Nutrition and Growth

Reader in Human Biology, Department of Human Sciences, Loughborough University, Leicestershire, United Kingdom introduction

It is self-evident that nutrition is essential for growth. Growth is, in this context, an increase in size and mass of the constituents of the body. The only way this can be achieved is from the environment. Nutrition is defined as the process whereby living organisms take in and transform extraneous solid and liquid substances necessary for maintenance of life, growth, the normal functioning of organs, and the production of energy.

The constituents of our bodies, the structure and chemical composition of our cells, tissues, and organs, are remarkably similar all over the world. Indeed, the similarity extends to much of the animal kingdom. The lean body mass typically consists of 72% water, 21% protein, 7% minerals, and less than 1% of carbohydrate and other nutrients. Yet, we have available and select or tolerate a very wide range of foodstuffs and diet types. This leads to the first truism of nutrition and growth, that a wide range of diet types is capable of satisfying nutritional needs and promoting optimal growth. What determines the particular diet we consume involves a myriad of factors. For much of human evolution and for some groups today, the physical environment and climate determined what could be procured or cultivated. However, technology and the economic power to develop and exploit it allow us to inhabit the polar regions, the ocean depths, and space, environments that do not naturally support food production. Economic and political systems are the major factors influencing food choice and whether food intake is sufficient to allow growth potential to be achieved.

This chapter has the following aims:

• To illustrate the importance of an understanding of methodology in a consideration of nutrition and growth.

• To recognize that, although growth is highly nutrition sensitive, for much of the growth period, the nutritional requirements for growth per se are only a small proportion of the total requirement.

• To demonstrate that growth perturbation is usually of multifactorial origin.

• To understand the need to assess the contribution of nutrition to growth perturbation in the total environment in order to identify the appropriate remedial actions.

nutrition for students of growth Importance of Methodology

Students of growth need an understanding of some of the basic principles of nutrition if they are to understand and critically evaluate issues in nutrition and growth. As in other fields, it is important to be able to assess the strengths and weaknesses of evidence. There are regular surveys of food intake in many countries and frequent research reports on dietary intake and nutritional status. However, a number of issues must be considered. The apparently simple task of collecting accurate, representative data of the food intake of people going about their everyday way of life is notoriously difficult, and that difficulty should not be underestimated. Much of the information may have been collected by pragmatic but not necessarily the most accurate techniques.

Similarly, it is not difficult for the student to find in textbooks, on websites, or in government and research reports accounts of what are the nutritional needs of the various members of the population. However, the bases to these figures and their correct use is not straightforward. If the reader is to be informed in order to make critical observation, thought, and decisions on nutrition and growth, it is important that these intricacies are known and remembered.

Measurement of Food and Nutrient and Energy Intake

The measurement of habitual food intake is an example of Heisenberg's uncertainty principle: The harder you try to measure it, the more likely you are to affect what you are trying to measure. Nutritionists are able to perform accurate, to less than 1% of the real value, measurements of energy and nutrient intake. The participants are housed in nutrition units and provided their meals. Duplicate meals and snacks are weighed and analyzed by good physical and chemical analytical methods. (It is true that you can never analyze what someone has eaten because, if they have eaten it, you cannot analyze it, and if you have analyzed it, they can not eat it.) The problems arise when we become interested in habitual food intake in people leading their everyday lives. It is difficult or inconvenient to weigh the food consumed in many circumstances. The inconvenience of weighing may influence the foods people eat or the number of meals and snacks they consume. A further problem in population studies is the need for statistical considerations of adequate numbers. There may be logistical and financial considerations and constraints, and these may influence the choice of method. Most dietary surveys adopt the simple methods of questionnaire, interview, or other subjective assessment to assess food intake and translate these to nutrients and energy intakes using tables of food composition. There are problems with the use of food tables, too. The data on composition in food tables may not match those of the food consumed.

There is, thus, a trade-off between ease of use and acceptability to participants and accuracy. In some cases, simple techniques may be appropriate. Some epi-demiological investigations may require individuals to be assigned only to a correct tertile of intake: high, medium or low. However, it is crucial when reading the literature to be able to decide if the nutritional methods used are fit for purpose. The literature abounds with studies purporting to show that children are inadequately nourished based on ignorance of either the limitations of the intake data or, more usually, the nature of figures for recommended intake.

