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.

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...