If you eat a varied diet, with good amounts of fresh fruit, vegetables, wholegrains, beans and, ideally, some meat and fish, it is possible to get all the vitamins you need from your diet.
A vegetarian diet can be lower in some vitamins, such as vitamin B12 and vitamin D, but is higher in others such as vitamin C and vitamin K. Having said that, national dietary surveys consistently show that significant numbers of the population do not get all the vitamins and mineral they need from their food. Even when average intakes seem adequate, it is important to remember that an average is just an average – half the population are getting more and half are getting less.
If you are cutting back on food intake to lose weight, or avoiding key foods because of intolerances, a vitamin and mineral supplement can act as an important safety net.
Vitamin A in the retinol form is obtained from eating animal and fish livers, kidneys, oily fish, eggs and dairy products. You can also convert some carotenoid pigments, found in dark green leaves and yellow-orange fruits, into vitamin A. Of around 600 carotenoids present in fruit and vegetables, around 50 can be converted into vitamin A, mostly in your intestines, and these are said to have pro-vitamin A activity. Of these, alphacarotene, betacarotene, gammacarotene and cryptoxanthin are the most prevalent in the diet. Recent studies show this conversion is less efficient that originally believed, and it takes around 12mcg betacarotene to yield 1mcg of retinol. In the average Western diet, plants provide less than 30% of dietary vitamin A intakes, while animal products account for over 70%.
B Vitamins are obtained from yeast extracts, wholegrains, seafood, meat, pulses, nuts, eggs, dairy products and green leaves. Pork and duck are especially good sources of vitamin B1 (thiamin), milk stored in cartons is a good source of vitamin B2 (riboflavin) but light destroys it, which is why little milk is now bottled in glass. Eggs are a great source of vitamin B3, royal jelly is one of the richest sources of vitamin B5 (pantothenic acid), while soy products, bananas, walnuts and avocado are good sources of vitamin B6 (pyridoxine). Vitamin B12 is mainly found in liver, kidney, oily fish (especially sardines) and red meats. The only plant products to contain consistent amounts of vitamin B12 are blue-green algae and fortified breakfast cereals. Folic acid is the synthetic (monoglutamate) form of the naturally occurring folate vitamin (polyglutamate form) found in green leafy vegetables and wholegrains. Biotin is widespread in the diet and is also produced by probiotic bacteria in the bowel, from which it can be absorbed.
Vitamin C is found in all fruit and vegetables, especially guava, citrus, berries, blackcurrants, kiwis, peppers, green leaves and sprouts. Most animals make their own vitamin C, but humans and other primates lack the enzyme (L-gulonolactone oxidase) needed for its synthesis. Why we lost, or never acquired, the ability to synthesize vitamin C remains one of the greatest mysteries of human biochemistry. It is thought to have resulted from a genetic accident millions of years ago. Because our ancient ancestors ate a vegetarian diet full of vitamin C-rich plants their vitamin C intake was several fold higher than ours (estimated at 392 mg a day) so this genetic accident did not pose a survival threat.
Vitamin D occurs in five different forms, the most important of which are vitamin D2 (ergocalciferol), derived from plants, and vitamin D3 (cholecalciferol) derived from animals. These two forms are collectively known as calciferol. Food sources of vitamin D3 include oily fish, fish liver oils, animal liver and fortified dairy products. You can also make some vitamin D3 in your skin from a reaction between 7-dehydrocholesterol and UVB ultraviolet sunlight (290-315 nm spectrum). This only occurs when the UV index is greater than 3 so there is a wide seasonal and regional variation in the amount of vitamin D produced by people in different countries. Those living at a latitude of 520N (which passes through the centre of the UK and Canada) receive too little UVB radiation to make vitamin D between the months of October and April. Those living at a latitude of 420N (which forms the northern limit of Spain and part of the border between Canada and North America) are unable to synthesis vitamin D between November to February. Low vitamin D status is therefore widespread at northern latitudes, and, with the exception of Norway (where intakes of vitamin-D rich fish are high) most Europeans have low vitamin D levels during winter.
Vitamin E consists of two groups of fat-soluble compounds, the tocopherols and tocotrienols. Until recently, eight fat-soluble substances were known to have vitamin E activity: four tocopherols (alpha, beta, gamma, delta) and four tocotrienols (alpha, beta, gamma, delta). In 2009, a ninth substance, delta-tocomonoenol, with vitamin E activity was identified in kiwifruit skin. Alpha-tocopherol is the main source of vitamin E in the European diet, while gamma-tocopherol is the most common form in the American diet. Food sources of vitamin E include oily fish, wheatgerm and olive oils, avocado, butter, wholegrain cereals, nuts, seeds fortified margarine and dairy products, liver and eggs.
Vitamin K occurs in three different forms, as phylloquinone (K1), menaquinones (vitamin K2) and menadione (vitamin K3). Ninety percent of your dietary intake is in the form of vitamin K1, and just 10% in the form of K2. Probiotic bacteria in the gut also produce some vitamin K2 which you can absorb and use. Good dietary sources include cauliflower, broccoli, and dark green leafy vegetables such as spinach and some lettuces which provide more than 100 mcg vitamin K1 per 100g. Useful amounts are also found in yoghurt (produced by the bacteria present), egg yolk, alfalfa, safflower, rapeseed, soya and olive oils, fish liver oils, liver, tomatoes, meat, potatoes and pulses.
For a full overview of each individual vitamin, click on the links above or see the A – Z of vitamins.
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|My Essential Guide to Vitamins, Minerals and Herbal Supplements offers a complete overview of each vitamin, mineral, dietary oils and herbal remedy.|
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