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Preface

This account of human nutrtion describes the basic facts in an clear andsimple way without the use of complicated details or much specialistlanguage. In the few places where more than this is necessary, elementaryexplanation are given. I believe that any averagely intelligent person willreadily gain a good knowledge of human nutrition from this book, which willalso be of value to students, teachers, nurses, doctors and healthprefessionals.

I would like to thank professors Anthony Angel and peter W. Andrews of theDepartment of Biomedical Scienc, University of shefiled, for the verygenerous facilities they provided during the several years it has taken to writethis book.Greald wiscman All the nenrgy needed for growth and repair of thebook. for ,usular activity of all kinds and for all the work done by cells comesfrom the metabolism of carbohydrate, fat, proten and alcohol. The numeriusother items of the diet, even though essenital for other reasons, do notprovide energy, although many are directly involved in the chemical reactionwhich vield enery (If the diet is adequate and properly balanced the energynormally comes chilfly from carbohydrate and fat, while most of the proteinis used for cell growth and repair. When there is not enough carbohydrateand fat, the protein is used for energy and is then not availabe for otherpurpoes. As dietary protein is generally less abundat than carbohydrate andfat and usually more expensive, using protein for energy is comparativelywastclit. In some communitics, however, there may be plentiful protein and itmay then be eaten in sufficient quantity to be used for both cell building andfor energy.

The intake of food is governed in health by the appetic which under ordinaryconditions controls the weitht of the body with remarkable precision. Manypeople taking only moderate care are able to keep their weight more or lessunchanged over several decades. If they take food in excess by only a smallamount, theat exvess energy can be disposed of as heat and thereby preventfat accumulation. This seems to work very efficiently in some people. It is,however, casy to over - ride the natural controlling mechanism and consumesubstantially more enery than is required. When this happens the excessenergy is stored in the body as fat.

During ageing there is a fall in the weight of the bones, due to loss ofminerals, plus a fall in the weight of the muscles, hence if the total bodywight remains constant there must be compensatory changes, mainly anincrease in the body fat.

Yhe ability of the body to override the mechanism which controls energyintake has survical value when the supply of food is unpredictable because itcnables fat to be accumulated when there is plenty of food and its energy tobe used later when food is scarce. How long a healthy adult can survicewithout food depends to a large extent on the fat stored: with adequatewater, prople have lived for many weeks. When people die during starvationthey often still have some fat in their body. They die because duringstarvation body protein is metabolised as well as body fat and it is the loss ofthe protein that is usually the cause of death.

Nutritional status

The nutriritional status of most people can be assecsed sufficiently well bytheir appearance, body weight and by simple questions about general health.For a more critical assesment their body mass index can be determined. Thisgives a witht for height ratio and is a good guide to underweight oroverweight in adults exvept for those who are extremely muscular or haveecessive accumulation of water in the body.

If weighting is not possible, an assessment can be made by measuring thecircumdrence of the upper arm with a tape - measure. A point midwaybetween the shoulder and the elbow is used with the arm at rest , preferablyhanging down. This simple measurement reflects the size of th3 underlying,uscles and the subcautaneous fat, as well as the bone and the skin. Inundernouished persons and in those overweight it will be the muscles and thefat which will change in bulk rather than the other tissues. For adult men ona satisfactory diet the circumference ranges from about 250-320 mm and forwomen from about 220-300 mm.

In children chronic energy lack causes a low height for age ratio, especially ifthe parents and siblings are of average height or more.

Energy content of food

1 g fat = 9 kral (38 kjoule)

1 g carbohydrate = 4 kcal (17 kjoule)

1 g protein = 4 kcal (17 kjoule)

1 g alcohol = 7 kcal (29 kjoule)

When carbohydrate, fat and alcohol are metabolised for energy in the bodythey are normally converted completely to carbon dioxide and water, withendrgy being released during the process. Protein metabolism nitrogencontatining substances in addition to carbon dioxide and water. By minickingthese ractions in laboratory experiment the energy value of any food can bemeasured and expressed as kilocablores (kcal or kilojoules (kjoule) per gramof the food. One kcal id equal to 4.18 kjoule. The energy values forcarbohydrate, fat and protein are approixmately 4 kcal per gram forcarbohydrate and protein and 9 kcal per gram for fat. For alcohol, the valueis 7 kcal per gram. Hence if the amounts of carbohydrate, fat, protein andalcohol in a meal are known, the energy value of the meal can be calcultedeasily.

