A conceptional framework for the study of nutrition
In the journey of discovery into nutrition science it will often be necessary to put new knowledge, or new applications of old knowledge, into the perspective of the holistic picture. For this, a conceptual frame-work of the multidisciplinary nature of nutrition science and practice may be of value. Such a conceptual framework, illustrating the complex interactions between internal or constitutional factors and exter-nal environmental factors which determine nutritional status and health, is given in Figure 1.1.
Figure 1.1 Conceptual framework for a holistic, integrated understand-ing of human nutrition.
On a genetic level it is now accepted that nutrients dictate phenotypic expression of an individual’s gen-otype by influencing the processes of transcription, translation, or post-translational reactions. In other words, nutrients can directly influence genetic (DNA) expression, determining the type of RNA formed (transcription) and also the proteins synthesized (translation). For example, glucose, a carbohydrate macronutrient, increases transcription for the synthe-sis of glucokinase, the micronutrient iron increases translation for the synthesis of ferritin, while vitamin K increases post-translational carboxylation of glu-tamic acid residues for the synthesis of prothrombin. Nutrients, therefore, influence the synthesis of struc-tural and functional proteins, by influencing gene expression within cells.
Nutrients also act as substrates and cofactors in all of the metabolic reactions in cells necessary for the growth and maintenance of structure and function. Cells take up nutrients (through complex mecha-nisms across cell membranes) from their immediate environment, also known as the body’s internal envi-ronment. The composition of this environment is carefully regulated to ensure optimal function and survival of cells, a process known as homeostasis, which gave birth to a systems approach in the study of nutrition.
Nutrients and oxygen are provided to the internal environment by the circulating blood, which also removes metabolic end-products and harmful sub-stances from this environment for excretion through the skin, the kidneys, and the large bowel.
The concerted function of different organs and systems of the body ensures that nutrients and oxygen are extracted or taken up from the external environ-ment and transferred to the blood for transport and delivery to the internal environment and cells. The digestive system, for example, is responsible for the ingestion of food and beverages, the breakdown (digestion and fermentation) of these for extraction of nutrients, and the absorption of the nutrients into the circulation, while the respiratory system extracts oxygen from the air. These functions are coordinated and regulated by the endocrine and central nervous systems in response to the chemical and physical composition of the blood and internal environment, and to cellular needs.
The health or disease state of the different organs and systems will determine the nutrient requirements of the body as a whole.
The central nervous system is also the site or “head-quarters” of the higher, mental functions related to conscious or cognitive, spiritual, religious, and cultural behaviors, which will determine, in response to the internal and external environments, what and how much will be eaten. What and how much is eaten will further depend on what is available, influenced by a host of factors determining food security. All of these factors, on an individual, household, community, national, or international level, shape the external environment.
During the first renaissance of nutrition, emphasis was placed on the study of nutrients and their functions. A medical, natural science or biological model underpinned the study of the relationships between nutrition and health or ill-health. During the second renaissance, these aspects are not neglected, but expanded to include the study of all other external environmental factors that determine what and how much food and nutrients are available on a global level. These studies are underpinned by social, behavioral, economic, agricultural, and political sciences. The study of human nutrition therefore seeks to understand the complexities of both social and bio-logical factors on how individuals and populations maintain optimal function and health, how the quality, quantity and balance of the food supply are influenced, what happens to food after it is eaten, and the way that diet affects health and well-being. This integrated approach has led to a better understanding of the causes and consequences of malnutrition, and of the relationship between nutrition and health.