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- Herausgeber: Crowood
- Kategorie: Lebensstil
- Sprache: Englisch
Nutrition for Marathon Running describes how the running body works and explains why performance in a marathon is so highly influenced by what a runner eats and drinks. The special dietary requirements of vegetarians, females, veterans and diabetic runners are all covered as well as information about running in hot and cold climates. By the end of the book, readers should be in no doubt just how important diet is in the training for, and running of, a marathon. The author addresses the following questions and many more: How can I keep hydrated on long training runs? What is the best thing to have for breakfast on the morning of the marathon? Are sports drinks really better than plain water? Why do I always seem to get a cold after running a marathon? What happens at the drinks station on route? The marathon is in the morning but I do all my training in the evening: will this matter?
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Veröffentlichungsjahr: 2012
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NUTRITION FOR MARATHON RUNNING
JANE GRIFFIN
First published in 2005 by The Crowood Press Ltd, Ramsbury, Marlborough, Wiltshire, SN8 2HR
www.crowood.com
This e-book edition first published in 2012
© Jane Griffin 2005
All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.
ISBN 978 1 84797 405 1
Acknowledgements
To my husband Chris for his love as well as his patience and support – again! To my children Daniel and Jessica for their love, interest and encouragement. To my brother Robin for sound advice. To the thousands of marathon runners I have met whose questions about diet inspired me to write this book.
‘An empty sack won’t stand up’
Old African saying
Contents
Title Page
Copyright
Epigraph
Introduction
1 The Running Body
2 The Training Diet
3 Training Fluids
4 The Marathon Countdown
5 Ergogenic Supplements
6 Special Considerations
7 Running into Problems
Appendix I: Dietitians in Sport and Exercise Nutrition (DISEN)
Appendix II: Useful Addresses and Websites
Index
Introduction
You may have picked up this book because you are thinking about running a marathon, have already started training for your first marathon or have run one that went very wrong. Your motive for running 26.2 miles may be to lose weight, generally get fitter and healthier, or raise money for charity. Perhaps you were inspired watching Tracey Morris run and win her first marathon at the 2004 Flora London Marathon and then go on and finish twenty-ninth out of eighty-one starters in the Women’s Marathon in the Athens Olympics. Whatever your reasons, I hope this book helps you to achieve your personal running goal.
Evidence continues to mount on the benefits of physical activity for the well-being of body and mind. The protective effects are as strong as not smoking and include reduced risk of heart disease, cancer, diabetes, osteoporosis and, of course, overweight and obesity. Regular running may help you cope better with the daily stresses of life and promote healthy sleep patterns. The running body is like a high-performance car. You would not use low-grade petrol and oil in such a car. You would not neglect to top up the battery or keep the tyres at the right pressure or the bodywork clean and sparkling. Similarly, the running body needs the right foods and fluids to provide energy and all the essential nutrients to keep the muscles and nerves functioning efficiently, the joints supple and flexible, and the body well-hydrated.
This book has a brief introduction to nutrition and then explains how the running body works. The next two chapters give practical advice about what, when and how much to eat and drink to maximize performance. Chapter 4 helps you to prepare for the marathon itself, including giving the low-down on carbohydrate loading, how to manage water stations efficiently, and what to do in the days after the marathon. After a short chapter on ergogenic supplements (things that may or may not help you to perform better), the next chapter covers specific issues you need to consider if you are a vegetarian, female, veteran, diabetic or a wheelchair marathoner, or if you are planning to run a marathon in a cold or hot climate. Finally, ‘Running into problems’ gives advice on dealing with certain difficulties that you hope will not happen.
When I ran the London Marathon, in 1986, at about 20 miles I encountered one of these difficulties. A huge amount of my training had been done in the evening and none before mid-day. My body was stunned by the early start and, although my legs knew what to do, my gastrointestinal system was totally confused. Accessible loos were few and far between. On a positive note, I have the marathon to thank for my husband’s interest in cooking. As my Sunday training runs got longer and longer, my children were in pyjamas before Sunday ‘lunch’ was ready. So Chris took over and to this day still cooks our Sunday lunch. Happy running and happy eating.
