Medium-chain triglycerides (MCTs) are fats with an unusual chemical structure that allows the body to digest them easily. Most fats are broken down in the intestine and remade into a special form that can be transported in the blood. But MCTs are absorbed intact and taken to the liver, where they are used directly for energy. In this sense, they are processed very similarly to carbohydrates.

MCTs are different enough from other fats that they can be used as fat substitutes by people (especially those with AIDS), who need calories but are unable to absorb or metabolize normal fats.

MCTs are also popular among athletes as a proposed performance enhancer, although there is little evidence as yet that they really work.

There is no dietary requirement for MCTs. Coconut oil, palm oil, and butter contain up to 15% MCTs (plus a lot of other fats). You can also buy MCTs as purified supplements.

Another double-blind study found similar results in 24 men with AIDS-related fat malabsorption.

The body depends on enzymes from the pancreas to digest fat. In one study, individuals with inadequate pancreatic function due to chronic pancreatitis appeared to be better able to absorb MCTs than ordinary fatty acids^. However, this didnt turn out to mean much on a practical basis, because without taking extra digestive enzymes they could only just barely absorb the MCTs; whereas, if they took digestive enzymes, they absorbed ordinary fats as well as MCTs without difficulty.

It is theorized that the ingestion of medium chain triglycerides, because of their rapid absorption and metabolism, provide the athlete with a fuel source that helps to spare the use of muscle glycogen (stored carbohydrate). Because depletion of muscle glycogen is a factor in fatigue, ingestion of MCT oil could provide the body with an immediate source of fat that is rapidly broken down for energy thus muscle glycogen stores are not used up as quickly and the onset of fatigue is delayed. MCTs are also thought to increase metabolic rate. Assuming that diet (calorie intake) and exercise (calorie output) remains the same; any increase in metabolic rate could result in a small, slow loss of body fat. MCTs are thought to be burned immediately for energy rather than being stored as body fat and used for energy at a later time. In theory, MCTs help to maintain muscle mass because they produce ketone bodies which are used for energy before the amino acids in muscle are used for energy.

MCTs have been proposed as an "ergogenic aid," an energy-boosting supplement to enhance athletic performance. During intense exercise, your body first burns up available energy from the blood (in the form of glucose) and then starts to use energy stored in the form of a larger carbohydrate called glycogen. When the glycogen is depleted, exhaustion begins to set in.

One solution to this is carbo-loading, the practice of taking large doses of carbohydrates prior to exercise in order to increase glycogen stores. Athletes can also sip carbohydrate-loaded drinks during exercise.

MCTs may provide an alternative. Like other fats, they provide more energy per ounce than carbohydrates; but unlike normal fats, this energy can be released rapidly.

The majority of MCT studies have looked at endurance cyclists to determine the effectiveness of MCTs on increasing endurance performance. Researchers have studied both the use of muscle glycogen when MCTs are consumed and the effect that MCTs have on cycling times. In these studies the endurance cyclists engage in moderate to intensive exercise while ingesting an MCT supplement, an MCT supplement plus a carbohydrate supplement, or a placebo (an inactive substance). The results of these studies suggest that MCT oil does not reduce the use of muscle glycogen or improve endurance performance. In one study, the use of 85 grams of MCT oil (a large dose) was associated with intestinal cramping, whereas smaller quantities of MCT oil did not effect gastrointestinal function.

Body builders have always searched for a supplement that will increase metabolic rate, reduce body fat and preserve muscle mass. This is especially important one week prior to a contest when calorie intake is decreased in an effort to reduce body fat to the lowest level possible without breaking down muscle protein. Despite the frequent claims for the benefits of MCTs in bodybuilding, there are no published scientific studies that have examined MCT use by body builders. The claims for MCTs are based on individual scientific studies that look at the use of MCTs in non-athletic populations. For example, a 1986 study found that the metabolic rate of seven healthy men increased by 12% with MCT ingestion versus only 4% with long chain fats. The authors speculated that weight loss would occur if MCTs were substituted for long chain fats and total calories remained the same. However, the rise in metabolic rate is small and the substitution of MCTs for long chain fats would have to be substantial for weight loss to be achieved.

There is clearly a need for more research regarding the role of MCTs in athletic performance. Some athletes have been experimenting with lower carbohydrate, higher fat “Zone” type diets (40% carbohydrate, 30% protein and 30% fat). It has been suggested that modifying a 40-30-30 diet by substituting MCTs for long chain fatty acids may help further reduce body fat but no scientific studies of athletes have been published to date.

MCTs are thought to be quite safe, but the safety of using them as a general fat substitute has not been established. Some people who consume MCTs, especially on an empty stomach, experience annoying (but not severe) abdominal cramps and bloating. People with diabetes should not use MCTs (or any other supplement) without a doctors supervision. The safety of MCTs in young children, pregnant or nursing women, or people with serious kidney or liver disease has not been established.