The Thermic Effect of Feeding Medium Chain Triglycerides

The Thermic Effect of Feeding Medium Chain Triglycerides

Even during rest the human body is constantly metabolizing energy to maintain itself. The rate at which energy is expended by the body, expressed in calories per hour, or normalized to calories expended per kg body mass per hour, is known as the metabolic rate. The basal metabolic rate (BMR) is the body's rate of energy expenditure while at rest. This represents the energy requirements of maintaining life, consisting mostly of maintenance of body temperature, heart rate, breathing, nerve transmission, and electrochemical gradients across cell membranes. The basal metabolic rate accounts for 65-75% of daily energy requirements (Van Zant, 1992). Other components of metabolic rate include the thermic effect of feeding (TEF; also referred to as diet-induced thermogenesis, or DIT), the thermic effect of activity (TEA), and adaptive thermogenesis (AT); Van Zant (1992). Metabolic rate is affected by many parameters such as eating (caloric consumption as well as dietary composition), activity (dependent on type, intensity, and duration of activity), lean body mass, age, sex, hormones, and drugs.

Since all of the energy expended by the body is ultimately converted to heat, except when work is performed outside the body, metabolic rate can be determined by the amount of heat energy liberated by the body (Guyton, 1976). A calorimeter can be used to directly measure the heat given off by the body. However, since greater than 95% of the energy liberated by the body is derived from the reaction of foods with oxygen, the metabolic rate can also be calculated from the rate of oxygen consumption (Guyton, 1976). In many studies metabolic rate, or energy expenditure, is expressed in terms of oxygen consumption.

After consuming a meal the food is digested, released into the bloodstream, and transported to all the cells of the body. There, it reacts with oxygen to produce energy. Some of the energy is captured in ATP, the energy source used directly by cellular machinery performing work. Calories consumed in excess of energy requirements will be stored as body weight. About 55% of the energy contained in food is liberated as heat during the process of ATP formation (Guyton, 1976). This release of heat energy from the oxidation of foods represents an increase in metabolic rate and is accompanied by increased oxygen consumption.

Feeding different dietary items while maintaining caloric intake affects oxygen consumption (Baba, Bracco, and Hashim, 1982). The fact that different foods, normalized for energy content, increase the metabolic rate to different extents probably reflects the tendency of a particular food to be burned for energy versus being stored as body weight, as well as its extent of digestion and absorption. That protein increases the metabolic rate more than carbohydrate and conventional fat suggests that certain amino acids may directly stimulate thermogenesis (Guyton, 1976). The increase in energy expenditure caused by feeding is known as diet-induced thermogenesis or the thermic effect of feeding (Van Zant, 1992). Medium chain triglycerides (MCTs) cause profound postprandial thermogenesis because they are very caloric dense and are absorbed and metabolized very rapidly. The rapid oxidation of MCFAs in the liver causes an increase in postprandial oxygen consumption, i.e. metabolic rate. The increase in metabolic rate resulting from MCT ingestion has been measured in humans as well as in rats, using long chain triglycerides (LCTs) as controls (Baba, Bracco, and Hashim, 1982).

Baba, Bracco, and Hashim (1982) observed that rats overfed MCT gained significantly less fat than rats fed an isocaloric diet containing LCT as the fat source. This was attributed to higher resting oxygen consumption (metabolic rate) in the MCT group. The authors explained this by pointing out that while conventional fats are transported as chylomicrons and are largely stored as body fat, MCTs are transported directly to the liver where they are oxidized extensively to produce energy. The rapid oxidation of MCTs results in increased oxygen consumption, increased heat generation, and increased metabolic rate.

In 1992 Van Zant and colleagues demonstrated in humans that a meal containing MCTs increased oxygen consumption 12% above basal levels for 6 hours following the meal, while the LCT-containing meal increased oxygen consumption by only 4%. This indicates that MCTs are burned faster than conventional fats and increase the metabolic rate more. The increase in energy expenditure accounted for 13% of the energy contained in the MCT meal and 4% of the energy contained in the LCT meal.

Hill and coworkers (1989) also compared the thermogenic effect of medium chain triglycerides with that of long chain triglycerides. Ten male volunteers were hospitalized and fed diets containing 30% of calories from either MCT or LCT. Metabolic rate was measured before, during, and after the experiment. Each subject was studied for one week on each diet in a double-blind crossover design. The thermic effect of food (TEF) is defined as the difference between metabolic rate during a six-hour period after eating and the resting metabolic rate. That is, it is a measure of the increase in metabolic rate caused by eating the test meal. On day one of the experiment, the TEF of the meal containing MCT accounted for 8% of the ingested energy, while the TEF of the LCT meal accounted for 5.5% of the ingested energy. On day six of the experiment, the TEF of the MCT meal had increased to 12% of ingested energy, and the TEF of the LCT meal was 6.6% of ingested energy. This means that the MCT

enhancement of the metabolic rate increased during the course of the experiment as the subjects became acclimated to the MCTs. On the last day of the trial the subjects were fed a liquid diet by continuous tube feeding. During this experiment it was found that the TEF of the MCT meal increased to 15.7% of ingested energy, and the TEF of the LCT meal was 7.3% of ingested energy. So the increase in metabolic rate was even greater when MCT was administered continually.

The chemical mechanism underlying this thermogenic effect is unknown at present, but several suggestions have been advanced. Hill and coworkers (1989) demonstrated that MCT overfeeding results in increased hepatic de- novo fatty acid synthesis in man. This process is energetically costly and could account for the lesser