Jane Griffin, Sports Dietitian and Nutrition Consultant
CHAPTER 1
The Running Body
Food provides the body with the energy and nutrients it needs for growth and development, to meet daily lifestyle requirements and maintain health. Food choices making up a runner’s daily diet must meet all these requirements, otherwise running performance will suffer, risk of infection and injury will increase, and there may even be long-term health implications. Not only do nutrients perform different functions in the body but the type and amount of nutrients varies from food to food. Understanding what energy and nutrients do in the body and which foods are good sources of particular nutrients will help a runner to build up the best possible diet. That diet should maximize performance, both in training and races, and keep the body fit and healthy. It must also be practical and fit into the lifestyle and daily schedule of the runner – and of course it must be enjoyable!
NUTRIENTS AND THEIR MAIN FUNCTIONS
Food is made up of carbohydrate, fat, protein, vitamins, minerals and water. In some foods, particularly fruits and vegetables, a very large proportion is water; in others, such as oils and fats, the water content is minimal. The amounts and indeed the presence of the different vitamins and minerals can vary considerably between foods, too. This is one of the reasons why health professionals are constantly encouraging the general public to eat a diet containing a wide variety of different foods.
Carbohydrate and fat are the major sources of energy or fuel for the body. Protein has the unique function of providing the material for the growth and repair of the body and is also an important component of enzymes, hormones and antibodies. When the diet contains more protein than is needed it contributes to the overall energy pool of the body. If insufficient carbohydrates and fats are available to meet energy demands, protein can be used to meet the shortfall.
Vitamins and minerals are essential components of the diet. Although the majority of them are needed in very small amounts they do play vital and often very different roles in the diet. Vitamins are a diverse group of substances that are needed for the regulation of chemical processes in the body. Although not a source of energy in themselves, many of them are involved in the release of energy from food. They cannot be made in the body in sufficient amounts to meet requirements and so must be provided by the diet. Minerals are also essential nutrients, which must be supplied by the diet. Like vitamins, they fulfil many functions. They help to control the composition of body fluids, and are constituents of bones and teeth and essential components of enzymes and proteins such as haemoglobin.
ENERGY
Terms Used
It is a myth that energy is ‘good’ because it helps to fuel activities such as running and calories are ‘bad’ because they are fattening. In that case, an energy bar will help a runner but a chocolate bar will make him or her fat! Energy can be measured in calories or joules. For the scientifically minded, one calorie is the amount of heat needed to raise the temperature of 1g of water by 1°C. The calorie is a very small unit, so the Calorie, which is 1,000 times greater than the calorie, has always been used for nutritional measurements. The kilocalorie is the same as the Calorie. This avoids any confusion between calorie and Calorie (initial capital ‘C’) and the possibility of printing errors on food packaging. Kilocalories have been replaced by the general unit for measuring energy, the kilojoule. Although it is more scientifically correct to use ‘kilojoules’, ‘kilocalories’ still remains popular with the general public and both units are used in food labelling.
CONVERTING CALORIES AND JOULES
Energy Needs
The actual amount of energy needed varies from individual to individual. It is primarily determined by the basal metabolic rate and level of physical activity. The basal metabolic rate (BMR) is the rate at which an individual uses energy to maintain all the bodily functions at rest. This includes the energy needed to keep breathing, keep the heart beating and the blood circulating, maintain the body temperature and the brain functioning. BMR is measured when the body is at complete rest.
