- The metabolism of nutrients by enzymes is dictated by the individual’s gene structure and the induction of enzymes and, in turn, by species and gender. These distinctions are complex, subtle and only partially understood.
- The nutrient needs and subsequent metabolism by the individual will be influenced by growth in the young and in pregnancy, and modified by disease, drugs, alcohol and tobacco.
- As the person ages there are important changes in the effectiveness of the absorption and utilisation of the nutrients consumed. It has been suggested that diet may affect behaviour.
- In some ancient cultures certain foods were thought to have magical qualities capable of giving special powers of strength, courage, health, happiness and well-being. It is possible that some food constituents may affect the synthesis of brain neurotransmitters and thus modify brain functions.
- It is therefore important to integrate dietary effects on brain chemicals into our wider understanding of human behaviour.
Until there is an understanding of such nutritional and metabolic mechanisms, confused advice of exercise being taken. Survival without any food at all, but with water, may be for 60–120 days, depending on the body stores.
- Females and those with considerable subcutaneous fat generally survive for longer than slightly built males. There are individual responses to nutritional deficiency and excess, although in general weight increase is associated with overall excessive eating and weight loss is associated with inadequate
- A failure to provide amino acids, fats, vitamins and trace elements leads to specific lesions which may progress to morbidity and death. There is no nutritional explanation for the apparent synthesis of essential vitamins by some individuals. When scurvy was a problem in the Royal Navy the fleet would come into land every 2 months to take on board provisions specifically to reduce the prevalence of scurvy. However, on the long sea voyages some individuals died quite quickly of scurvy, whereas others appeared to be unaffected.
- Similarly, the different types of beri-beri suggest individual metabolic responses to thiamin deficiency. In general, the body copes better with an excess than with a deficiency of nutrients, with the exception of alcohol. Consequently, there is an inclination
to eat somewhat more than is required.
- The body copes less well with an excess of dietary fatty or fat-soluble compounds than an excess of watersoluble dietary components. Fatty nutrients, e.g.lipids, are stored and, if the storage load becomes excessive, then the body is disadvantaged.
- Watersoluble dietary excesses may be excreted, metabolically modified or unchanged in the urine. Excess dietary protein and lipid intakes may be metabolically modified to structural or storage tissues, or possibly be excreted in bile and urine.
- The variable pathways whereby these processes occur will be determined by the range of variants of the same enzyme (isoenzymes) that forms the metabolic enzyme structure of the individual.