Some foods ar energy - rich because the contain little or no water, fibre orother material which does not yield energy: examples ar metabolizable sugarsfats and oils. Foods with much water and dietary fibre are usually energy -poor. For example 100 g table suger (sucrose) will provide 400 kcal, whereas100 g of items such sd lettuce, timatoes of cucumber, which contain about 95per cent water plus fibre, will provide only about 20 kcal. Eating most saladitems instead of sugar, fats and oils reduces the energy intake.

The amount of carbohydrate, protein and fat in the diet varies graatly butan average national picture in 1983 showed that about 12 per cent of thedaily calories came from protein, about 46 per cent from carbohydrate andabout 42 per cent from fat. From then until 1996 the protein intake wasmore or less. constant, the carbohydrate fell by a few percent, while the fatcaten rose slightly, despite repeated advice that the fat content of theaverage diet was exvessive. A much healthier intake would be about 12 percent of calories from protein, about 58 per cent from carbohydrate and only30 per cent from fat. Many changing their habits. The food industry does notproduce on a mass scale a sufficient variety of attractive low - fat foods,particularly snacks.

Energy expenditure

Part of the energy produced by the body may be used for the production ofextra tissue during growth or tissue repair and this energy does not appear asheat. It is locked in the new tissue and although it can be estimated it is oftenignored. In Contrast, the rest of the metabolic energy, which in adults isvirtually all of it, does appear as heat and can be measured accurately inspecialized laboratories. The technique is called direct calorimetry. Becausethis requires special expensive apparatus and is very time consuming, theenergy produced by the body can also be calculated from the amount ofoxygen taken up and the carbon dioxide tiven off in the breath. This method,called indirect calorimetry, is easy, cheap and relatively quick. yhese basicexperiments on energy production were first carried ouit at the end of thenineteenth century and since then many measurements have been made ofthe energy produced by adults and children white resting or engaged in allsorts of activities.

The results of these investigations show that almost all normal adults needabout 500 kcal for the usual eight hours of sleep. yhe energy needed for eighthours of work and for eight hours of non - work, however, variesconsiderably, as would bd expected. People who do sedentary work requiringlittle physical activity need about 2200 kcal per 24 hours, those who domoderately active work need about 2500 kcal per 24 hours, while the fewwho undertake heavy work require 3000-3500 kcal per 24 hours. Moderatelylight housework needs only about 2000 kcal per 24 hours. Moderately lighthousework needs only about 2000 kcal per 24 hours but this goes up if thereare young children to care for, whon the amount of physical activity may begreatly increased.

The values given here and elsewhere for the energy expended duringdifferent activities are only guidelines and may vary greatly from subject tosubject and often in the same subject the same thing at different times.

Ennect of body weight

Tghe energy requirement of overweight people is usually less than that ofthin people of similar age. This is partly because in the overweight thethicker layer of fat under the skin reduces the bodys heat loss, so that lessheat production is needed to keep the body temperature normal, requiringless food to be metabolised. In addition, overweight people tend to be lessactive and therefore need to produce less energy. However, when overweightpeople are active, the extra weight they carry needs extra energy and theirfood requirement may go up very markedly.

Effect of age

During the process of ageing the energy requirment gradually decrascs. Inpart this is because in older people some muscle, which is metabolically veryactive, is ofthen replaced by fat. Yhe agening process is also accompained bya fall in the hormones which normally keep the metabolic rate high. etweenthe ages of about 20 years and 80 years the resting energy requirement fallsby an average of about 15 per cent. The total energy requirement of olderpeople may also decrease because many become less mobile, althought somedo remain remarkably active and may need a higher food intake than somemuch younger people.

Effdct of exercise

Thd amount of energy used during exercise is closely related to movement.Frequently moving the body greatly increases the energy used, especially ifthe body is lifted rather than merely moved horizontally. For using up energyand thereby using up body fat, most forms of exercise are not very good.

Effect of undernutrition

When food is plentiful the energy expended is replenished by eating. This iscontrolled by the appetite. Under these conditions it is easy to expend moreenergy if necessary. When food is scarce, however, and energy intake limited,physical activity is reduced to more or less match the reduced energy intak.Under very severe conditions the physical activity may fall so much the evenneccessary tasks may be abandoned and survical thereby threatened. It isclearly not useful to ask starving people to work harder, nor somebodydieting on a very low calorie intake to do more exercise.