In adults, the BMR is proportional to the lean body mass. As men tend to have more muscle than women, their BMR is usually higher. Older adults usually have a lower BMR than younger adults as a result of the decrease in muscle (normally) with increasing age. This of course is one of the many advantages of maintaining a high level of physical activity throughout adult life. Muscle is an active tissue and therefore has an energy requirement. This means that older runners will need to eat and enjoy more food than their more inactive friends and relatives. On average, the BMR accounts for about 75 per cent of an individual’s energy needs.
The thermic effect of food (TEF) represents the increase in energy expenditure that results from digesting, absorbing, metabolizing and storing food during the day. This is the source of the myth that says that it takes more energy to digest an egg than the egg actually contains. TEF accounts for about 10 per cent of the twenty-four-hour energy expenditure. Adaptive thermogenesis (AT) is the smallest component of total energy expenditure (although it would have been a larger component hundreds of years ago before central heating was invented). It represents the energy needed in times of environmental stress such as seasonal temperature changes. The amount of energy expended being physically active is the most variable. Sitting and sleeping for most of the day will probably require no more than 100kcal whereas an average of 2,800kcal will be needed to complete a marathon distance.
Dietary Energy
All the energy needed comes from diet. Food is digested, absorbed and metabolized to release energy, which the body can then use. On a weight basis, carbohydrate, fat, protein and alcohol provide variable amounts of energy. Water on the other hand has no calorific value. 1g of protein provides 4kcal (17kJ), 1g of fat provides 9kcal (37kJ), 1g of carbohydrate provides 4kcal (17kJ) and 1g of alcohol provides 7kcal (29kJ). Almost all the weight of a food is made up of these components, plus water. Therefore foods that contain a large percentage of water, such as fruits and vegetable, will have relatively fewer calories. Fatty foods, such as butter, margarines and oils, which contain little water, will be rich in calories. In fact, most foods are a mixture of nutrients and the total energy value of a food is the sum of the energy from each of the nutrients.
Energy Storage
Carbohydrate can be stored in a limited amount in the liver and muscles as glycogen. Protein in excess of the daily requirement cannot be stored but instead may be converted in the liver into glucose and used as an immediate energy source, stored as glycogen for later use, or stored as fat. Excess fat is stored as adipose tissue. The capacity for the body to store fat is unlimited. Fat cells can just get bigger and bigger.
CARBOHYDRATES
Carbohydrates occur in the diet as simple carbohydrates or sugars and complex carbohydrates or starches. Most dietary carbohydrate is plant in origin, the exception being lactose, the sugar found in milk. The main sources of simple carbohydrates are fruits and fruit juices, milk and milk products, honey and sugar. They are identifiable by their sweet taste. Sources of complex carbohydrates include bread, rice, pasta, potatoes, breakfast cereals, pulses and sweetcorn. Runners have a much greater requirement for carbohydrate than the general public and as training increases more carbohydrate is needed. Most runners will find that they need to include some of the more sugary sources of carbohydrate as well as the more bulky starchy varieties.
After digestion, glucose from sugars and starches is absorbed into the bloodstream and transported to the liver. Some of the glucose passes directly to all the cells of the body to be used for energy. Some is converted into glycogen and stored in the liver and muscles as a readily available source of energy. Some may be converted into fatty acids and stored as adipose tissue. Carbohydrates are the main source of energy for exercising muscles, the brain and the central nervous system. The brain needs a regular and constant supply of glucose to function. When carbohydrate availability is low, through starvation, very prolonged submaximal exercise or a minimal intake of carbohydrate, ketones released from the metabolism of fat are used and they can make up to, but no more than, 50 per cent of the brain’s energy requirement. Many experts see very low carbohydrate intakes as positively harmful. High levels of ketones in the blood lead to the abnormal state of ketosis, which, apart from the health aspect, can leave the dieter feeling nauseous, light-headed and suffering from rather nasty halitosis (bad breath). If the diet is low in carbohydrate a greater percentage of dietary protein is used to provide glucose so there is less protein available for growth and repair of body tissues. Carbohydrate therefore has a protein-sparing effect, which is particularly important for those involved in regular physical activity. Runners following the fashionable low-carbohydrate high-protein diets are likely to feel tired, lethargic and irritable as they become deprived of stored carbohydrate, the body’s best source of energy for exercise.
