Food: The Definitive Guide - Brossura

Coultate, Tom; Davies, Jill

Brossura

ISBN 10: 0851864317 ISBN 13: 9780851864310

Casa editrice: Royal Society of Chemistry, 1994

Riassunto

This book is a definitive guide to food, nutrients and diet and sets out in a clear, concise manner everything needed to provide an in-depth introduction to the field. It provides a single point of reference and is an indispensable aid to all those studying food-related subjects. Food: The Definitive Guide is unique in addressing both the key issues concerning diet and the scientific facts about the constituents of food. The book introduces the techniques of dietary self-evaluation and provides a listing of weights of food portions, with their nutrient content. Food: The Definitive Guide is written in a highly readable manner and will appeal to students of home economics, catering, nutrition, dietetics, and food science and technology, as well as to lay persons with a healthy interest in what they eat. It will also be of great value and interest to teachers, health professionals and those wanting to know about the scientific background to present day dietary advice.

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Food The Definitive Guide

By Tom Coultate, Jill Davies

The Royal Society of Chemistry

Copyright © 1994 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85186-431-0

Contents

1 Introduction, 1, 
2 What are people eating?, 4, 
3 The quality diet, 15, 
4 Choosing food, 24, 
5 Fats (and oils), 33, 
6 Carbohydrates, 50, 
7 Proteins, 69, 
8 Minerals and vitamins, 82, 
9 The non-nutrients, 105, 
10 Not just nutrients, 127, 
11 Evaluating your diet, 142, 
Index, 163, 

CHAPTER 1

Introduction

Nowadays more and more people appear to be moving into the pollsters 'Don't Know' classification when it comes to the great issues of the day; but there is one issue that is an exception – Food. Quite rightly, almost everyone has an opinion on the food they eat, or don't eat. We have likes and dislikes, fads and fancies, compounded with views on what is 'good for you', what is 'full of vitamins', what 'rots the children's teeth', what is 'full of chemicals' and so on.

We acquire these opinions from a host of sources, starting with the food we ate as children. It may be illogical to reject cabbage for the rest of one's life just because of what the School Meals Service used to to it 30 years ago, but this is no sillier than any of the other innumerable prejudices that colour our approach to life, and certainly no easier to change. As we go on we add opinions from many other soruces. These include school teachers, cookery books, government booklets, supermarket pamphlets, food product labels, advertising and articles in magazines and newspapers. This is where the problem starts.

When, in previous centuries (the 'good old days') most of the people of this country were underfed, issues of diet and health were very straightforward. The best advice was simply to eat as much as you could of whatever you could get! Nowadays a high proportion of our more affluent population is in a position to afford more than they actually need, and informed choices must be made. The accuracy and nature of the information upon which our choices are based have become critical to the relationship between health and diet.

The basic information must come from scientists. Some do carefully designed experiments that test the effects of this or that additive on a collection of laboratory animals. Others review the incidence of diseases, such as cancer, in different communities in relation to what is eaten, to illuminate the links between diet and health on a broader scale. These scientists' findings are published in obscure scientific journals or intimidating government reports which do not provide the consumer with the information needed in a palatable form; but are the alternatives any better? In spite of the soundness of content and excellent presentation there is often a tendency amongst the public to ignore government leaflets, not least because of our healthy cynicism towards all authoritarian pronouncements. At the other extreme are the well-meaning evangelists whose books on 'health foods' are too often based on a blend of half-baked science and unsubstantiated anecdotes.

Some of the best information is to be found in the leaflets at the supermarket checkouts, but we are wary of them because we take for granted a vested interest in promoting their own products. Television programmes can also get accurate information across to consumers, especially when the science is blended with other popular issues of consumerism and new recipes! Perhaps the problem is that scientists are frequently poor communicators, and the best journalists and other media folk rarely have A level chemistry on their c.v. This book is an attempt by two scientists, and food enthusiasts, to present the scientific basis of food in a style that consumers will find 'digestible'.

To start with we need to appreciate just what constitutes today's diet, and understand the factors that influence the choice we make in the supermarket, the canteen or the 'take-away'. We need to be able to recognize what should be in what used to be called a 'good balanced diet' and how this can be achieved in the context of a wide range of ancient and modern 'isms'. The language of diet uses terms like 'non-starch polysaccharides' and 'eicosapentenoic acid', but this essentially chemical terminology cannot be dismissed purely because we cannot pronounce the words, let alone understand them. Scientists discuss with each other the effects of particular substances such as these on the properties of individual foodstuffs, but we all tend to forget that farmers do not grow 'substances'. Neither do we ship 'substances' to famine- stricken countries nor entertain our friends to an evening of 'substance' intake. Contrary to the futuristic dreams of a few years ago, the only people who do consume synthetic mixtures of nutrients, divorced from traditional concepts of what food should be like, are hospital patients whose normal digestive processes are out of action. Even astronauts eat food the rest of us would recognise.

