It's not rocket science: Eat fewer calories than you burn and you'll lose weight.

But somehow it seems almost impossible to do. How could something so simple be so hard? The dilemma we all face is how to eat less without feeling hungry and deprived. The solution, while not as important for short-term weight loss, is essential to weight maintenance. Any diet that keeps us hungry is doomed to fail over the long haul.

For decades, scientists have been digging for clues about what influences the decisions to start and stop eating. They have found that our bodies have a complex physiological signaling system that tells us when we're hungry and when we've had enough. When you're eating, nerves in the stomach wall detect your stomach is stretching. They send satiation messages to the brain and you start to feel full. When food is sensed in the intestines, several substances are activated there as well, including nerve regulators and hormones such as cholecystokinin (CCK).

The physiological sensation of hunger has been more difficult for researchers to pin down. Emerging research points to ghrelin (pronounced GRELL-in), a hormone sent into your bloodstream by the stomach when it is empty.

The level of ghrelin "goes up before meals -- making you hungry -- while the other satiating hormones go up after eating [with ghrelin going down], causing you to stop," says David E. Cummings, associate professor of medicine at the University of Washington and the Veterans Affairs Puget Sound Health Care System. "With these two systems, you have physiological control over pre-meal hunger and post-meal satiety."

Of course, we're not automatons governed strictly by physiology. The decision to eat is also governed by things we can control.

"In humans, ghrelin may not be the only driver. Initiation of meals is a complex process that deals with social cues, smells, sights and more," says Phil Smith, co-director of the Office of Obesity Research at the National Institute of Diabetes & Digestive & Kidney Diseases.

Evidence suggests that many factors are related to your satisfaction with a meal and ultimately, your ability to lose and maintain weight:

Portion size In a series of widely known experiments over the past five years, researchers found that the amount of food you are served will affect how full you feel and how much you eat. In the studies, people were given varying amounts of macaroni and cheese, submarine sandwiches and bags of popcorn or chips. In each case, when they were served smaller portions, they ate those portions and felt satisfied. But when they were given larger portions, without realizing it, the subjects (including children, in other similar studies) ate significantly more -- sometimes 50 percent more.

This effect persisted. Over two days, when portions were 50 percent larger at each meal, subjects ate 16 percent more (328 extra calories in women and 522 extra calories in men per day). When the portions were 100 percent larger, subjects ate 26 percent more (531 extra calories in women and 806 extra calories in men).

Those extra calories, added daily over the course of a year, would pack 50 pounds more on women and 80 pounds more on men.

"This is a case where physiological satiety cues are overridden by environmental cues such as large portions and the easy availability of food," says Barbara Rolls, co-author of the studies and a professor of nutritional sciences at Pennsylvania State University in University Park. "When served larger portions, people adjust their level of satiety to accommodate greater calorie intakes."

Fortunately, studies also have found that the reverse is true. When good-tasting, lower-calorie foods or portion-controlled meals are available, people will eat those and feel just as satisfied. In fact, studies of successful weight loss maintainers find they easily adjust to smaller, more appropriate portions of higher-calorie foods.

An interesting study showed that preschool-age girls who regularly overeat can be taught how to change their behavior by learning to pay attention to their natural hunger and satiety signals.

Water and air content Rolls and colleagues found that as long as the volume of the food is high, people can feel full with fewer calories. In a study published in 2000, participants who drank milkshakes blended with more air (compared with the same shakes containing less air) ate 12 percent less at the next meal without realizing it.

In the most recent experiment, which will be published in the Journal of the American Dietetic Association in October, the researchers served salads of various sizes and calorie levels before a main course to determine the effect on the calorie intake of the whole meal. Participants consumed the fewest overall calories -- 100 calories fewer -- when they were served the largest, lowest-calorie salad before a meal.

Researchers surmise that a large food volume caused by water or air, even without added calories, influences satiety in a variety of ways. It causes stomach stretching and slows stomach emptying, stimulating the nerves and hormones that tell you when it's time to put down the fork. Also, seeing a large volume of food can increase your ability to feel satisfied by it.

Finally, the larger a meal is and the longer a meal goes on, studies show, your satisfaction declines and you lose interest in completing it.

"Water is the component in food which has the largest influence on how much you eat," says Rolls, who co-authored "The Volumetrics Weight-Control Plan" (HarperTorch, 2003). "These studies show eating a high-water-content, low-calorie first course enhances satiety and reduces calorie intake at the next course."

Type of diet New research on ghrelin reveals it can work against weight loss. Apparently, losing weight under most conditions -- low-calorie diets, anorexia, binge-purging and high levels of exercise -- can increase ghrelin that's circulating in the bloodstream and cause hunger. This is the body's evolutionary way of holding on to body fat and surviving during the famines that threatened our ancestors, according to Cummings. But his research found one diet in which weight loss did not increase ghrelin levels: a low-fat one. After three months, people on a 15-percent-fat diet lost significant weight. But their ghrelin levels stayed the same.

Cummings cautioned that levels of fat higher than 15 percent -- say, 20 or 25 percent -- as well as other diet methods, may also prevent rises in ghrelin; not every situation has been tested.

But these results help explain studies of successful weight loss maintainers, most of whom eat a low-fat (less than 24 percent of calories) diet.

There's a lot more to learn about why we eat. Researchers are studying types of foods, diets, meal timing, meal patterns, habits learned from childhood, genetics and sensory factors. But in the meantime, there is plenty that we do know to help us eat the right number of calories but still feel full and satisfied.

Katherine Tallmadge is a Washington nutritionist and author of "Diet Simple" (Lifeline Press, 2004). Send e-mails to her at