DIETARY FIBRE
Fibre was originally referred to as ‘roughage’, its technically correct term is ‘non-starch polysaccharides’ (NSP), but ‘dietary fibre’ is the term with which most people are familiar and this remains the term that is most commonly used in food labelling. Fibre is the major component of plant cell walls. It is resistant to the action of digestive enzymes.
Most dietary fibre comes from fruit, vegetables and cereals. In wheat, maize and rice the fibre is mainly insoluble. Insoluble fibre helps to keep the bowels functioning healthily and regularly. As it absorbs water, it tends to swell in the gut, making the gut contents heavier and as a result causing them to move more quickly through the digestive system. This can help to relieve constipation and other bowel disorders. Soluble fibre occurs more frequently in oats, legumes, leafy vegetables and some fruits, particularly apples. It is thought to help reduce blood cholesterol levels and to slow down the absorption of blood glucose in some types of diabetes.
Although those in the general population are being encouraged to increase their intake of dietary fibre, runners who are already including more carbohydrate in their diet may find that their intakes of dietary fibre are already enough without adding a lot of high-fibre foods. A gut that works very quickly is not always a benefit to a runner. In some instances it can be a positive embarrassment.
GLYCAEMIC INDEX
The glycaemic index (GI) of food is a measure of that food’s effect on blood glucose levels. The concept was first developed in 1981 by Dr David Jenkins, a professor of nutrition at the University of Toronto, Canada, to help patients with diabetes minimize the rise in their blood glucose after a meal. The glycaemic index classification is described as a ranking of foods based on their actual postprandial blood glucose response compared to a reference food, either glucose or white bread. It therefore reflects the speed of digestion and absorption of a carbohydrate-rich food.
However, the level of the GI of a food is not the complete picture as it is not related to portion size. For example, parsnips have a high GI but 400g of parsnips (the amount needed to provide 50g carbohydrate) would have been used for the measurement. The average portion size of parsnips is only 65g, which will contain about 8g carbohydrate. Such a small amount of carbohydrate would not have a particularly significant glycaemic effect. A more practical way of using the GI is therefore to take into consideration the usual portion size of the food.
The glycaemic load (GL) is the GI of the food multiplied by its carbohydrate content in grams. Certain foods may be more suitable for eating before running, others more suitable for refuelling after a run or race, depending on their GI or GL. The practical aspects of using the GI and its relevance to a runner’s diet will be dealt with in more detail in the next chapter.
FAT
Fat is an essential nutrient and no attempt should be made to exclude it from the diet. It is an important source of energy and it supplies essential fatty acids such as linoleic acid and alpha linolenic acid, which the body cannot manufacture (hence the term ‘essential’). Fat provides insulation and cushioning for the internal organs and serves as a carrier for fat-soluble vitamins (vitamin A, D, E and K) and fat-soluble antioxidants such as beta-carotene and other carotenoids. Certain essential fats are vital for the formation of hormones.
Food would taste bland without some fat in the diet as many of the flavours, smells and textures are linked to the fats in food. However, an excess amount of fat in the diet is increasingly recognized as one of the risk factors influencing the development of chronic diseases. The main concern centres on its potential role in contributing to obesity and all the associated health risks of obesity, such as heart disease. Runners who maintain a high level of physical activity still need to keep a check on their total fat intake. Enjoying a high-fat diet could mean that vital carbohydrates get pushed out of the diet. If a high carbohydrate intake is being achieved, together with a high fat intake, then even a runner will not be immune to weight increase as overall energy intake exceeds energy output.