If our food is going to continue to satisfy both our biological needs for nutrients, and our human needs for food that tastes, smells, looks and feels good, then we need to understand how the 'substances', the chemical components of foodstuffs, can meet both types of need. We must also understand the effects, both good and bad, of operations such as cooking and preservation.

Diet is personal and the information included in this book is designed to help readers to review their own diets. However, the data is not intended to do more than guide. As the reader will discover, a full-blown study of a person's total nutrient intake is rarely practicable and requires far more background data than can be accommodated in a book of this type. Nevertheless, guidance is provided to enable changes to be made towards a healthier diet.

CHAPTER 2

What are people eating?

Does it matter what we eat? What is the use of knowing about food intake? In the very early days of surveying food intake the focus was on food requirements, with particular concern for the classic nutritional diseases. More recently, the emphasis has been on nutritional epidemiology, having regard for possible associations between diet and diseases such as cancer of the gastrointestinal tract and cardiovascular disease. As a result of this interest in food requirements and the association of diet with disease, methods have been developed to assess the food intake of individuals and of groups of people. These methods are called dietary surveys.

Dietary surveys

It may sound surprising, but there is no generally accepted method of measuring the dietary intake of people following their daily pattern according to their own life-style (free-living individuals)! However, it is agreed that to find out what people are in the habit of eating, their diets must be those to which they are accustomed and chosen freely. Some methods for assessing food intake are not practical for use with free-living individuals.

There are five main methods that are commonly used to assess the dietary intake of free-living people:

1 record of food intake with food weights

2 record of food intake with estimates of food weights

3 24-hour recall

4 diet history

5 food frequency questionnaires

The first two methods provide records of actual food consumption, made when the food was eaten. The three remaining methods focus on food intake in the past. The 24-hour recall method gives information about food intake the previous day. Diet histories and food frequency methods provide information on recent food intake or food intake in the distant past. Before we move on to discuss some of the inherent problems associated with the methods identified, in terms of obtaining accurate results, it is probably helpful to give a brief resume of what the different methods entail.

Record of food intake with weights

The most accurate method for assessing food intake involves the measurement and recording of the weight of food consumed at the time it is eaten. The recording of food intake at the time of consumption involves the subject in weighing their food before they eat it. It is necessary for subjects to be provided with a sturdy and convenient set of scales. Ideally the scales should be electronic, weighing up to 2 kilograms and accurate to the nearest 1 gram. Scales with a taring facility are not recommended as they can lead to confusion. It is very important to demonstrate the weighing procedure to the subject using their own plates, bowls and cups. This is usually done by the process of cumulative weighing. The individual foods to be eaten are put onto, or into, an appropriate serving vessel such as a plate, bowl or cup, which is positioned on a dietary scale set at zero, and a record is made of the cumulative weights in a diet record diary. From the resulting figures it is possible to calculate the quantities of the foods served.

Taking account of the fact that people do not always eat everything that is served, a record is made in a similar fashion of the food items remaining. So, for example, if some peas and roast potatoes remained on the plate after a meal of roast beef, Yorkshire pudding, roast potatoes, peas, carrots and gravy, it would be necessary to record the weight of the plate with the food items on, then remove the potatoes and note the reduction in the weight, and remove the peas and again note the change of weight.

People taking part in dietary assessment exercises need to state clearly what the food eaten was. It would be of little use if the roast beef described above was merely recorded as meat and the roast potatoes as just potatoes! In the case of home-made foods it is essential to keep a record of the recipes used, as these may differ from those given in standard tables of food composition. In instances when branded foods are eaten it is important to keep a record of the brand and if possible the actual food label. Food record diaries vary in design, but essentially they need to facilitate the recording of individual recipes and details about food brands, and even the name of a retail outlet where a particular food was purchased.

It is usual to give participants a written set of detailed instructions so that everything is crystal clear from the outset right through to completion. As a further safeguard the subject is visited by the investigator, or they go back to the investigator during the course of the record to make sure that all is in order. At the end of the period under investigation the investigator checks the food record diary with the subject. It is important to do this immediately afterwards while things are relatively fresh in the subject's mind.

So far so good, but what about food eaten away from home? Students of nutrition will know only too well of the attention that weighing foods publicly in the refectory and other similar venues brings. Moreover, the burden of carrying portable scales around all day long is a nuisance. Therefore, in this type of survey it is usually acceptable to have a description of foods eaten away from home. Better to have a description than no record at all, and such flexibility within the procedure reduces the risk of interference with normal eating habits. The likely weight of the food can usually be established by purchasing and weighing a duplicate portion of the food in question.

A new technique which involves the use of a portable electronic set of tape recording scales (PETRA) is available. The procedure is straightforward as it is only necessary to press a button and dictate a description of the food items into a microphone. The machine has a capacity of 2000 by 1 gram, and food can be served onto normal plates and the cumulative weight is recorded automatically. This method is particularly useful for people who are not able to write. Moreover, once the information is recorded it is difficult for any tampering of results, as may occur with diet record diaries when subjects are ashamed of what they have eaten!

