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Vitamins as micronutrients

Vitamins are classified as a micronutrient – a substance that is required in very small quantities for normal health and development of an organism. This is in contrast to macronutrients, which are required in larger quantities, such as carbohydrates, lipids and proteins, that are required for structural components and as an energy source. Micronutrients include vitamins and microminerals (also known as trace elements), such as iron, cobalt, iodine and zinc. The role of vitamins is to provide various functions in the organism and a deficiency of a particular vitamin can result in illness or even death.

Most vitamins cannot be synthesised by the body and must be consumed from food. Fruits and vegetables are rich sources of many vitamins and a diet with a lot of variety is needed to ensure that all vitamins are being consumed. Other sources of vitamins come from supplements, or tablets, that contain either one or a mixture of different vitamins and minerals. It is possible to 'overdose' on a vitamin by consuming too much, resulting in a condition known as hypervitaminosis. For some vitamins, the body can excrete the excess in urine, but for other vitamins, the excess builds up in the body. For example, carrots are a common source of vitamin A and overconsumption of carrots can result in liver damage.

Types of vitamins

Vitamins are classified by letters – A, B, C, D, E and K. There are variations in the type of vitamin and each type performs different functions.

Vitamin A

Vitamin A compounds are known as retinol, retinal, retinoic acid and carotenoid. The structure of vitamin A compounds generally consists of a hexagonal ring with three methyl groups and a long conjugated chain consisting of nine carbons with a hydroxyl group on the end. Conjugated chains are hydrocarbon chains with alternating single and double carbon-carbon bonds. Conjugation generally results in a compound that is capable of absorbing visible or ultraviolet light, depending on the length of the chain. Since vitamin A has extensive conjugation in its long chain of nine carbons, many vitamin A compounds are yellow or orange. Plant sources of vitamin A include yellow-orange fruits and vegetables, such as carrots, sweet potatoes and mangoes.

Vitamin A has very low solubility in water, due to the presence of only a single polar hydroxyl group. Therefore, vitamin A is considered to be a 'fat-soluble' vitamin and is better absorbed from foods when taken with lipids. Excess vitamin A cannot be excreted in the urine, which is an aqueous solution, because vitamin A is nonpolar. Therefore, excess vitamin A is stored in fat tissues, such as the liver. Animal sources of vitamin A include butter, liver and cod liver oil, which are nonpolar.

Vitamin A performs many functions, the most important being vision health. The form of vitamin A that supports eye health is retinal, the word derived from the part of the eye that it supports, the retina. Retinal binds to proteins in the eye that are responsible for the growth and development of the rods and cones, the parts of the eye that respond to light. Vitamin A, in the retinoic acid form, is important in DNA gene transcription – the process of reading the genetic code in preparation of making proteins – and for skin health.

Vitamin C

Vitamin C is also known as ascorbic acid. The structure of vitamin C consists of a five-membered ring of four carbons and one oxygen. Attached to the ring are a carbonyl group, two hydroxyl groups and a short hydrocarbon chain that also contains two hydroxyl groups. Many plants and animals are capable of synthesising vitamin C from monosaccharides, but humans cannot, so vitamin C must be consumed. Plant sources of vitamin C include citrus fruits, berries and broccoli.

Since vitamin C contains many polar functional groups, it dissolves well in aqueous solutions, making it a 'water-soluble' vitamin. The hydroxyl groups form hydrogen bonds with water molecules, making it very soluble in water. Excess vitamin C is easily excreted in the urine, so an overdose of vitamin C is not common. Some excess vitamin C is stored in the liver, so animal sources of vitamin C include liver and some seafood, such as oysters. Vitamin C is insoluble in fat as it lacks a long hydrocarbon chain usually present in fat-soluble vitamins such as vitamin A.

Vitamin C plays an important role in overall health because it acts as an antioxidant. Antioxidants are compounds that quench reactions involving free radicals. Free radicals are chemical species that have an unpaired electron, which are responsible for damage to cells and tissues. The action of vitamin C as a reducing agent slows down the damage by providing electrons to free radicals, allowing the body to repair itself.

Vitamin D

Vitamin D consists of two compounds, D2 and D3, that consist of a steroid base with a hydroxyl group and hydrocarbon chain. Recall that steroids consist of four fused rings, but in vitamin D, one of the rings is broken. Very few foods contain significant levels of vitamin D; rather, it is easily synthesised from cholesterol, catalysed by light from the sun. Vitamin D supplements can be taken in a tablet form and some food items, including milk and breakfast cereals, are often fortified with extra vitamin D, although this varies from country to country. A natural source of vitamin D is eggs, which have the vitamin concentrated in the egg yolk.

Vitamin D has very low solubility in water, due to the presence of only a single hydroxyl group. The steroid base and hydrocarbon chain result in a compound that is mostly nonpolar, making vitamin D a 'fat-soluble' vitamin. As a result, vitamin D is carried to the liver by transport proteins for storage after it is synthesised in the skin. When vitamin D is later required, transport proteins carry it in the bloodstream from the liver to the destination.