SOURCES OF FAT
Cheese
Nuts, olives, avocado pearsHydrogenated fats and vegetable shortening (check food labels)Whole milk – silver or gold topSome types of cakes and biscuitsCreamEggsCreamy puddings and cheesecakeFat on meat, poultry skinMeat products – pies, pasties, sausages, burgers, pate and salami, tinned meatsMayonnaise, salad cream and creamy saucesOily fishChips, crisps and roast potatoesPeanut butterChocolate, toffee, fudgeGood and Bad Fats
Advice has always been to reduce the amount of saturated fats in the diet because they are primarily responsible for raising blood cholesterol levels. Although this still holds true, importance is now also being placed on the inclusion of other types of fat because of their positive health properties. The body uses cholesterol to build cell membranes as well as brain and nerve tissue. However, the body gets all it needs from cholesterol made in the liver and transported round the body in blood. Because blood is mainly composed of water, which does not mix with fat, cholesterol travels around the body attached to specific proteins or lipoproteins.
There are two types of lipoproteins: low-density lipoproteins (LDL) and high-density lipoproteins (HDL). Cholesterol is carried in the blood by low-density lipoproteins (LDL) from the liver to various tissues in the body but on the way cholesterol can also be deposited in the arteries. It is these deposits that lead to an increased risk of developing heart disease. High-density lipoproteins (HDL) carry cholesterol from the tissues back to the liver, where it is broken down. The balance of LDL and HDL cholesterol is as important as the total blood cholesterol level. Low levels of LDL and high levels of HDL cholesterol are best. Regular physical activity helps to increase the level of HDL cholesterol in the body.
Saturated Fats
A high intake of saturated fats is associated with an increase in blood cholesterol. Main sources of saturated fats are butter, cheese, meat, meat products such as sausages and hamburgers, full-fat milk and full-fat yoghurt, pies, pastries, lard, dripping, hard margarines and baking fats, coconut and palm oil.
Monounsaturated Fats
Monounsaturated fats lower LDL and so help to reduce the risk of heart disease. However, this beneficial effect may simply be the result of these fats replacing the saturated fats in the diet. Main sources of monounsaturated fats are olives, rapeseed, nuts (pistachio, almonds, hazelnuts, macadamia, cashew, pecan), peanuts, avocados and their oils.
Polyunsaturated Fats
These fats can be further divided into the omega-6 and omega-3 families. Omega-6 fats are derived from linoleic acid. Omega-3 fats, which include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), are produced from alpha-linolenic acid (ALA). Linoleic acid and ALA are called essential fatty acids as the body cannot make them and the diet must provide them. A diet rich in omega-6 fatty acids and low in saturated fat has been linked with a reduced risk of coronary heart disease. Advice is now also to eat more omega-3s as these are thought to have a positive impact on heart health as well as having an important role in brain and eye function. Good sources of omega-3 polyunsaturates are salmon, mackerel, herring, sardines, trout (all of which are particularly rich in EPA and DHA), eggs laid by hens consuming diets rich in omega-3 (for example, Columbus eggs), and walnuts, rapeseed, soybean, flax seed, and their oils (all particularly rich in alphalinolenic acid). Good sources of omega-6 polyunsaturates are sunflower seeds, wheatgerm, sesame seeds, walnuts, soybean, maize and their oils and certain margarines; information is given on the tubs. Of particular interest to runners will be the anti-inflammatory properties of omega-3 fatty acids and their potential to relieve symptoms of stiffness and pain in joints (seeChapter 7).
Unsaturated fats can also come in different chemical structures – a bent ‘cis’ form or a straight ‘trans’ form. Most unsaturated fats come in the cis form but in the meat and milk of ruminants (cows, sheep and deer), and in products containing industrially hardened oils, some of the unsaturated fats will exist in the trans form. These are called trans fatty acids and, like saturated fats, they increase blood cholesterol levels. Some frying and baking fats (hydrogenated vegetable oils) used in biscuits, cakes, pastries, dairy products and fatty meat from beef and sheep will contain these trans fatty acids.
