1.1 Introduction: The defining characteristic
The enabling science for nutritional investigation embraces many disciplines, including mathematics, physics, chemistry, biochemistry, physiology, physical and mental development, disease prevention, clinical care, food production, food processing, food marketing, consumer behaviour and choice, social relations, micro- and macroeconomics, policy and planning. This invites the important question of whether nutrition is a discrete discipline in its own right. If these threads are to be brought together to make a whole, then it is important to be clear about the particular or defining characteristic of that whole that represents nutrition research. This requires an agreed conceptual framework based on fundamental principles. These principles underlie every aspect of nutrition and should be explicitly acknowledged and taken into account at all times.
By the end of this chapter you should be able to recognise that:
- Nutrition is a demand-led process, the study of which draws on many disciplines to understand how energy and nutrients are made available to the cells of the body to enable function.
- By its nature nutrition is integrative and complex, because it relates to how cells, tissues and the body are organised, as well as how people organise themselves in society, to ensure ongoing availability of a sufficient and appropriate mix of energy and nutrients.
- This complexity comes about because of the nature of two major systems and their interaction: the (internal) metabolic system of the body, and the (external) system through which food is acquired from the environment.
- Each system in its own right is inherently complex and their interaction adds an additional level of complexity, thus each system requires explicit investigation of how the component parts act in concert to satisfy the demand.
- Diet plays a pivotal role in these processes, but is only part of the story. The dietary intake is the ultimate source of energy and nutrients for the body, but in addition cells and tissues satisfy their needs for nutrients through integrated processes involving endogenous formation of nutrients, mobilisation of nutrients from structural and functional pools and the availability of nutrients as products of the gut microbiome.
- Research in nutrition requires a focus on a particular aspect of these complex interactions, but that focus needs to be interpreted as an integral part of the wider whole.
Therefore, research in nutrition can be seen at one level as quite simple, but at another as increasingly complex. The diet contains many components, some of which are necessary for metabolism (nutrients), others which have biological activity but are not metabolically essential or may be toxic, and still others which cannot be utilised. Consumption of too little, or of too much, of the nutrients themselves can have adverse effects in terms of deficiency or toxicity, respectively. Thus, there is a preferred range of consumption for each nutrient. Extreme changes in any one aspect of nutritional exposure can induce dramatic and readily measurable change. More modest variability within the usual or preferred range of consumption also has effects, but these may be more subtle and less immediately obvious, as the integrated system operates to maintain an assured and balanced supply of nutrients to the cells of the body.
This chapter outlines some of the fundamental principles of nutrition, how they contribute to building and maintaining the complex systems that represent the function of the body, and how the systems through which food is extracted from the environment can and do exert influence on this. The principles outlined here act as a foundation on which it is possible to consider different aspects of nutrition research methodology. At the same time, we highlight specific limitations of our conceptual understanding that need to be addressed.
1.2 Simplicity to complexity
The nature of nutritional science
We live, survive and thrive in a world of almost infinite variety and complexity. Like other living organisms, we are able to do this by maintaining a constant internal environment. Maintaining this constancy requires the behaviour of all cells, tissues and organs, which maintain structure and function and protect against challenge from the external environment, to be integrated and regulated. The integrity of the body depends on the complex interaction of physical and chemical processes that together characterise metabolism. This is supported by energy derived from the oxidation of macronutrients contained in food. These chemical reactions take place within an aqueous environment, so water is the major constituent of individual cells and the body as a whole. Hence, a fundamental feature of life is that we have to draw continually from the environment around us the oxygen, water and food that we require to stay alive. These are dynamic processes operating within a dynamic system and they are fundamental to the maintenance of health at all ages.
From the time of conception, normal growth and development through childhood to adulthood create a demand for energy and nutrients and depend on their availability, as well as on the body’s ability to perform the processes of normal metabolism that allow it to utilise them. Healthy infancy, childhood, adolescence and pregnancy are characterised by a positive energy and nutrient balance, with ordered net tissue deposition leading to increased capability in both structure and function. Health in adulthood is generally represented by an energy and nutrient balance, with constant height and weight and similar body composition over extended periods of time, consistent with physical, mental, intellectual and social function.
By its nature, nutrition is an integrative discipline. A core feature of nutritional science is the exploration of how chemicals, most of which comprise the substance of other living organisms, are drawn from the environment to provide the energy, substrates and co-factors needed to support and enable life. Nutrition occupies the space between the food we eat and the health we enjoy. Its core is the endogenous environment, including the colonic microbiome, where a varied and inconstant intake is transformed into a constant and appropriate supply for normal function. Nutrition is by nature multifaceted, embracing a wide spectrum of biological and other human experience.