Vitamin D plays a role in the absorption of certain microminerals, including calcium, zinc, iron, magnesium and phosphate. Without vitamin D present, these minerals cannot be absorbed in the intestine effectively, leading to malnutrition and poor bone development. Vitamin D also seems to play a role in increasing immunity against some viral infections, although the explanation is unclear.

Vitamin B

Vitamin B includes a class of eight different compounds, some of which are known as thiamine (B1), riboflavin (B2), niacin (B3) and folic acid (B9). The structures of the eight different compounds vary greatly, but all are water-soluble and many are found together in the same foods. Vitamin B compounds are generally used to catalyse metabolic processes by acting as a coenzyme, a compound that is required for an enzyme to function. Other B vitamins are chemical precursors to make coenzymes. For example, thiamine is a coenzyme required for the catabolism of carbohydrates and amino acids, and folic acid is required for proper neural tube development in unborn babies.

Plant sources of B vitamins include legumes, whole grains and bananas, and animal sources include turkey and tuna. The processing of whole grains, such as wheat, causes much of the B vitamins to be lost, so many processed grain products, like flour, are 'enriched', which means that the vitamins have been added back after processing.

Vitamin E

Vitamin E includes a group of compounds known as tocopherols. The chemical structures of the vitamin E compounds vary slightly, but generally consist of fused rings with a long hydrocarbon chain. The nonpolar nature of the compound makes vitamin E fat-soluble. Vitamin E compounds work as antioxidants and are found in plant-based oils, such as corn oil, soybean oil and wheat germ oil.

Vitamin K

Vitamin K is a group of fat-soluble compounds that play a very important role in blood clotting. Vitamin K compounds are found in dark green leafy vegetables, such as kale and spinach. Bacteria in the large intestine also synthesise some vitamin K compounds that can be absorbed. The structure of the vitamin K compounds consists of two fused aromatic rings and a long hydrocarbon chain, making the compound nonpolar and fat-soluble.

Summary

Vitamin deficiencies

When a balanced diet containing complete sources of both macro- and micronutrients is not available, the body does not have the nutrients required to undergo metabolic processes that are needed for growth and development, repair for damaged cells and tissues or to combat illness. When vitamins are not available in sufficient quantities, a vitamin deficiency results, which can lead to temporary or long-term health effects.

Sometimes a deficiency occurs when there is no available food source for a particular vitamin. When a crop suffers damage or poor growth from weather or disease, it can affect the global market for that particular fruit or vegetable, leading to vitamin deficiencies. Other causes come from the selection of foods that individuals choose to eat, due to medical, religious or economic reasons or problems with availability. Some diets are low or incomplete in certain nutrients, so care must be taken to ensure that those nutrients are available through supplementation.

Some storage and cooking methods also can have an effect on the concentration of the vitamin in the food. For example, many vitamins are broken down at high temperatures. The higher temperatures allow the vitamins to undergo oxidation reactions at a greater rate than they would at lower temperatures, changing the compound. Cooking methods that involve exposure to high temperatures for extended periods of time can result in a final food product that has destroyed the important vitamins it contains. Studies have consistently shown that frying, microwaving and boiling fruits and vegetables result in greater vitamin C loss than steaming.

Other factors include food storage or processing methods. Exposure to oxygen will also result in greater oxidation, so it is better to keep fruits and vegetables uncut until just before consumption. Exposure to light can also result in oxidation, so some foods are better kept away from light in paper bags or coloured glass or opaque containers. When fruits and vegetables are processed by drying, canning or freezing, the quality of the vitamins can also be affected.

Vitamin A deficiency

Vitamin A is critical in proper vision development and deficiency in vitamin A is the leading cause of blindness in children worldwide. Since vitamin A must be consumed from food sources, deficiencies are most common in developing countries where consumption of meat is low and there are few crops available that are high in vitamin A content, such as carrots, sweet potatoes or squash. Vitamin A deficiency can also occur when the liver is damaged from excess alcohol consumption.

There are several global initiatives by worldwide non-profit companies to eradicate childhood blindness by providing vitamin A supplementation. Vitamin A deficiencies can be improved with supplementation of oral or injectable vitamin A. Some foods that have a high fat content can also be fortified with vitamin A, a fat-soluble vitamin, such as oils, margarine and milk. There are also some genetically modified foods that have been developed that have increased levels of vitamin A, the most famous being 'golden rice', but the use of these foods is controversial. Genetically modified foods are a newer development and their health effects after long-term use are still unknown.

