ave you ever wondered what makes you feel hungry - why that persistent growl in your stomach drives you to leave your comfortable place on the sofa and ravage the fridge for tempting morsels? Scientists have long tried to understand what drives our craving for food, and they have come up with a simple answer: hormones.

Hormones are your body's messenger molecules: they're made in one organ (sometimes a few) and travel elsewhere to prompt your body to respond in some way. In the case of hunger, hormones made in the digestive system travel all the way to the brain to make sure that you seek food to make your stomach happy. Most of us know all too well just how effective those hormones can be.

Scientists are still working out the details of this complex communication system between your gut and your head, but they have sketched in some of the important players that let your body know whether it should track down food or stay on the sofa. One of the most important hormones in triggering hunger is called ghrelin. Made by the stomach when it is empty, ghrelin sends a signal that says "I'm hungry" to the brain. Neurons in the hypothalamus (a part of the brain that regulates appetite, as well as sexual desire and sleep) respond by instructing the body to seek food (which usually means a quick trip to the fridge or turning the car into the nearest drive-thru).

Putting the Brakes on Eating

Clearly, if you succumbed to the powers of ghrelin too easily, serious weight gain would result. That's why it's good that you also have hormones to put the brakes on your appetite and send the "I'm full" signal to your brain. These satiety hormones are called PYY (peptide YY) and CCK (cholecystokinin). PYY and CCK are produced toward the end of a meal and are responsible for giving you that satisfied, full feeling (see the sidebar "Hormones - The Key to Appetite Suppression"). Of course, we all know that these signals can be overridden - just think, for example, of that bloated "I-wish-I-hadn't-eaten-all-that-food" feeling you get following a Thanksgiving feast.

MCTs Help Suppress Appetite

The "I'm full" signal is triggered more effectively by some foods than others. In general, fats are the most effective dietary ingredients for inducing the production of PYY and CCK, thus reducing the desire to eat. Not all fats are equally effective in suppressing appetite, however. Medium-chain triglycerides (MCTs), for example, appear to be more effective than the far more common long-chain triglycerides (LCTs) when it comes to ratcheting down those hunger pangs. The inclusion of MCTs in your diet - in the form of salad oil or cooking oil - can thus offer yet another aid in your battle for weight control.¹

MCTs are different from most dietary fats. Compared with LCTs, which likely account for most or all of your current fat intake, MCTs have a lower caloric content; they typically don't get stored in fat cells; and they stimulate thermogenesis (energy output in the form of heat) by raising the body's basal metabolism rate, which is a measure of the amount of energy expended in performing basic involuntary functions, such as breathing, digestion, and heartbeat.

MCTs Increase Satiety

MCTs work by several different mechanisms to help you rein in your weight. One has to do with satiety, the feeling of satisfaction brought on by a full stomach. Researchers in France fed different groups of people breakfasts that were either low in fat or rich in one of several types of fat: MCTs, saturated LCTs (e.g., lard), and unsaturated LCTs (e.g., olive oil); they then observed behavioral and metabolic responses to these different meals.²

During the study, all the subjects were restricted to rooms that contained no time cues: they were kept under artificial light, with no windows and no time-keeping devices, and they were allowed to request food when they were hungry again. The length of time before the next meal requests (lunch and dinner) was monitored, as was the amount of food (caloric content) consumed at each of those meals.

The researchers found that the low-fat breakfasts resulted in significantly shorter time intervals between meal requests than did the fat-rich breakfasts. This is not surprising, considering that fats are best at triggering the satiety response. It follows that diets with the most fat would induce the greatest satiety and allow the subjects to remain food-free for the longest intervals.

MCTs Result in Less Food Intake

With respect to breakfasts rich in either MCTs or LCTs, there was no difference in the time intervals between breakfast and lunch. However, the amount of food consumed by the MCT group at lunch was significantly less than the amounts consumed by either the LCT group or the low-fat group. The total caloric content of the lunch consumed by the MCT group was 18.4% lower than that of the saturated-LCT group, 16.5% lower than that of the low-fat group, and 5.4% lower than that of the unsaturated-LCT group. The authors concluded that MCTs modified food intake in these subjects via a "postabsorptive mechanism" that enhanced satiation and decreased food consumption at the next meal.

Hormones - The Key to Appetite Suppression Scientists have known about the function of the hormone CCK for a long time. The small intestine starts secreting this molecule at the end of a meal and primes the digestive system to start breaking down fats (the last portion of a meal to enter the intestine from the stomach). For example, CCK stimulates the gallbladder to contract and release bile into the small intestine to help digest fats. But that's not all it does - it also enters the bloodstream and is transported to the brain, where it acts to suppress appetite. In fact, the common problem of reduced appetite in the elderly is believed to be caused in part by elevated levels of CCK.1 The hormone PYY has received much more press recently, thanks in large part to an article that appeared in the British journal Nature.2 This research demonstrated that injecting PYY into the bloodstream of healthy volunteers reduced their food intake by one-third over a 24-hour period. PYY is normally produced in the gastrointestinal tract following a meal, and the amount produced appears to correlate closely with total caloric intake. So the more you eat, the more PYY you produce, and the less food you eat at the next meal. PYY sounds like a great candidate for a supplemental appetite suppressant, but it is likely to be at least another decade before our knowledge of it can be turned into a useful weight-control therapy. References MacIntosh CG, Morley JE, et al. Effect of exogenous cholecystokinin (CCK)-8 on food intake and plasma CCK, leptin, and insulin concentrations in older and young adults: evidence for increased CCK activity as a cause of the anorexia of aging. J Clin Endocrinol Metab 2001;86:5830-7. Batterham RL, Dowley MA, Small CJ, et al. Gut hormone PYY(3-36) physiologically inhibits food intake. Nature 2002;418:650-4.