The next leading question is the duration of the study period in order to obtain meaningful results. This depends on factors such as which nutrients are of particular interest, whether the study is focusing on individuals or groups of people and, if the latter, how many people? In clinical investigations, if the energy-yielding nutrients are under scrutiny, a 7-day record is sufficient. Other dietary components including vitamins, minerals and fibre (non-starch polysaccharides) require longer periods of observation, of the order of about 16 days. This does not have to be done as a single run, 4-day records at four different times are acceptable. When groups of people are the subject of investigation for the purpose of research, it may be adequate to record food intake for 3 days, randomised to cover each day of the week.

Record of food intake with estimates of food weights

As an alternative to the method of weighing it is possible to estimate the weight of food eaten. This can be done by using models of foods, standard portion sizes, food replicas and household measures. It is very easy to underestimate or overestimate food portion sizes when this method is used.

24-hour recall

The 24-hour recall method involves asking the subject questions such as 'what did you have for breakfast today?' This method for assessing the diet in the recent past is not highly regarded in nutritional circles, on a point of accuracy. In recent studies looking into this it was found that 12 to 35% of the daily nutrient intake was overlooked.

Diet history

The taking of a diet history is deemed to be practical when it is not feasible to study people at home. The method consists of two main stages:

1 Assessment of the overall pattern of eating, together with a 24-hour recall of foods actually eaten, is made during an interview. The quantities are usually recorded in household measures, and a whole day is covered in this way.

2 A 'cross-check' comes next, involving the subject in looking at a detailed list of foods and questions are asked about likes and dislikes and use of particular food items. This serves to verify information given in stage 1.

The actual interview takes about an hour and a half. The final result is generally representative of the person's average intake for the period being investigated.

Food frequency questionnaires

This method of dietary assessment is commonly used and it involves the subject in filling in a questionnaire without any supervision. The emphasis is on a particular nutrient or food as opposed to all nutrients. It is well known that there is a lack of agreement between measured dietary intake and that estimated from questionnaires. Despite this, they are used routinely in epidemiological studies.

What is particularly interesting about the various methods of dietary assessment is that not one of them is flawless. Errors creep into all of the techniques. Table 2.1 (adapted from the work of Sheila Bingham) summarises the likely sources of error.

Assessing the nutrient content of the diet can be very demanding and tedious for the investigators and subjects alike. In nutritional circles debates on issues such as: food portion sizes; tables of food composition, nutrition computer programs and food labels are, to say the least, hot!

Food portion sizes

The idea of using average or standard food portion sizes in place of recorded food weights is certainly not new. In terms of practicality, it is far less demanding on participants. Weighing foods can have a significant effect on the choice of food eaten as some foods are more convenient to weigh than others and therefore this in itself will bias the results. In addition to this, hot foods may become cold and this is surely not good for the high degree of cooperation that is required of subjects.

Tables of food portion sizes have been published, and these are used as tools in assessing food intakes. Bingham and Day obtained their values retrospectively from records of dietary surveys undertaken in the community and Crawley's figures were derived in similar fashion from the Ministry of Agriculture, Fisheries and Food (MAFF) Household Survey. Food portion sizes obtained retrospectively from dietary surveys are open to question on the grounds that varying numbers of observations may be used to construct one average portion, the size of portions may alter as a result of bias and extremes of portion size tend to be recorded in dietary surveys.

A different approach to obtaining food portion sizes was discussed at length by us some years ago, and this culminated in a book called the Nutrient Content of Food Portions. The food portion sizes were derived from a carefully planned study. Foods were made up according to standard recipes in tables of food composition or they were simply purchased and used as such. To estimate portion size three different people were asked independently to serve what they considered to be a small, medium and large portion. Suitable serving vessels were selected for this procedure, for example dinner plates, soup bowls and cups, and the scales used were accurate to the nearest gram. For each food item there were nine weights: three small, three medium and three large. The mean value of the medium size portion was used in the tables. When foods purchased were already in unit form such as one lamb cutlet, one white bread roll, one Danish pastry, three different brands were used and the mean value taken as the weight of the item. In the case of fruit such as oranges, individual purchases were made of three small oranges, three medium oranges and three large oranges, and the mean value of the medium orange was used.

Tables of food composition

Having an idea about the amount of food eaten is just the beginning; what is in the food also needs to be known. To this end we recommend the use of food composition tables produced by the Royal Society of Chemistry in collaboration with MAFF. The present tables have been developed over the years since the 1920s, and the dedication of both McCance and Widdowson has been the driving force behind this.

(Continues...)

Excerpted from Food The Definitive Guide by Tom Coultate, Jill Davies. Copyright © 1994 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Dettagli dell?edizione

• Editore: Royal Society of Chemistry
• Data di pubblicazione: 1994
• Lingua: Inglese
• ISBN 10: 0851864317
• ISBN 13: 9780851864310
• Rilegatura: Copertina flessibile
• Numero di pagine: 176
• Contatto del produttore: non disponibile
• Persona responsabile: non disponibile