Dietary Recommendations
Most European guidelines now suggest that overall fat intake should be no more than 30–35 per cent of total calories, with no more than 10 per cent of calories coming from saturated fats. This means that the remaining 20–25 per cent of calories should come from mono- and polyunsaturated sources. It is also important to make a positive effort to include more omega-3 polyunsaturated fats in the diet and keep trans fats to a minimum.
CHOLESTEROL
Cholesterol is used by the body to make hormones (including the sex hormones), vitamin D, bile salts and to maintain the structure of cell membranes and it is also involved in protecting nerve fibre. Sources of cholesterol in the diet are egg yolks, offal (liver and kidney), shellfish and fish roes. Although some foods are rich in cholesterol, up to 95 per cent of the cholesterol in the body is made from dietary saturated fat. In terms of lowering blood cholesterol levels, general advice is to lower saturated fat intake rather than try to eliminate all dietary cholesterol.
PROTEIN
Proteins are an essential constituent of virtually every cell in the body, accounting for about one-fifth of the total body weight. They perform vital structural functions in the body and are constituents of muscle, connective tissue (bone, cartilage, tendons and ligaments), skin and hair. Proteins are responsible for growth and development and on a daily basis they are involved in rebuilding, repairing and maintaining vital tissues. They also have a regulatory role. Enzymes and hormones are proteinous by nature and together they regulate tissue and cell metabolism. For example, insulin is a protein that monitors blood glucose levels. Some proteins (immunoglobulins) are important in the functioning of the immune system and so help to fight off infection. Other proteins work as transporters, moving fats and minerals around the body. Oxygen is transported in the blood to all cells by the protein haemoglobin.
Although these are the primary and unique functions of protein, it can also be a source of energy. Protein cannot be stored in the body so if more is consumed than the body needs, some of the protein molecule is broken down and excreted in the urine as urea and the rest is either used for energy or converted to fat and stored.
SOURCES OF PROTEIN
Amino Acids
Amino acids are the building blocks of protein. Like carbohydrates and fats, amino acids contain carbon, hydrogen and oxygen but they also contain nitrogen and occasionally sulphur. Amino is the chemical name for the combination of nitrogen and hydrogen in these compounds. All the proteins needed by the body can be made from just twenty different amino acids. Some amino acids can be made from others (these are called non-essential amino acids) but there are some that cannot be made by the body, which have to be supplied by the diet. These are known as essential amino acids. Cysteine and tyrosine are sometimes called semi-essential amino acids as they can only be made from the essential amino acids methionine and phenylalanine.
CLASSIFICATION OF AMINO ACIDS
Sources of Protein
Some foods contain more protein than others but actually quality is just as important as quantity. It is not just the amount of protein that matters, but also which amino acids the protein contains and whether they are present in sufficient amounts. Plant proteins tend to be low or deficient in one or more essential amino acids and therefore generally have a lower biological value than animal proteins. Animal foods (for example, meat, fish, eggs, milk and cheese) have high protein contents with high biological value. Pulses (for example, soybean, kidney beans, chickpeas, lentils and peanuts) are foods with a very high protein content with high (soya) or medium biological value. Cereals (wheat, rice, barley, maize and oats) are foods with medium protein content with medium (rice) or low biological value. Nuts (hazelnut, cashew, almond, walnut) are foods with high protein content but low biological value. Starchy roots (cassava, potato, yam and sweet potato) are low in protein and have negligible biological value. Vegetables and fruits are low in protein and are not really considered dietary sources of protein, although of course they make a valuable contribution to the diet in other ways.
Excess Protein
A high-protein diet increases the workload of the kidneys because of the extra nitrogen that must be excreted. This does not seem to be a problem in otherwise healthy people but might be problematic in physically active individuals who already have increased fluid losses through sweating.
