MCTs in general
MCTs or medium chain triglycerides are an important source of energy and an alternative or addition to the use of long chain triglycerides (LCTs). The human body is able to use (metabolise) MCTs to generate energy thereby contributing to the overall requirements for energy supplied by carbohydrates, fats and proteins. MCTs are usually derived from palm kernel oil or coconut oil.
MCTs are interesting in that they are absorbed in the gut by a different route from LCTs. LCTs are absorbed into what is known as the lymphatic system before reaching the blood circulation and the liver. The route taken by LCTs in the lymphatic system is a long and precarious one. After absorption into tiny lymphatic channels in the bowel wall these amalgamate to form larger channels draining the bowel into a single channel – the thoracic duct containing a fluid called chyle. This joins the blood stream at the base of the neck on the left hand side at the junction of the subclavian vein and jugular vein. MCTs are absorbed mostly unchanged directly into the blood stream via the portal vein and go directly to the liver for further handling. MCTs therefore bypass the usual complicated route used by LCTs. Also, MCTs do not need to be broken down by enzymes within the gut (although they can be) into smaller components as does ordinary fat (LCTs) which has to be broken down by enzymes before being absorbed into the lymphatic system. The enzymes required for digestion of LCTs are found principally in pancreatic secretions although there is a small amount secreted in saliva too. This means that when the quantity of enzymes available for digestion is markedly reduced (by pancreatic disease, surgical removal or bypass of the pancreas), MCTs can be used to maximise the contribution of fat based energy to the overall needs of the body. If LCTs are used as in the most commonly used feeds, then the lack of enzymes leads to failure of digestion and malabsorption of fats. These then appear in the stools or stoma bag undigested except by bacteria and do not contribute to energy needs.
Another situation in which LCT is not effective is when there is a leak or blockage of the lymphatic system. This occurs when the thoracic duct is cut by trauma or at surgery (chyle leak) and when the duct is blocked by tumours as in lymphomas and other intrathoracic growths. Under these circumstances, LCTs do not contribute to energy needs but MCTs can be used instead as they do not follow the thoracic duct route to the liver and the rest of the body. Chyle leaks are typically treated by a reduction in LCT fat content in favour of MCT feeds.
MCTs are neither better not worse than alternative fats or oil based energy sources – instead they offer an alternative when LCTs are not absorbed efficiently although attention must be paid to the provision of adequate essential fatty acids from LCTs under dietetic guidance.
MCTs and enteral feeds
For the above reasons, specialised feeds or food supplements containing MCTs are often used to provide extra energy to patients with malabsorption due to pancreatic insufficiency, a short bowel or a chyle leak or blockage. For those able to eat properly, an MCT supplement can be advised by your dietitian. For those unable to swallow ordinary foods by mouth, an enteral nutrition product containing MCT rather than LCT can be used. These are especially useful when there is fat malabsorption or leakage of chyle. Those with short bowels often find absorption of MCTS easier and less intrusive to bowel movements or stoma output, perhaps because MCTs require no digestion before absorption so they can be immediately absorbed across the remaining bowel wall.
MCTs and parenteral feeds
When MCTs are given by the intravenous route, they are also utilised for energy provision having passed through the liver first. MCT is a component of some lipids prescribed for TPN and is thought as part of a package of lipids to be less damaging to the liver (hepatotoxic) than the first generation lipids which largely comprised of LCT. The latter are important to have in some amounts because the provide Essential Fatty Acids without which, unpleasant deficiency symptoms can arise. Dietetic advice should be sought to avoid essential fatty acid deficiency when MCTs are the predominant fat source. Not everyone uses these newer preparations but as more evidence is published to show the relative merits of newer MCT containing PN feeds, it seems probable that more will be used. We will not further discuss the relative merits of various oils or fat solutions used in parenteral nutrition in this article although this would make a good topic for the VNT to tackle!
“Is glucose better than maltodextrins?”
