world. The tubers contain a large amount of starch and little protein of poor biological value. They are a good source of vitamin C and yellow varieties are a good source of carotene.

Root Crops
Sugar beet is a root crop, which is processed to sucrose (table sugar).

Cassava (manioc) is the staple food of many people in the tropics, despite the need to prepare it very carefully to prevent toxicity because it contains cyanogenic glycoside linamarin. The roots are traditionally ground, washed and dried to hydrolyse the major part of this toxin. The fresh root contains 50-75% of water, 1% of low biological value protein, and the remainder is starch.

Other root vegetables include turnips, carrots, beetroot, and parsnips. They have rather high water and low protein contents. The carbohydrates include simple sugars and some starch in the more mature roots. They contain relatively low amounts of dietary fibre and most other nutrients. Carrots are an exception - they are very rich in carotenoids. Beetroots are very rich in anthocyanin pigments.

Other Vegetables
Leafy vegetables are low in energy, high in carotenoids, vitamin C, folates, B vitamins, vitamin K, potassium, magnesium, and trace elements. Cooking in water leads to leaching and thermal loss of some of the vitamin C. Freezing involves minor losses of labile vitamins, canning produces greater losses and drying produces total loss.

Some foods commonly considered as vegetables are botanically fruits. Vegetables consumed as fruits include cucumber, courgette, marrow, pumpkins and squashes that have high water content, provide minor amounts of sugars and dietary fibre, in most other types the vitamin C content is low.

Fruits
Fruits are high in water which increases the bulk of food. They contain sucrose, glucose and fructose (in varying proportions in various fruits) and only negligible amounts of fat (avocado being an exception). The fruits as well as vegetables are primarily seen as a source of vitamins, especially C, although the concentrations in various fruits show wide variations. Yellow and red fruits are also a rich source of beta carotene - a precursor of vitamin A.

Fruits and vegetables are seen as protective foods preventing micronutrient deficiencies (such as scurvy) that may also lower the risk of some of the widespread non-communicable diseases. Depending on the type, vegetables and fruits also contain fibre and a number of substances not included among nutrients (e.g. salicylates, carotenoids, lycopene, polyphenols, phytoestrogens, flavonoids, and alginates) that may contribute to their protective role. WHO estimated in 2002 that low intake of vegetables and fruits contributes to 31% of ischaemic heart disease, 11% of stroke and 19% of gastrointestinal cancer worldwide. Low in energy with minimum of fat, they may replace some other higher-calorie food and thus may be useful in helping to lower energy intake and thus prevent obesity.

Meat
Meat is not an essential component of the diet as societies who have adopted vegetarian diet do not show signs of malnutrition, provided that the total supply of a variety of foods is adequate. The nutrition of millions of people in the world would however substantially improve if at least a small amount of animal foods were added to their diet.

Cattle, sheep, pigs, and poultry are the major sources of meat. Composition of all meats depends on its fat:lean ratio, which determines the energy value and the concentrations of all nutrients, because the nutrients are present in different concentrations in the fat and the lean. For this reason it is difficult to give typical nutrient values for meats as a whole. Triglycerides form the fat stores in subcutaneous adipose tissues, the abdominal fat, and the marbling fats between the muscles; phospholipids are in cell membranes and nervous tissues; glycolipids are in brain and other neural tissues; and lipoproteins in many tissues. The fat composition of ruminants is affected by the activities of the microflora in the rumen, which hydrogenate much of the ingested fat, so that the fats of ruminant animals are usually highly saturated. The fat composition in non-ruminants depends on the composition of the fats in the animal's diet. The fat content of wild animals is usually lower than that of comparable domesticated species.

Lean meat is an important source of high quality protein (around 20 g/100g). Meats are high in Mg, P and Fe (if not bled out at slaughter), zinc, and copper; they are relatively low in sodium and calcium. The micronutrients are highly bioavailable. Meats contain most of the B vitamins; they are especially important as a source of vitamin B12. The fat soluble vitamins are present in the fat, their concentration depends on the diet eaten by the animal.

