Enteral feeds or liquid diets were by products of the space race and were originally developed by NASA to feed astronauts. They are made from the basic building blocks of foods, so that the nutrients are easily available for absorption. Feeds contain carbohydrates and sugars, oils to provide fats and proteins, single amino acids or short chain peptides. Vitamins and minerals are then added as required to make them nutritionally complete.
This crucial first stage of dietary treatment appears to be essential for success. Special liquid diets are the best treatment for Crohn’s disease (CD) currently known; in our experience 85-100% of patients will enjoy a full remission. This is considerably better than pharmacological treatments such as corticosteroids or Infliximab. Most (95%) people drink the liquid diets but some prefer administration by a fine tube through the nose into the stomach.
The special liquid diets are sometimes referred to as ‘enteral feeds’ or the process as ‘exclusive enteral nutrition (EEN)’. They can be divided into three different types: elemental diets, semi-elemental diets, polymeric diets. The difference depends on the size of the protein molecules and how broken down the protein is. The building blocks which make up proteins are amino acids so an elemental feed has the protein completely broken down into individual amino acids. Semi- elemental feeds will contain short chains of amino acids, while a polymeric feed has whole proteins where none of the protein has been broken down into its smaller constituents.
There is a difference in the time required for the diets to be effective: an elemental diet is normally taken for 2-3 weeks, whereas a polymeric diet is recommended to be taken for 6- 8 weeks. Typically, no other food or drink is allowed during this time, hence the term exclusive enteral nutrition.
It is now also appreciated that one of the key factors governing the success of liquid diets in CD is not the form in which protein (or nitrogen) is provided, but rather the amount of fat in the feed. Fats are divided up according to the length of the chain of carbon atoms in their molecules. Thus there are short chain fatty acids (SCFAs) such as acetic, proprionic and butyric acids which respectively have 1-, 2- or 3-atoms of carbon in the chain. Medium chain triglycerides (MCTs) have 12 or 14 carbon atoms.
MCTs are absorbed directly into the blood stream from the gut, which makes them readily digestible. Long chain fatty acids (LCTs) however, have long carbon chains which may contain anything from 16 or 18 carbon atoms or more. LCTs are absorbed into the lymph channels of the gut and are more likely to be absorbed poorly, leaving a residue to pass down to be fermented by the bacteria lower in the gut. Feeds which contain more than 15% of their energy in the form of LCT have been shown to be less effective in managing CD.
Elemental diets are made from the basic building blocks of foods and are pre-digested so that the normal processes of digestion in the gut are not required and virtually 100% of the nourishment is absorbed into the body high in the small intestine, leaving little if any residue to be metabolised by the bacteria which live lower down in the intestine.
However, elemental diets contain large amounts of small molecules, like sugars and amino-acids, so present a high osmotic pressure to the gut. As the gut is used to having less pre-digested content at any one time, this means that when elemental feeds pass into the intestine they draw water into the bowel and in some patients this may be sufficient to cause diarrhoea. This can be avoided if the elemental diets are slowly introduced to enable the gut to get used to this different pressure. Semi-elemental diets, where the amino-acids are linked together, have a lesser osmotic effect than the elemental diets, but may also need to be built up slowly
The choice of liquid diet for use in CD is therefore quite complicated. Many different feeds are listed in the British National Formulary, but the ones we have found to be best in careful clinical trials are: Elemental 028 Extra, if an elemental diet is required and Pepdite 1+ when it is necessary to use semi-elemental diet. These are manufactured by Nutricia.
A dietitian would determine the amount of diet needed, based on individual requirements, and the volume would be achieved gradually over several days; most patients need 2-3 litres daily. No other food or drink is allowed except bottled water (although this advice may vary between hospitals). The feed should be sipped throughout the day and is better if kept chilled. Symptoms are recorded regularly and when they have resolved the patient is ready for Stage 2 of dietary treatment.
At present no test exists to allow us to determine which foods will provoke symptoms. Each new food item must be eaten to see what effects it produces.
The simplest way to do this is an elimination diet. One new food is introduced each day and consumed in generous quantities 2-3 times on that day; if symptoms develop it is subsequently avoided whilst foods that cause no problems are left in the diet. In this way a ‘safe’ diet is gradually built up. Some foods need to be tested for longer periods as the onset of reactions may be slow. The enteral feed is gradually reduced in amount as the diet widens. The elimination diet is slow and often tedious; it may take 2-3 months to complete the testing process.
With increasing experience it has been possible to simplify and shorten the process by means of the LOFFLEX diet. This stands forLOw Fat, Fibre Limited EXclusion diet, which is discussed in more detail in the ‘The LOFFLEX Diet’.
When all foods have been introduced those which caused problems are retested to confirm that the reaction was genuine and not mere coincidence. If so the food is excluded for at least 6 months, but it is sensible to retest from time to time as food intolerances slowly become less severe and after 5 -10 years usually disappear completely.
Nutritional assessment is essential to check that the final diet is adequate. Foods which have been excluded must be replaced with suitable alternatives and vitamin and mineral supplements such as calcium may be required.
The construction of a diet for CD is thus a complex procedure; it is essential it is done under medical and dietetic supervision. Eliminating foods without proper testing and dietary assessment can result in a nutritionally inadequate diet.