VITAMINS AND MINERALS
Vitamins
Vitamins are complex organic substances that are needed in very small but vital amounts for many of the processes that go on in the body. Although only a few milligrams (mg) or even micrograms (μg) are needed each day they are absolutely essential for health. Most vitamins cannot be made in the body and must therefore be supplied by the diet. The exception is vitamin D, which is manufactured in the body by the action of sunlight on the skin. In addition, small amounts of niacin, a B vitamin, can be made from the amino acid tryptophan.
Vitamins perform a variety of functions in the body; some are co-factors in enzyme activity, some are antioxidants and vitamin D is a pro-hormone. A consistently poor intake of vitamins over a period of time can result in the development of a deficiency disease. Vitamin-deficiency diseases are rare in the developed world, although they still occur in some parts of the developing world. Vitamins are traditionally grouped into two categories: fat-soluble, which are stored in the body (vitamins A, D, E and K), and water-soluble, which cannot be stored in the body (vitamin C and the B complex vitamins). Any water-soluble vitamins consumed in excess of requirements are normally excreted via the kidneys in the urine.
Minerals
Minerals are inorganic substances that are needed by the body for a variety of functions. They help to build and maintain strong bones and teeth, transport oxygen around the body, regulate water and acid–base balance, activate and form essential parts of enzymes and hormones, fight infections, maintain healthy levels of haemoglobin in the blood, release energy from food, transmit nerve impulses and relax and contract muscles. Most minerals are excreted and must therefore be supplied regularly in the diet.
While deficiencies in essential nutrients can be harmful so, too, can excesses. Excessive intakes of some minerals can lead to problems ranging from nausea and vomiting to hypertension, irregular heartbeat or skin and kidney damage. An imbalance in the proportion of one mineral may cause a corresponding deficiency in another one.
How Much Is Enough?
In 1991 the UK Department of Health published guidelines for the intake of nutrients using a new term, the Reference Nutrient Intake (RNI) (Department of Health, Dietary Reference Values for Food Energy and Nutrients for the United Kingdom, London: HMSO, 1991 (Report on Health and Social Subjects; No. 41)). The RNI was defined as the amount of a nutrient that is enough for almost every individual, even someone who has significant needs for that nutrient. The amount is therefore considerably higher than that which most people need. You are most unlikely to become deficient in a nutrient if you consume the RNI.
VITAMINS
Vitamin A is found in animal foods as retinol. Plant foods contain beta-carotene, the precursor of vitamin A. Richest sources of vitamin A are fish liver oils (cod liver oil) and animal liver (lamb, calf and pig). Good sources of vitamin A include oily fish (mackerel, herring, tuna, sardines and salmon), egg yolk, full-fat milk, butter, cheese and fortified margarine. Good sources of beta-carotene are fruit and vegetables, especially orange ones (carrots, apricots), dark green ones (spinach, watercress and broccoli) and red ones (tomatoes and red peppers).