Any individual’s needs vary with age, gender, physiological state, lifestyle and behaviour, as well as in comparison to other individuals. At the same time, the foods we consume are also very varied. This contrasts with the relative constancy of the internal environment within and among individuals. Such constancy is enabled through regulatory processes and assured through adaptive mechanisms. There is a need to organise our understanding, and our approach to scientific investigation, more effectively to determine more clearly the nature of our nutritional demands and how they might be adequately met across all contexts.
Maintaining integrity – the science of nutrition
The cell is the basic unit of all life. The maintenance of cellular integrity ensures integrated organ and tissue function, keeping intact the body’s defences with appropriate inflammatory and immune responses. An intrinsic feature of health is the ability to cope with a changing environment associated with the usual challenges of everyday life, and unusual stresses from time to time, as well as the ability to recover. The stresses may be biological, such as infections with bacteria or viruses, or physical trauma; behavioural or psychological, as with smoking, alcohol, inactivity or poor mental health; or social, as associated with poverty, deprivation or lack of personal control. The ability to maintain the internal environment is called homeostasis; the ability to cope with external stresses, allostasis. The summary of all the internal and external stresses that tax the ability of the organism to maintain constancy represents the allostatic load. The ability of the organism to adapt – to accommodate changes in the internal and external environment – is fundamental to survival and is central to the achievement of homeostasis and allostasis. For this to be achieved in the face of the uncertain nature, extent, severity and duration of changes in the internal or external environment requires a reserve capability that can be drawn on as required, and that is known as resilience. Thus, the nutritional integrity of the organism requires the ability to maintain the usual function, but is intimately linked to the adaptive responses. An insufficient intake of food or a poor-quality diet constrain the adaptive responses, leading to loss of resilience and vulnerability to both internal and external perturbations and hence susceptibility to ill-health.
The balance and amount of energy and nutrients needed depend on this range of experiences with which the healthy body copes as a matter of course. The underlying objective of nutrition research is to understand the needs of individuals, groups and populations for energy and nutrients, and how these might best be met from dietary intake and through other processes.
Structured organisation
How can we manage this complexity, which extends from the molecular, through the cellular, to tissues and whole organisms interacting to maintain the body’s function in different environmental contexts? One helpful way of conceptualising the complexity is to consider it as the interaction of two systems, with the whole body as the point of interaction. One system is within the body: its regulated biochemical, physiological and metabolic processes, which maintain the integrity of the system as a whole. However, the body itself is part of a wider social system with even higher levels of organisation within family and community, which incorporate complex social systems and interactions operating at national and global levels. Nutritional science in its broadest sense embraces the way in which each of the systems is organised and functions, but also the way in which they interact. Nutritional science seeks to understand how to ensure the ongoing availability of an adequate diet, in terms of quantity and quality of the pattern of foods consumed, to meet the needs of every individual in the population.
As with all systems, these relationships are dynamic and have multiple regulatory feedback loops. Think of riding a bicycle. It is easy to maintain control so long as the bicycle is in a dynamic state, moving forwards, but it becomes particularly challenging if the bicycle is stationary. The challenges for research in nutrition are to understand how the many relationships are held in dynamic tension, within and between levels of organisation; to be able to comprehend, measure and manage these relationships; and to be aware of the preferred point of equipoise to achieve a dynamic equilibrium for the component parts and the system(s) as a whole.
Simplified models have been developed to describe, measure and help understand this process. One of the best known is the UNICEF framework, which characterises the different levels of organisation that have to be considered in relation to the factors leading to ill-health and their interplay: proximal, intermediate and distal. This type of model is of value in helping to conceptualise the relationships. The bigger challenge is to quantify the dynamics within and across different levels of organisation. Central to this task is the ability to make valid measurements of the critical factors, and their interactions, and in particular the demand for energy and nutrients in relation to what is available, the supply.
1.3 Structure and function: Appreciating complexity
The individual as an organised system
Understanding the synthetic or integrative nature of nutrition presupposes knowledge and understanding of the component parts. While life starts with a single fertilised egg, ultimately each individual exists as an organised dynamic system, as a body that has boundaries in relation to the outside world. The dynamism within the system creates demands for energy and nutrients on an ongoing basis, which have to be satisfied and are ‘topped up’ intermittently, most obviously from the dietary intake. Foods represent the vehicle through which energy and nutrients are drawn from the environment. The heart of nutritional science is determining the energy and nutrients that are required to maintain the integrity of the organism, which are formally articulated as the Dietary Reference Values (DRVs), Recommended Daily Allowances (RDAs) or Population Reference Intakes (PRIs).