This is a good question but it depends on the context in which the question is posed. A number of vital organs depend on the monosaccharide glucose as their basic source of energy and nothing else is as good. Thus the brain, heart, kidneys and red cells all use glucose as their primary energy source. Muscles prefer glucose too although they also can utilise lactic acid or free fatty acids. When the body needs more glucose, it is able to break down proteins to amino acids and then glucose in the liver.
However, we rarely eat foods containing free glucose except in honey and sports drinks. Instead, we eat polysaccharides such as starch from cereals, rice, potatoes and other vegetables. We eat fruits which contain fructose, also a monosaccharide. We also eat a lot of the disaccharide, sucrose or common household sugar which is a mixture of glucose and fructose. Milk contains lactose, a combination of galactose and glucose. Maltose (malt) is the disaccharide of glucose. So glucose is everywhere and extremely important.
Maltodextrins are derived from polysaccharides. We digest polysaccharides (starches) to produce a mixture of maltodextrins or oligosaccharides, trisaccharides, disaccharides and monosaccharides. This process starts in the mouth with the enzyme amylase secreted by the salivary glands. The process stops in the stomach where acid prevails but restarts in the duodenum in an alkaline environment and continues throughout the small bowel. Here, the pancreatic enzyme amylase is present in vast amounts. The resulting mixture of sugars is further digested in the bowel wall or mucosa by enzymes sited there. The end result is to reduce all the sugars to a single unit or molecule of glucose, fructose or galactose. These are then absorbed into the blood stream (portal vein) and transported to the liver and beyond. Larger sugar molecules are not absorbed in significant amounts.
To illustrate the relative merits of glucose v maltodextrins, you may like to hear of some interesting work done by NASA early in the manned space programme in the 1970’s. It was realised that astronauts would find it difficult to pass much in the way of stools while in space for obvious reasons. Reducing the amount of “waste” can be achieved by eliminating fibre and using a liquid diet. It was thought that a solution containing food already pre-digested to its smallest component parts would be most efficient. Thus a prototype liquid diet or “Space diet” was designed and trialled on possible astronauts. They all vomited profusely and the diet was abandoned in favour of one with glucose polymers (maltodextrins). The reason is that small molecules exert a high osmotic effect which sucks fluid into the gut and causes vomiting. Maltodextrins avoid this problem as they are much larger molecules. The so called Space diet never went into space but lived on as the commercial feed, Vivonex® containing maltodextrins and free amino acids. This was much better tolerated although its taste was not always popular!
Humans can digest and absorb maltodextrins as efficiently as free glucose so there is no reason to prefer glucose in liquid diets used for enteral feeding. Maltodextrin feeds are less sweet or syrupy and flow well down small tubes. All modern enteral feeds contain maltodextrins as their primary carbohydrate source. Even sports drinks and energy gels use the same principals.
Maltodextrins and parenteral feeds
Intravenous preparations for total parenteral nutrition all contain glucose as their principle carbohydrate source. Maltodextrins are not efficiently or sufficiently utilised when given directly into the blood stream. This is a shame as maltodextrins would help lower the osmolality of parenteral feeds and help reduce the inflammation to the lining of veins caused by high concentrations of glucose. It is also important to note that glucose given intravenously is handled less efficiently than when it is given via the gut. This causes higher blood sugar levels in normal patients and instability in diabetic patients. The amount of glucose which can be administered is limited by the ability of the insulin response and the oxidative capacity of that individual. The rest of the energy needs are supplied by fats such a LCTs or LCT/MCT mixtures.
We hope this answers the interesting questions posed by the PINNT member and helps to inform all members. Please note that patients and family members can always and should always discuss their concerns and questions with a member of the team caring for the nutritional aspects of their care. The medical staff, dietitians and pharmacist members of nutrition teams are always happy to advise. We cannot advise on individual problems but we are happy to respond to questions such this one which covers matters of general interest.