Fish and Sea Food
Fish and sea foods are important sources of a good quality protein. Lean white fish, crustaceans and molluscs contain little fat. Fatty fish, especially sea fish are a substantial source of long chain polyunsaturated fatty acids important in the prevention of cardiovascular diseases (PUFA n-3) and fat soluble vitamins A and D. Sea fish and sea foods are a major source of iodine, and fluorine accumulated from the sea water. Fish eaten with bones (e.g. sardines) are also a good source of calcium. Fish and other sea creatures accumulate toxic substance such as heavy metals if they live in polluted waters. Oysters and some other sea creatures are an excellent source of Zn.

Eggs
Eggs are a rich source of excellent quality protein (similar amino acid composition as required for humans), lipids (phospholipids, high polyunsaturated fat, saturated fat) and (cholesterol 280mg/egg). They contain a wide range of vitamins and minerals. Protein avidin binds to biotin making it unavailable to man. Cooking denaturises avidin abolishing its effect.

Milk and dairy products
Milk and dairy products are rich in high quality protein (major protein is casein, milk also contains lactalbumin, immunoglobulins). Proteins are high in lysine, so milk products complement cereals. They are also a source of fat. Cow's milk fat content is about 3.9 g/100 g with a significant proportion of short chain fatty acids and low content of unsaturated fatty acids. Human milk also contains longer-chain polyunsaturated fatty acids.

Milk contains disaccharide lactose and is the only known source of this sugar in the diet. During the fermentation of milk the lactose changes to lactic acid. The increase of acidity inhibits the growth of many pathogenic organisms. Milk and dairy products are an important source of calcium, they also contain Na, K, Mg, P and both fat  and water soluble vitamins (especially vitamin B and C, but most of the vitamin C is lost during storage and heating). Concentrations of fat soluble vitamins are proportional to the fat content in milk and milk products - they are stable during the processing of milk. Concentrations of minerals in milk products are proportional to the protein concentrations.

Raw milk can be easily contaminated For this reason it is treated with pasteurization (heated to 72oC for 15 seconds), which is sufficient to kill all non sporing pathogens. Pasteurized milk needs to be stored in a refrigerator. Ultra high treatment (UHT) with temperature exceeding 135°C for 1 second kills even spores and the product has a long storage life. As raw milk spoils quickly, many milk products that could be stored and transported easily evolved over the times. Cheese making was one of the earliest ways of converting milk into concentrated food that could be stored. In cheese making, the milk is initially treated to form a curd, which is then processed in various ways. There are a very large number of different types of cheese and even the same type of cheese may substantially vary depending on the producer, so it is not possible to generalize about their composition. Some of the cheeses contain a substantial amount of salt (2-5 and even more g salt per 100g). Fermented milk products (yoghurt, kefir and many other) contain all constituents that are in milk with the exception of lactose. Separated milk fat is used to produce different types of cream. Butter is produced by churning milk to separate the fat.

Fats and Oils
Dietary fats are of both animal and plant origin. Chemically they are predominantly triacylglycerols (one glycerol molecule esterified with three fatty acid molecules). Oils are fluid at ambient temp while fats are solid. The melting point depends on the proportion of unsaturated fatty acids: the more unsaturated fatty acids, the higher melting point. For this reason most fats of animal origin are solid and of plant origin fluid. Energy content is 36kJ/g, making fats the richest source of energy. Fats and oils are a source of essential fatty acids (for example linoleic acid). They contribute to absorption of fat soluble vitamins. They improve palatability of food which constrains compliance when giving nutritional advice to reduce total fat intake.

Beverages
Water is essential in hydration of tissue, composition of extra-cellular fluid, prevention of renal damage and nephrolithiasis. Soft drinks are very high in energy. Fruit juices provide vitamin C and improve availability of Fe in a meal. Tea, coffee, cocoa contain caffeine (stimulant). Alcohol may provide good source of available iron whose absorption is enhanced by alcohol, it is however a substance of abuse.