Main functionsEssential for healthy skin. Maintains healthy mucous membranes in the throat and nose. Protects against poor vision in dim light. Antioxidant properties.DeficiencyVery rare in the UK. In developing world, deficiency is a major cause of blindness.RequirementsReference Nutrient Intake (RNI) for adult men is 700μg per day and for adult women 600μg per day.Excessive intakesRegular intakes of retinol should not exceed 7,500μg for adult women, 9,000μg for adult men and 3,300μg for pregnant women. Women who are or might become pregnant are advised by the Department of Health not to take vitamin A supplements or eat liver, as excessive amounts can be toxic and dangerous to the unborn child.Vitamin B1 (thiamin)Major food sourcesCereal products such as breakfast cereals, bread, pasta and rice, lean pork and peas, beans and lentils.Main functionsRelease of energy from carbohydrate. For normal functioning of nerves, brain and muscles.DeficiencyVery rare in the UK. Causes beri-beri, which affects the heart and nervous system.RequirementsRNI for adult men is 1.0mg per day and for adult women 0.8mg per day. (Dependent on the energy content of the diet; RNI is set at 0.4mg per 1000kcal for most groups of people.)Excessive intakesChronic intakes in excess of 3g per day are toxic in adults.Vitamin B2 (riboflavin)Major food sourcesMilk, egg yolks, liver, kidneys, cheese, wholemeal bread and cereals and green vegetables. Sensitive to light.Main functionsRelease of energy from carbohydrate, fat and protein.DeficiencySores in the corners of the mouth. Severe deficiency unlikely in the UK.RequirementsRNI for adult men is 1.3mg per day and for adult women 1.1mg per day.Excessive intakesAbsorption of riboflavin in the intestine is limited, so toxic effects unlikely.Niacin (nicotinic acid, nicotinamide, vitamin B3)Major food sourcesMeat, poultry, fortified breakfast cereals, white flour and bread, yeast extracts.Main functionsRelease of energy from protein, fat and carbohydrate.DeficiencyRare.RequirementsRNI for adult men is 17mg per day and for adult women 13mg per day.Excessive intakesVery high intakes in the region of 3–6g per day may cause liver damage.Vitamin B6 (pyridoxine)Major food sourcesMeat, particularly beef and poultry, fish, wholemeal bread and fortified breakfast cereals.Main functionsNeeded for protein metabolism, central nervous system functioning, haemoglobin production and antibody formation.DeficiencyDeficiency signs are rare.RequirementsRNI for adult men is 1.4mg per day and for adult women 1.2mg per day.Excessive intakesHigh intakes have been associated with impaired function of sensory nerves. Amounts involved have varied from 50mg per day to 2 to 7g per day.Vitamin B12 (cyanocobalamin)Major food sourcesFound only in food of animal origin (liver, kidney, meat, oily fish, milk, cheese and eggs). Some breakfast cereals are fortified with vitamin B12. Some vegetarian foods are fortified, for example, soya protein, soya milks, yeast extract.Main functionsRed blood cell formation, maintenance of nervous system and protein metabolism.DeficiencyPernicious anaemia (blood disorder).RequirementsRNI for adult men and women is 1.5μg per day.Excessive intakesExcreted in the urine and therefore not dangerous.Folic acid (folates)Major food sourcesLiver, kidney, green leafy vegetables, wholegrain cereals, fortified breakfast cereals and breads, eggs, pulses, bananas and orange juice.Main functionsRed and white blood cell formation in bone marrow. Essential for growth. Protection against neural tube defects (spina bifida) pre-conceptually and in early pregnancy.DeficiencyMegaloblastic anaemia (blood disorder).RequirementsRNI for adults is 200μg per day. Women who might become pregnant or pregnant women during the first twelve weeks of pregnancy are recommended to take an extra 400μg per day.Excessive intakesDangers of toxicity are very low.Biotin and Pantothenic acidMajor food sourcesWidespread in food.Main functionsRelease of energy from fats, carbohydrates and protein.DeficiencyUnlikely.RequirementsNone set.Excessive intakesNo danger.Vitamin CMajor food sourcesFruit and vegetables especially blackcurrants, strawberries and citrus fruit, raw peppers, tomatoes and green leafy vegetables. Potatoes because of the amount eaten.Main functionsFor healthy skin, blood vessels, gums and teeth, wound healing, iron absorption and formation of antibodies. Important antioxidant.DeficiencyScurvy. Mild deficiency leads to tiredness, bleeding gums, delayed wound healing and lowered resistance to infection.RequirementsRNI for adults is 40mg per day.Excessive intakesIntakes at levels of twenty times RNI or more have been associated with diarrhoea and increased risk of oxalate stones in the kidney.Vitamin D (cholecalciferol)Major food sourcesFortified margarines and spreads, fortified breakfast cereals, oily fish, egg yolks, full-fat milk and dairy products. Main source of vitamin D is the action of UV light on the skin.
