Control of Food Intake and Body Weight  The body is in a continual state of hunger, which is intermittently relieved by eating. This perpetual drive to eat is periodically suppressed by inhibitory impulses generated by such things as the presence of food in the gastrointestinal tract, the flow of nutrients into blood and other factors. After these "satiety factors" have dissipated, the desire to eat returns. Why is it important to understand the factors that control food intake? At least two major areas of import come to mind:

• Obesity is the most prevalent nutritional disease of humans, dogs and cats in affluent societies such as ours, exceeding by far the number of nutritional deficiency diseases.
• Metabolic demands of people and animals increase with sickness or trauma, often in conjunction with anorexia. Sickness combined with anorexia leads to accelerated starvation.

Before going on, take a minute to reflect on observations you have already made about food intake, body weight and similar topics. You may have noticed, for instance, that:

• Most animals as adults maintain a remarkably constant body weight.
• When it's cold, animals and people eat more than when it's hot.
• Children maintain energy balance with wildly varying intakes of food per meal.

These kinds of observations suggest a very complex system in charge of regulating energy balance and body weight. What is known about control of food intake is often discussed in terms of short-term and long-term controls. This discussion will focus on the following areas:

• Role of the central nervous system
• Pregastric factors
• Gastrointestinal and postabsorptive factors

Role of the Central Nervous SystemFor many years, the hypothalamus was thought to be the key to control of food intake. This view derived from classic experiments in which food intake was studied in rats with lesions in various areas of the brain. Such studies clearly identified two regions in the hypothalamus that dramatically influence feeding behavior:

• Lateral hypothalamus (hunger center): animals with lesions in this area become anorectic and lose weight.
• Ventromedial hypothalamus (satiety center): animals with lesions in this area overeat and become obese.

 Pregastric FactorsWe all know of "environmental" conditions that can dramatically affect food intake. Consider which of the following items are likely important to animals, humans or both:

• Appearance of food: humans like or dislike certain meals based on visual appearance, but does your cat appreciate your buying fish-shaped food?
• Taste and/or odor of food: this is extremely important in all species.
• Learned preferences and aversions: Almost everyone has an aversion to one or more types of foods, and they also affect companion animals.
• Psychologic factors: mental states such as fear, depression and social interactions often affect food intake.

.Postabsorptive FactorsThe degree of gastrointestinal fill is the most important signal from the digestive tract per se - a full stomach and intestine induce satiety, probably via the vagus nerve relaying that fact back to the hypothalamus. Additionally, the enteric hormone cholecystokinin is well documented to induce satiety in experimental settings, while the hormone ghrelin seems to be a potent stimulator of appetite. As nutrients such as glucose and amino acids are absorbed, their concentrations in blood rise, as do the concentration of several hormones (cholecystokinin as mentioned above, but also insulin and glucagon). These changes also have been linked to the sensation of hunger or satiety.RAJESH KUMAR