A major challenge within nutritional science is the inherent variability of many of the factors involved. Ultimately, molecular interactions enable cellular structure, function, replication, terminal differentiation and apoptosis. There are higher levels of complexity for organs and tissues and their interactions. From conception, through fetal life, infancy and childhood to adulthood, growth is based on the acquisition of energy and nutrients as tissues (structure), accompanied by the refinement of function with maturation. Increases in organ size lead to greater functional competence. Enhanced intellectual capability enables progressively increasing efficiency in the processes through which food is made available, thereby creating the space and time for individuals and groups to engage in wider creativity, exploration and discovery.
The demand
The dynamic state at every level of organisation is a reflection of continuing turnover and exchange. The cellular and physiological environment is maintained within relatively narrow limits by integrated metabolic regulation. Any individual’s dietary intake is inherently variable from day to day, as is their physical activity. DRVs are captured as references: amounts that need to be taken in the diet to maintain health in otherwise healthy people. Their determination recognises the obvious physiological variability associated with age, sex, growth, maturation, pregnancy, lactation and different levels of physical activity. While it allows for environmental protection, it does not consider the effects imposed by infection, trauma or other exceptional stresses, the demands of recovering from such a challenge, or the need to repair the system subsequently. Nor can the values for individual nutrients take account of potentially simultaneous variations in other essential nutrients. Ongoing cellular replication is a critical feature of the usual environmental protection, for the maintenance of external boundaries (skin) and internal mucosal boundaries (gut, respiratory tract, bladder), as well as immune surveillance and continuing protection. All of these create demands.
In terms of energy, the body’s ongoing demand under standardised resting conditions represents the basal metabolic rate. The demand comprises three major functions: membrane transport (especially brain and nervous tissue), macromolecular turnover (especially liver and muscle) and internal mechanical work (cardiac, respiratory, gastrointestinal and so on). These are continuous processes and the energetic need is met through the oxidation of macromolecules (protein, lipid and carbohydrate) drawn from the availability within the body itself. This resource is intermittently replenished through dietary intake, which for active individuals is exquisitely well regulated so that energy expenditure is matched by energy intake. The availability of macromolecules for oxidation is a regulated process ensuring that the pattern of macronutrients oxidised exactly matches that taken in the diet, which achieves balance and constancy of weight and body composition.
Meeting the demand (supply)
Food (in)security is the term that has been adopted to characterise a country’s food supply status. Its use has evolved and it is now applied to the situation of individual countries, communities or even households; conceptually it can also be applied to individuals. A related but separate term is nutrition security, which similarly has been applied at different levels of organisation. The purpose of both is to capture as a simple, single statement the extent to which food and/or nutritional needs are adequately provided for in terms of dietary supply. Availability is considered, but this in itself is not enough, because the food also has to be acceptable and accessible for consumption. Although the degree of food security is important for different groupings, it is ultimately a summary of the adequacy of provision, availability and consumption at the level of the individual.
Food security consists of two elements, one quantitative and the other qualitative. The quantitative element is a statement of the extent to which food supply adequately meets nutritional needs. The qualitative element embraces the extent to which the pattern of nutrients available from the diet meets an individual’s pattern of demands.
For all practical purposes, the quantitative aspect of the diet is determined by the amount of food that has to be consumed to satisfy an individual’s energy needs. The maintenance energy needs are determined by that individual’s basal energy needs (the basal metabolic rate or BMR) and the level of physical activity. The energy expenditure from most forms of physical activity is proportionate to the BMR, which varies with height, weight, sex, age, maturity and physiological state (pregnancy and lactation). The energy needs are met from the oxidation of amino acids, lipids and carbohydrates, in the same proportions as they are present in the diet. The quality of the diet is determined by the mix of foods and their nutrient composition; that is, the proportions of amino acids, lipids, carbohydrates, minerals, vitamins, trace elements and water.
In general terms, those who are less physically active expend less energy and need less food to meet their requirements. However, if less food is consumed, then the intake of all nutrients is likely to be less. Therefore, if a diet is marginal in any nutrient, that potential limitation is likely to become obvious sooner or more readily in those who are less active (unless they are eating more food than they need). This is one problem for subsistence societies where the quality of the diet might be poor. At the higher levels of physical activity associated with rural life, food consumption is likely to be higher to meet the higher energy requirements, so whatever diet is usually consumed is more likely to meet the needs for nutrients. With urbanisation, decreased energy expenditure and a reduction in the need for food consumption to meet the requirements for energy, any nutrient limitation in the diet is more likely to become exposed.