Dietary Requirements and Recommended Intake

Determination of Dietary Requirements and Recommended Intakes

We have nutritional needs, because being in a state of turnover, we have loss of body constituents and because at certain times of the life span, such as growth and pregnancy, we have a net gain of tissue. An individual's dietary requirements can be ascertained by measuring these losses and gains. Under most circumstances, such determinations are not practical and our judgments have to be based on the available experimental and other evidence from other individuals. However, individuals vary in their dietary requirements, even after taking into account age, sex, and size. Therefore, we can make statements about only the probability of what an individual's requirement for a particular nutrient might be or the probability that a particular intake will be adequate or inadequate. This has led nutritionists to develop a series of values, including their best estimates of the average requirement for a given group of healthy individuals given the circumstances under which they live. The series usually includes a high level, thought to represent the needs of most of the group, and a lower level, below which most individuals would have an inadequate intake. In some cases, a high level, above which problems of tox-icity may be expected, is also identified.

Evidence on nutrient requirements has been gained in a variety of ways. Toward the end of the nineteenth century, medical scientists investigated the types and amounts of foods associated with good health or that would lead to the reversal of signs of nutrient deficiencies. They labored under the difficulty of the hegemony of the germ theory of disease following the work of Pasteur and Koch. Around the turn of the twentieth century, physiologists conducted animal and human experiments on artificial and deficient diets with nutrients fed at a variety of levels to ascertain needs. These were essentially balance experiments, with allowances for growth and production. A variation has been the factorial approach, which measures all the avenues of losses from the body—urine, feces, sweat, secretions, and other emanations—to calculate the total losses. A further approach has been to determine the level of intake associated with high or maximum levels of the nutrient in the body. This invariably produces higher estimates of requirements than the other approaches.

A problem with the balance approach is that the body can be in balance but in a state of over- or undernutrition that may be hazardous to health owing to changes or adaptations that occur to varying planes of nutrient intakes. Two difficulties emerge. First is the identification of the range of intakes where balance is not associated with risk. The second is whether recommended intakes should be set to maintain the status quo or at levels that are normative and lead to balance at the lower risk range. The problem is most acute for energy. We may calculate an allowance for children to take up aerobic exercise to promote cardiorespiratory fitness and body composition. However, if they are not active, the recommendation would be a prescription for weight gain and obesity. As usual, a decision has to be made according to the context. The FAO/WHO/UNU1 recommendations for energy intended to be applicable to individuals all over the world include components for desirable but discretionary activities; that is, they are normative. The United Kingdom recommendations do not.2 They are values to maintain the status quo.

Nutrient Requirements and Recommended Intake

Many countries have drawn up their own recommendations for nutrient intake. They may have been drawn up and published in slightly different ways, but there is some communality in their types.

The estimated average requirement (EAR) is what it says it is, except in the United States, where it is actually a median not an average or mean. It refers to a particular group according to age, sex, possibly body size and composition, and in some cases, lifestyle. It is used as one factor for assessing the adequacy of intake of an individual or groups and for planning intake of groups. Approximately half the members of the group will need more and half less than the EAR.

The upper level, at or above which daily intake of a nutrient will meet the needs of most individuals in the specified group, is usually set at the average or median requirement plus two standard deviations (SD). Where distributions of requirements are know to be skewed, the 97.5 percentile may be used. This level is called reference nutrient intake (RNI) in the United Kingdom and recommended dietary allowance (RDA) in the United States. It is intended as a goal for the intake of healthy individuals. It is not intended to be used to assess individual or group intake, as most of the population require, and can stay healthy, on an intake lower than these.

One of the commonest mistakes or misleading actions in nutrition is to assume that, if an individual or group consumes less than the RNI or RDA, the intake is inadequate. There is no shortage of examples of this occurrence. The U.S. National Institute of Child Health and Human Development website has a table titled "Calcium: Who Gets Enough," showing that approximately 50% of children under 12 years old were getting the 1989 RDA.3 The implication is that the other 50% were not getting enough. But 97% could be consuming less than the RDA but be getting enough! All that can be said is that, as the intake becomes a lower proportion of the RDA, the risk of dietary nutrient inadequacy increases. Students of growth should have less of a problem with this point, as they are accustomed not to expect every child to be at the 97.5th percentile for height. Similarly, not every child's nutrient intake needs to be at the 97.5th percentile of requirements. It may be prudent to be at that level, but prudence should not be confused with proof of nutrient inadequacy.

A further point is that, as recommendations vary among countries, given intakes can represent much greater percentages of the reference intake in some countries than others. Such discrepancies do not reflect inherent biological differences but the different approaches and conclusions of the national advisory committees. Table 7-1 compares the United Kingdom's RNI and the United States' RDA for selected nutrients for children 1-10 years old. In the main, the RDAs are higher than RNIs. The United States' RDAs are some of the highest in the world, surpassed only by those from Germany.

The certainty about statements of dietary inadequacy would be increased if a lower level were used. The United Kingdom has a lower reference nutrient intake

table 7-1 The United Kingdom (UK) Reference Nutrient Intakes and the United States (USA) Acceptable Intakes of Selected Nutrients for Children 1-10 Years Old

Age Group

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