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Nutrients are often essential which means that the body needs them but
cannot synthesise them or cannot synthesise them in sufficient quantities
to meet the daily requirements even under the best circumstances. Essential
nutrients hence need to be supplied in adequate quantities.
Some nutrients can be essential under certain circumstances, e.g. during
growth or in disease, though in other circumstances they are not. These
nutrients are referred to as conditionally essential (sometimes also
as semi-essential).
Proteins
Proteins have an average calorific value of 4 kcal/g and are the
only nutrients with a substantial amount of nitrogen, on average 16 g N per
100 g of dry weight. They contain up to 20 different amino acids,
8 of which are essential. Also the nitrogen as such is essential.
Carbohydrates
Carbohydrates have an average calorific value of 4 kcal/g and are
not essential in the specified sense of the word. However, because the
body has a very limited reserve of glucose in the form of glycogen and
because once consumed the only major precursor for glucose in the body
is protein, a sufficient amount of carbohydrate is normally part of the
diet.
Lipids
Lipids (more precisely triglycerides) have an average calorific value
of 9 kcal/g if they are long-chain triglycerides (LCT), and 8.3 kcal/g
if they are medium-chain triglycerides (MCT). Two fatty acids are known
to be essential, linoleic acid (ω6) and α-linolenic acid
(ω3).
Anabolism
After a normal meal the gut breaks down the foodstuffs to nutrients and
these are absorbed. The body receives here a lot more nutrients than
it actually would need to cover its basal needs over the period of
absorption. Hence, most of the nutrients are put into stores. The underlying
processes are called anabolic and the main hormone of anabolism is
insulin.
Glucose is converted to its reserve form, glycogen, in the liver and
in muscle. Triglycerides end up in the adipose tissue, the main energy
store of the body. Amino acids are used for protein synthesis.
Catabolism
Once absorption is finished, the body starts to reverse the anabolic
processes in a situation referred to as catabolism. The main hormone
of catabolism is glucagon. The result is a release of glucose from
the glycogen stores, of fatty acids and glycerol from the triglyceride
stores and of amino acids from protein breakdown. In catabolism the
bulk of these amino acids are used to synthesise glucose (gluconeogenesis).
In a normal human being, a prolonged starvation will lead to metabolic
changes with the aim to reduce the consumption of calories and especially
the breakdown of protein, which is always functional. Because protein
is the only major glucose precursor (liver glycogen constitutes a very
small glucose reserve that is exhausted after one day) and because the
brain consumes about 150 g/day oxidatively, the mechanism for protein-sparing
is by offering the brain alternative energy substrates. In a prolonged
starvation these alternative energy substrates are produced from fatty
acids in the liver, the so-called ketone bodies. They can largely, but
not completely, substitute for glucose in the brain.
Injury and infection
In injury and infection the catabolism that ensues is totally different
in nature. There is a hypercatabolic state with increased energy expenditure
and a specially pronounced increase of the gluconeogenesis ( Long
et al.). This leads to a rapid deterioration of the body´s
protein status and hence functional mass. The consequences are lowered
resistance to infections, impaired wound healing and deterioration
of organ functions.
In clinical settings where normal oral food and fluid intake is not
possible clinical nutrition must be implemented. This can be achieved
either enterally with formula diets or parenterally with nutrient solutions
and lipid emulsions.
Aim of clinical nutrition
The general aim of clinical nutrition is to avoid nutrient deficiencies
and their related complications. In detail there are some additional
objectives. In children the aim is to achieve normal growth and development.
In an adult with a good nutritional status and no hypercatabolism the
aim is to maintain this condition. In hypercatabolic patients the aim
is to minimise the protein losses in order to avoid their deleterious
effects, though protein equilibrium cannot be achieved during the acute
phase of hypercatabolic states.
Routes of clinical nutrition
Enteral nutrition is the preferred form of clinical nutrition, as it
maintains the functionality of the gut. For it to be feasible the intestine
must be working appropriately so that nutrient demands can be met.
If nutrient demands cannot be met by the enteral route alone a parenteral
supplement must be given. When the gut is not working or the tolerance
to enteral nutrition is very poor the only viable route of administration
is parenteral. In this situation it may still often be possible to
give a minimum of nutrients enterally. This is important in order to
stimulate the gut and maintain its integrity. Some 10-20 ml/h
of a formula diet are enough (Arenas-Márquez
et al.).
It is beyond the scope of this presentation to go into the details of
enteral nutrition and in the following only parenteral nutrition will
be considered.
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