MCTs - A Better Kind of Fat All fats are not created equal. We know that unsaturated fats (both monounsaturated and polyunsaturated forms) have substantial health benefits compared to saturated fats (think of the Mediterranean diet rich in monounsaturated olive oil). And of these, the omega-3 fatty acids (fatty acids are precursors to fats) found in coldwater fish oils are especially beneficial with respect to mental function and cardiovascular health. All fats consist of a molecule of glycerol to which up to three hydrocarbon chains are attached (when it's three, the molecule is a triglyceride, the most common form of fat). These chains, of varying length, are derived from fatty acids, which are found naturally in our foods. Our bodies process different kinds of fats differently, depending in part on the length of the hydrocarbon chains as well as on their degree of saturation with hydrogen atoms. Most dietary fats, such as those mentioned above, are long-chain triglycerides (LCTs), with hydrocarbon chains typically containing 14 to 22 carbon atoms. Medium-chain triglycerides (MCTs) are smaller, with chains of 6 to 12 carbon atoms, but with a strong preponderance of 8-carbon and 10-carbon chains. MCTs are easier for the body to deal with - they can be absorbed directly from the gut and sent to the liver for rapid processing. By contrast, LCTs must be subjected to digestive enzymes in the gut, converted to a new form in the intestinal cells, and incorporated into molecular transport vehicles for delivery to the liver for processing. This helps explain why MCTs are a good fat source for people with malabsorption syndrome, in which ordinary dietary fats are poorly absorbed by the body owing to various dysfunctions of the pancreas, liver, digestive system, or lymphatic system. Another important difference between MCTs and LCTs is that MCTs are less energy-rich than their long-chain cousins. This means that if you consumed the same weights of MCTs and LCTs, you would be getting fewer Calories from the former: 8.2 Calories per gram of fat from MCTs, versus 9.0 from LCTs (a 9% reduction).1 Over time, a difference like that will really make a difference. Ingle KL, Dreidger A, Traul KA, Nakhasi DK. Dietary energy value of medium-chain triglycerides. J Food Sci 1999;64:960-3.

In terms of weight control, these are exciting results, suggesting that MCTs may play a significant role in suppressing appetite. It is important to keep these results in perspective, however, and remember that they derive from a study that focused on a single day's food intake. Further research is needed to determine whether sustained dietary intake of MCTs has a long-term impact on appetite, as well as to shed more light on how specific hormones are involved in this appetite-suppression response.

MCTs Increase Energy Expenditure

Longer studies with MCTs have also been performed, but with different objectives in mind. Canadian researchers focused on energy expenditure in individuals maintained on either MCT- or LCT-enriched diets; they investigated the impact that MCTs had on 12 nonobese premenopausal women over a 2-week period.³ For 14 straight days, all the women were fed the same types of food, but one group's meals were prepared with LCTs, while the other group's meals were prepared with MCTs. That was the only difference, and the total amount of food served was the same for both groups.

On days 7 and 14 of the trial, the researchers recorded the women's total energy expenditure and basal metabolism rate. On day 7, both measures were significantly higher in the MCT group: the basal metabolism rate had increased by 4.4%, and the total energy expenditure had increased by 3.1%.

To put this in perspective, the amount of energy expended increased by 52 and 45 Calories per day, respectively.* Over the 7-day period, women on the MCT diet burned over 350 more Calories than those on the LCT diet. This increased caloric output was due solely to differences in the dietary fat, not to any other factor, such as increased exercise. On day 14 of the study, the total energy expenditure in the MCT group continued to be higher than that in the LCT group, but the increase was not as dramatic as that observed on day 7.

*If you remember your high school science, 1000 physical calories = 1 nutritional calorie, the unit of measure used to rate foods. Scientists abbreviate nutritional calorie to Calorie (or just Cal) with a capital C to differentiate it from the physical calorie, but laymen don't usually bother with this distinction. In the context of nutrition, one can assume that the spelling calorie means Calorie.

MCTs - A Recipe for Success

In a marketplace that touts miracle diets, overnight weight loss, and "ripped abs" without the bother of exercise, the above results may seem unremarkable. But when you add up all the individual benefits of MCTs, it becomes clear that including this type of fat in your diet may offer significant benefits. Consider, for example, that MCTs have a lower caloric content than LCTs on a per weight basis (see the sidebar "MCTs - A Better Kind of Fat"); they can reduce overall food intake in the short term; and they increase the rate at which your body burns Calories. That sounds like a recipe for success that we can all benefit from in the age-old battle against an increasing waistline.

References

1. St-Onge M-P, Jones PJH. Physiological effects of medium-chain triglycerides: potential agents in the prevention of obesity. J Nutr 2002;132:329-32.
2. Van Wymelbeke V, Himaya A, Louis-Sylvestre J, Fantino M. Influence of medium-chain and long-chain triacylglycerols on the control of food intake in men. Am J Clin Nutr 1998;68:226-34.

3. White MD, Papamandjaris AA, Jones PJH. Enhanced postprandial energy expenditure with medium-chain fatty acid feeding is attenuated after 14 d in premenopausal women. Am J Clin Nutr 1999;69:883-9.

Dr. Jensen is a cell biologist who has conducted research in England, Germany, and the United States. He has taught college courses in biology and nutrition and has written extensively on medical and scientific topics.