After a peeiod of severe undernutrition the body may accumulate water(oedema), making body weight and inadequate guide to he degree ofwasting. When such a starving person is given a good diet the accumulatedfluid is exvreted in the urine causing a fall in body weight, which may alarmthe subject who, unless warned, expects to gain weight as soon as extra foodis eaten.

همة‌ انرژي‌ موردنياز براي‌ رشد و بازسازي‌ بدن‌ و اعم‌ از همة‌ انواع‌ فعاليتهاي‌ عضلاني‌ ونيز براي‌ سلولها انجام‌ يمش‌ ود از متابوليسم‌ كربوهيدراتها - چربي‌ - پروتئين‌ و الكل‌ناشي‌ مي‌شود. همه‌ نوع‌ كاري‌ كه‌ توسط‌ چندين‌ مورد ديگر از مواد غذايي‌ حتي‌ آنهايي‌ كه‌به‌ دلايل‌ ديگر ضروري‌ هستند، انرژي‌ فراهم‌ نمي‌كنند. هرچند بسياري‌ از آنها مستقيماً دربازتابهاي‌ شيميايي‌ كه‌ توليد انرژي‌ مي‌كند درگير هستند. اگر رژيم‌ غذايي‌ مناسب‌ بوده‌ وبطور صحيح‌ تنظيم‌ متعادل‌ نشده‌ باشد بطور طبيعي‌ بيشتر انرژي‌ از كربوهيدراتها وچربي‌ تأمين‌ مي‌شود درحاليكه‌ اكثر پروتئين‌ها براي‌ رشد و بازسازي‌ سلولها به‌ كارمي‌روند. زماني‌ كه‌ كربوهيدرات‌ و چربي‌ بطور كافي‌ وجود نداشته‌ باشد پروتئين‌ جهت‌تأمين‌ انرژي‌ مصرف‌ مي‌شود و لذا به‌ منظور برآورده‌ ساختن‌ اهداف‌ ديگر در دسترس‌نخواهد بود. همان‌ طور كه‌ مي‌دانيم‌ پروتئين‌ غذايي‌ نسبت‌ به‌ كربوهيدرات‌ و چربي‌ عموماًوفور كمتري‌ دارد و گرانتر است‌. استفاده‌ از پروتئين‌ به‌ منظور تأمين‌ انرژي‌ نسبتاً كارولخرجانه‌اي‌ است‌. در برخي‌ جوامع‌ با وجود اين‌، ممكن‌ است‌ پروتئين‌ فراوان‌ باشد و لذا به‌مقدار كافي‌ مصرف‌ گردد و براي‌ هر دو هدف‌ ساخت‌ سلول‌ و تأمين‌ انرژي‌ استفاده‌ گردد.

جذب‌ غذا از نظر بهداشتي‌ توسط‌ ميل‌ و اشتهايي‌ كه‌ تحت‌ شرايط‌ معمول‌ وزن‌ بدن‌ را بادقت‌ قابل‌ ملاحظه‌اي‌ كنترل‌ مي‌نمايد تعيين‌ مي‌شود و تحت‌ تأثير قرار مي‌گيرد. بسياري‌ ازمردم‌ تنها به‌ طور متعادل‌ عهده‌دار مي‌شود و قادرند وزنشان‌ را طي‌ چند دهه‌ كم‌ و بيش‌بدون‌ تغيير حفظ‌ نمايند. اگر آنها تنها يك‌ مقدار جزئي‌ اضافه‌ به‌ آن‌ غذا مصرف‌ نمايند. آن‌انرژي‌ اضافي‌ مي‌تواند در معرض‌ گرما قرار گرفتن‌ و به‌ موجب‌ آن‌ از تجمع‌ چربي‌جلوگيري‌ نمايد.

اين‌ به‌ نظر مي‌رسد كه‌ در برخي‌ از مردم‌ بسيار مؤثر عمل‌ مي‌نمايد. با اين‌ وجود آن‌ آسان‌است‌، تحت‌الشعاع‌ قراردادن‌ مكانيسم‌ و كنترل‌ قابلي‌ در مصرف‌ اساساً انرژي‌ بيشترنسبت‌ به‌ آنچه‌ كه‌ لازم‌ است‌ زماني‌ كه‌ اين‌ امر رخ‌ مي‌دهد انرژي‌ مازاد در بدن‌ به‌ شكل‌چربي‌ ذخيره‌ مي‌شود.

