Nutrition (11 page)

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Authors: Sarah Brewer

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BOOK: Nutrition
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In addition to rickets, scurvy, a disease caused by lack of vitamin C, is also making a comeback. In 2004 to 2005, 61 children were admitted to hospital with scurvy in England alone. By 2007 to 2008, this had risen to 94 – a 50 per cent increase.
Another example of micronutrient deficiency involves the mineral selenium. Intakes in Britain almost halved from 60 mcg in 1974 to just 34 mcg per day in 1994 as a result of using European flour for bread-making in place of selenium-rich flour from the United States. Although the full effects of this lack are not yet visible, it is known from other parts of the world with low intakes, such as areas of China, that lack of selenium lowers considerably immunity against viral diseases (e.g. influenza) as well as increasing the risk of Alzheimer’s disease, heart muscle weakness and many cancers. In fact, a scientific review of over 150 clinical trials published in the
Journal of the American Medical Association
found that lack of many vitamins is a risk factor for heart disease, stroke, cancer, birth defects, osteoporosis, bone fractures and other major chronic health problems. In an accompanying paper, the authors actually state that: ‘Pending strong evidence of effectiveness from randomized trials, it appears prudent for all adults to take vitamin supplements for chronic disease prevention.’
So although severe deficiency diseases such as scurvy are rare, this does illustrate why attention to nutrition is important to help reduce the niggling symptoms (tiredness, dry skin, reduced immunity) that can accompany a minor lack of key micronutrients. Take note of national guidelines, aim to eat your five portions a day of fruit and veg, eat more fish and wholegrains and select locally grown produce that is fresh from the field. If you know your diet is not as good as it could be, a vitamin and mineral supplement is a useful nutritional safety net. You may be surprised to know that, having analysed over a hundred food diaries, I’ve yet to find someone who gets all the vitamins, minerals and essential fatty acids they need from their food – despite dieticians advising patients that this is possible! In an ideal world, maybe it is. In the real world, it doesn’t seem to be the case.
What are vitamins?
Vitamins are molecules that are vital for life. They act as essential intermediaries, or catalysts, to support the enzymes involved in all metabolic reactions in the body. If your intake of any vitamin is low, metabolic deficiencies will eventually result. How quickly these develop depends on the state of your body’s stores. You store very little folic acid, for example, so deficiency symptoms may occur within weeks. On the other hand, vitamin B12 is stored in your liver, and it can take years for dietary deficiency to be detected. In general, the fat-soluble vitamins (A, D, E and K) are more readily stored than those that are water-soluble and are easily lost in the urine (vitamin C and most B-group vitamins apart from B12).
The EU Recommended Daily Amount (EU RDA), which is also believed to supply the needs of most (up to 97 per cent) of the adult population are shown in
Table 9
, together with average UK dietary intakes, and upper safe limits suggested by the UK Expert Group on Vitamins and Minerals (EVM) in 2003. The upper safe limits are the maximum daily amount of each vitamin that is considered safe to take long-term in the form of food supplements. These are in addition to the amounts typically obtained from your food, unless otherwise stated. Different RDA values may be used in non-EU countries, and during pregnancy and breastfeeding.
Nutrient
EU RDA
Average dietary intakes from food
EVM upper safe level for long-term use from supplements
Vitamins
 
 
 