Vitamin D deficiency

Vitamin D is necessary for the absorption of several microminerals, including calcium, which is required for proper bone development in babies and young children. Since the majority of vitamin D is produced from cholesterol in the skin and catalysed by sunlight, exposure to low levels of sunlight or certain medical conditions can result in low levels of vitamin D synthesis in the body. When sufficient vitamin D is not present, calcium cannot be absorbed and children can develop a condition known as rickets, where the bones remain soft and flexible and weight-bearing bones can bend. In adults, vitamin D deficiency can result in osteoporosis, a decrease in bone strength, and has also been associated with greater risk of depression and certain cancers.

Since the body is able to synthesise vitamin D from cholesterol, a deficiency in vitamin D is not very common, but can occur in breast-fed infants and individuals that avoid the sun. Vitamin D deficiency can be treated with supplements and sensible exposure to the sun during daytime hours with uncovered skin. The use of sunscreen has not been associated with decreased vitamin D levels, but covering the skin with clothing and avoiding sun exposure does impair vitamin D production. There are not many food options that have significant levels of vitamin D, but eating oily fish or fortified milk can improve levels of vitamin D.

Vitamin C deficiency

Low levels of vitamin C can quickly result in a condition known as scurvy, one of the diseases classified by the World Health Organisation as a disease of malnutrition. Historically, scurvy affected sailors and soldiers that were away from sources of fresh fruits and vegetables for long periods of time, but still exists today as a disease of malnutrition. Vitamin C is an antioxidant that is important in providing electrons to the free radicals that cause oxidative damage to cells and tissues, so a deficiency in vitamin C causes effects all over the body. Early symptoms of scurvy include fatigue and pain in the gums, later progressing to loosening of the teeth, bone pain and poor wound healing, and eventually resulting in death.

Since vitamin C must be consumed from food sources, deficiency is most common in developing countries where food sources are limited, and also in industrialised countries where poor food choices are made, due to poverty, poor education or diets that contain restricted foods. Treatment includes supplementation with vitamin C and regular prevention by including vitamin C-containing foods in the diet, including citrus fruits, red peppers and berries.

Vitamin B deficiencies

The B vitamins are a group of compounds that work to catalyse a variety of metabolic processes in the body. Since many vitamin B compounds are found together in foods, poor nutrition can lead to a deficiency in one or more B vitamins, resulting in a variety of health problems.

Beriberi

Beriberi is a condition caused by a deficiency in thiamine (vitamin B1) and is part of a group of four diseases characterised by weight loss, oedema (swelling), weakness and impaired sensory perception. Historically, beriberi was observed in places where 'polished rice' was commonly eaten, that is, rice where the outer husk has been removed, leaving only the white grain. More recently, beriberi outbreaks have occurred in prisons where the prisoners had been fed a diet high in polished rice. Treatment includes supplementation with thiamine.

Pellagra

A deficiency in niacin (vitamin B3) results in a condition known as pellagra, one of the diseases classified by the World Health Organisation as a disease of malnutrition. Symptoms of pellagra include the 'three Ds' – diarrhoea, dermatitis (inflammation of the skin) and dementia (a decrease in mental performance and memory) – among other symptoms. If untreated, pellagra can result in death in 4-5 years, but treatment with supplementation is very effective in reversing many of the symptoms.

Historically, pellagra was first noted when maize (corn), originating in North America, became cultivated worldwide. Epidemics of pellagra seemed to follow wherever corn was planted, to the mystery of doctors, yet it was rarely seen in Mexico, where corn is a staple of the Mexican diet. The ancient practice of soaking corn overnight in lime water to soften it was routinely done in Mexico, but this tradition did not follow with the crop worldwide. Only later was the link made between soaking the corn in an alkaline solution and the prevention of pellagra, as the alkaline lime water released the niacin, making it better absorbed by the body. Like beriberi, outbreaks of pellagra have more recently occurred in prisons and orphanages where individuals are on a very limited diet that is incomplete in nutrients.

Vitamin E deficiency

A deficiency in vitamin E is not very common; however, it can occur with certain medical conditions where fats may not be absorbed properly. Since vitamin E is a fat-soluble vitamin, absorption of fat during digestion is required for absorption of vitamin E also. Since vitamin E is an antioxidant, a deficiency can result in various health problems including poor immunity to disease, vision problems and anaemia. Treatment for a deficiency generally involves supplementation.

Vitamin K deficiency

A deficiency in vitamin K can cause serious issues with blood clotting. Evidence of a vitamin K deficiency includes easy bruising of the skin, petechiae (red and purple spots on the skin) and uncontrolled bleeding. Since intestinal bacteria produce vitamin K, the occurrence of a deficiency is relatively uncommon and generally associated with certain medical conditions that involve poor absorption of vitamin K or a decreased ability for bacteria to colonise in the intestine. Newborn babies in the first days of life are at risk of a deficiency because they have not had adequate time to develop a healthy colony of intestinal bacteria, so it is standard practice in some countries to administer a vitamin K supplement to all newborn babies.