در طول‌ فرآيند پيرشدن‌، كاهش‌ وزن‌ استخواني‌ به‌ علت‌ كاهش‌ مواد معدني‌ علاوه‌ بركاهش‌ وزن‌ عضلات‌ وجود دارد از اين‌ رو اگر وزن‌ كلي‌ بدن‌ ثابت‌ باقي‌ بماند بايد تغييرات‌جبراني‌ - اساساً افزايش‌ در چربي‌ بدن‌ - بوقوع‌ بپيوندد.

توانايي‌ بدن‌ جهت‌ فايق‌ آمدن‌ به‌ اين‌ مكانيسم‌ كه‌ جاذب‌ انرژي‌ را كنترل‌ مي‌نمايد (انرژي‌حياتي‌ دارد) زماني‌ كه‌ تأمين‌ غذا غيرقابل‌ پيش‌بيني‌ است‌ زيرا آن‌ چربي‌ را قادر مي‌سازدكه‌ وقتي‌ مقدار زيادي‌ غذا وجود دارد تجمع‌ يابد و انرژي‌ آن‌ زماني‌ كه‌ غذا به‌ حد كافي‌موجود نيست‌ مصرف‌ گردد. اين‌ كه‌ افراد بالغ‌ تا چه‌ مدت‌ بطور سالم‌ مي‌توانند بدون‌مصرف‌ غذا (كه‌ معمولاً وقتي‌ چربي‌ زيادي‌ وجود داشته‌ باشد آب‌ كاهش‌ مي‌يابد) باقي‌بمانند تا اندازة‌ زيادي‌ به‌ چربي‌ تجمع‌ يافته‌ بستگي‌ دارد كه‌ به‌ همراه‌ آب‌ كافي‌ افراد تاهفته‌هاي‌ بسياري‌ زنده‌ مانده‌اند.

وقتي‌ كه‌ مردم‌ در طول‌ دورة‌ گرسنگي‌ كشيدن‌ مي‌ميرند آنها اغلب‌ هنوز مقداري‌ چربي‌ دربدنشان‌ دارند. يكي‌ از دلايلي‌ كه‌ اينها مي‌ميرند اين‌ است‌ كه‌ پروتئين‌ بدنشان‌ هم‌ مانندچربي‌ متابوليزه‌ مي‌شود دليل‌ مردن‌ آنها كمبود پروتئين‌ است‌. قبل‌ از اينكه‌ پروتئين‌هاي‌بدن‌ شما تمام‌ شود شما خود تمام‌ شده‌ايد) آنها مي‌ميرند به‌ دليل‌ اينكه‌ در طول‌ مرحلة‌گرسنگي‌ كشيدن‌ پروتئين‌ بدن‌ و نيز چربي‌ بدن‌ متابوليزه‌ مي‌شود و آن‌ كاهش‌ پروتئين‌است‌ كه‌ معمولاً سبب‌ مرگ‌ مي‌شود. كنترل‌ وزن‌ بدن‌ در فصل‌ 2 مورد بحث‌ قرار مي‌گيرد.

وضعيت‌ تغذيه‌اي‌: وضعيت‌ تغذيه‌اي‌ بيشتر مردم‌ مي‌تواند بطور كامل‌ خوب‌ از طريق‌ظاهر آنها وزن‌ بدني‌شان‌ و از طريق‌ پرسيدن‌ سؤالات‌ ساده‌اي‌ در مورد وضع‌ بهداشت‌عمومي‌ آنها ارزيابي‌ گردد. آنها مي‌تواند تعيين‌ گردد. اين‌ شاخص‌ كه‌ از تقسيم‌ وزن‌ بدن‌ بربلنداي‌ قد حاصل‌ مي‌شود يك‌ راهنماي‌ خوب‌ جهت‌ اضافه‌ وزن‌ داشتن‌ يا كمتر از وزن‌طبيعي‌ بودن‌ در افراد بالغ‌ به‌ استثناء آنهايي‌ كه‌ بي‌نهايت‌ عضلاني‌ هستند يا آنهايي‌ كه‌ ازتجمع‌ مفرطي‌ از آب‌ در بدنشان‌ دارند است‌. استفاده‌ از BMI در فصل‌ 2 چاقي‌ شرح‌ داده‌خواهد شد. اگر وزن‌ كردن‌ مقدور نباشد ارزيابي‌ مي‌تواند از طريق‌ اندازه‌گيري‌ محيط‌پايين‌ با يك‌ باندكشي‌ دستها صورت‌ پذيرد. نقطة‌ وسط‌ بين‌