Vitamin A (retinol equivalents) (betacarotene)
800 mcg
520 mcg
1,500 mcg
1
7 mg
Vitamin B1 (thiamin)
1.1 mg
1.5 mg
100 mg
Vitamin B2 (riboflavin)
1.4 mg
1.8 mg
40 mg
Vitamin B3 (niacin)
16 mg
34 mg
500 mg
Vitamin B5 (pantothenic acid)
6 mg
5.4 mg
200 mg
Vitamin B6 (pyridoxine)
1.4 mg
2mg
10 mg
Vitamin B12 (cobalamin)
2.5 mcg
6.2 mcg
2,000 mcg
Folic acid (folate)
200 mcg
2
260 mg
1,000 mcg
Biotin
50 mcg
33 mcg
900 mcg
Vitamin C (ascorbic acid)
80 mg
64 mg
1,000 mg
Vitamin D (cholecalciferol)
5 mcg
3
3 mcg
25 mcg
Vitamin E (tocopherol)
12 mg
8.5 mg
540 mg
Vitamin K
75 mcg
68 mcg
1,000 mcg
1
This upper safe level refers to total intakes from both diet and supplements
2
400 mcg folic acid for women who could become pregnant
3
For adults confined indoors (no sunlight exposure) and those aged 65 and over, 10 mcg per day is preferable
The following sections look at each of the vitamins in turn, explaining what they do, how much you need and the symptoms that can occur when you get too little from your diet. The average dietary intakes quoted are taken from the
Safe Upper Levels for Vitamins and Minerals
report, 2003, from the UK Expert Group on Vitamins and Minerals.
Vitamin A
Vitamin A is not just one, but a group of fat-soluble substances called retinoids. These include retinol and retinyl esters found in animal-based foods, and the plant-based precursors that you can convert into vitamin A such as the yellow pigment betacarotene.
Retinol was named after its first recognized function in the retina of the eye. Here, it is converted into the pigment rhodopsin (visual purple), which is involved in light detection. Vitamin A also acts as an antioxidant, but one of its most important functions is to enter the nucleus of cells where it binds to retinoid nuclear receptors and regulates which genes are switched on to make proteins. So many genes are controlled by vitamin A that it is essential for normal growth, development, immunity, sexual health and fertility. It helps to maintain healthy skin and moist mucous membranes in the lining of the eyes, nose, mouth, lungs and genital tract. It is also needed for healthy bones, teeth, wound healing and the production of immune cells.
Dietary sources of vitamin A
Normally, around 80 per cent of vitamin A in your diet is absorbed in the small intestines, along with dietary fats. Foods containing preformed vitamin A (retinol) include animal and fish liver; meat; oily fish and cod liver oil; eggs; milk and dairy products; butter and fortified margarine.
Foods containing pro-vitamin A carotenoids such as betacarotene, include yellow-orange fruit and vegetables and dark green leafy vegetables.
Vitamin A is easily destroyed by exposure to light, while betacarotene is destroyed by heat and overcooking. Boiling or frying food sources can reduce vitamin A content by 40 per cent after one hour and by 70 per cent after two hours.
Vitamin A is so important for child development that, in many countries including the UK, health officials recommend that children should ideally take a supplement providing vitamins A (together with C and D) at doses appropriate for their age up until at least the age of five. This may not be necessary where their diet is known to provide sufficient amounts of these micronutrients, for example from using a vitamin-enriched formula milk.
INTERNATIONAL UNITS
Vitamin A levels are sometimes given on labels as an International Unit (IU), which is a measure of biological activity rather than weight. This system was developed to take into account the fact that two or more substances may have vitamin A activity in the body (e.g. retinol, betacarotene), and measuring the presence of just one of these would give a false account of dietary intake.
1 IU of Vitamin A = 0.3 mcg retinol
1 mcg vitamin A = 3.33 IU
As it is fat-soluble, vitamin A is readily stored in your liver. Your blood levels are tightly controlled and do not alter substantially until liver stores are severely depleted.
Deficiency
As vitamin A is needed to make a pigment vital for sight (rhodopsin, or visual purple), one of the first signs of lack of vitamin A is loss of sensitivity to green light. This is followed by difficulty in adapting to dim light (night blindness). More severe retinol deficiency leads to dry, burning, itchy eyes, plus corneal hardening and ulceration – a condition known as xerophthalmia. Lack of vitamin A also increases the risk of cataracts. It is estimated that as many as half a million people worldwide go blind from vitamin A deficiency each year. Other symptoms of vitamin A deficiency include growth retardation, reduced male fertility, impaired hearing, taste and smell, increased susceptibility to infection, scaly skin with raised, pimply hair follicles and flaky scalp, loss of appetite and possibly kidney stones.
Vitamin A deficiency is uncommon in developed countries, except in people with reduced absorption, chronic liver disease and alcohol dependency. Worldwide, it is a public health concern in more than half of all countries, especially those in Africa and Asia where general malnutrition is rife and diets include few animal sources of retinol. Worldwide, an estimated 250 million preschool children have a vitamin A deficiency and, as a result, up to half a million children become blind every year, with one in two dying of severe infections within a year of losing their sight. This makes it a leading cause of preventable blindness and death in children. Where deficiency is present, high-dose vitamin A drops can reduce mortality from infections by a quarter overall, while halving the number of deaths associated with acute measles.
Toxicity
Vitamin A has a narrow therapeutic window and intakes of just double the recommended daily amount can cause health problems. As it is fat-soluble, retinol enters the nervous system and excess can cause headache, irritability, blurred vision, nausea, weakness and fatigue. Other symptoms of retinol poisoning include abdominal pain and loss of appetite. In the long term, excess vitamin A may cause hair loss, reduced bone-mineral density and increased risk of liver cirrhosis.
Vitamin A is vital for normal healthy development in the womb, but high intakes during pregnancy (3,000 mcg per day or more) is associated with an increased risk of birth defects, especially when exposure occurs during the first seven weeks of pregnancy. Pregnant women are therefore advised not to take supplements containing preformed retinol (including cod liver oil) or to eat liver products. High doses may also interfere with the function of vitamin K.
DANGER!
Avoid eating polar-bear liver, which contains so much retinol that consuming just 100 g can prove lethal.
Vitamin B1
Vitamin B1, also known as thiamin or thiamine, is a water-soluble vitamin that is readily lost via the kidneys. Most people only have stores sufficient to last one month. A regular dietary supply is therefore essential.
Vitamin B1 plays a central role in metabolism as it is needed for an enzyme (pyruvate dehydrogenase) to process pyruvate during the metabolism of glucose (glycolysis). The more carbohydrate you eat, the more thiamin you need. It is also involved in the synthesis of some amino acids, and the function of pancreatic beta cells (which secrete the hormone insulin). It also has a role during nerve-cell stimulation and may be important for regulating mood.
Dietary sources of vitamin B1
Food sources rich in vitamin B1 include unrefined wholegrains and oats; meat products – especially pork and duck; seafood; fruit, nuts and vegetables; dairy products; eggs; pulses, soybean flour; yeast extract.
Vitamin B1 is readily lost into cooking juices during food processing such as chopping, mincing, liquidizing, canning and preserving. Sulphur dioxide, a common preservative in minced meat, destroys 90 per cent thiamin content within two days, while sulphites used to keep processed potatoes white reduce their thiamin content by over 50 per cent. Flour is therefore fortified with thiamin in some countries to replace losses incurred during production. Both baking and toasting bread loses almost a third of its vitamin B1 content, while using baking powder increases losses to 50 per cent.

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