The science of nutrition aims to define the overall quantity and individual nutritients required by the human population.
Humans deal uniformly badly with a deficiency of a nutrient whether this be the very short term (oxygen), the short term (water) or longer term (food ). Survival times depend upon body stores at the onset of deprivation. The consequence of a deficiency of single or several nutrients from the diet may be lethal over a period of time.
Modern western populations are faced with an excess of food. This may lead to an over-generous intake of nutrients. The manner in which each individual deals with this is in part dependent upon their genetic and isoenzymic constitution. The basal metabolic rate and level of physical activity are also important. The excess intake may be in all nutrients or be confined to a few specific nutrients. The stage of life and health status are also important.
Growth is frequently used as a non-specific term to describe the changes associated with the development of form and function. Growth may readily be measured as length, weight and chemical composition. By such measurements, normal and abnormal growth can be identified.
Growth requires nutrient utilisation to be directed to energy storage, cell multiplication and skeletal utilisation. These metabolic fuels are co-ordinated in their role in growth by systemic and local regulatory factors. Growth includes:
• whole body growth
• individual organ growth
• cellular growth and replication
• tissue turnover and repair
- cell death ( apoptosis)
These overlap and are all involved in the achievement of functional maturity. The amount of dietary energy and protein necessary for growth varies between species, and is influenced by the time taken for the growth phase. Differences between species and the genetically determined rate of proliferation will influence cell number and body mass.
Hypertrophy is the reversible increase in the size of a cell through the accumulation of more structural components. Hyperplasia is an increase in the number of cells in a tissue or organ, and an increase in the organ size.
Maternal nutrient supply to the foetus in the latter part of gestation will affect the rate of growth and the composition of the infant. Hypertrophy becomes more significant in the more mature foetus, as does the differential growth of different cells, tissue and body protein mass and organs.
The increase in weight from birth to adult maturity is determined by the proportion of the life span spent in maturation and growth. The development of reproductive maturity usually means the end of growth. Humans are somewhat different from other mammals in that body weight increases 15-fold from the birth weight. Protein deposition continues for a quarter of the time between birth and death.
1. Growth is an increase in weight, height, and composition of body organs, cells and tissue repair. This requires appropriate nutrient intake of energy and protein.
2. Hypertrophy is a reversible increase in cell size. Hyperplasia is an increase in the number of cells. Organ growth is in three phases, the development of cell lines, differentiation and maturation to mature function.
3. Growth at cellular and organ level is a coordinated process, controlled by various specific signalling molecules, many of which are hormones. The growth process at maturity is in response to need. Disuse leads to atrophy.
4. Growth is only possible if energy intake exceeds expenditure. Height is dependent upon sex and ethnic, genetic, dietary and environmental factors
- Catch-up growth requires sufficient nutritional intake including high quality protein and energy content. This growth spurt is seen in children who, following failure to achieve their age growth norms and are subsequently well.
- A multicellular organism must balance cell generation and cell death to maintain a constant size. Senescent, damaged or abnormal cells are removed. Physiological cell death, apoptosis, is directed towards scattered individual cells rather than all the cells in a particular area.