Appetite is controlled by many factors (Figure 3.1). Processes in the body tell people when they are hungry and when they are satiated (full). Neurotransmitters (chemical messengers in the brain) such as norepinephrine, serotonin, and dopamine tell the brain that the stomach feels full (Figure 3.2). Drugs that increase the levels of these neurotransmitters cause a decrease in appetite by relaying the message that the stomach is full, thus telling the brain to send a “stop eating!” signal. These drugs are effective because, in general, appetite decreases when a person feels full. Drugs that increase more than one of the neurotransmitters may be more effective for weight loss than those that increase only one. Some drugs that work in this way also have the effect of increasing the number of calories burned (thermogenesis).

Dopamine, norepinephrine, and serotonin have other responsibilities in the body besides dictating hunger. For example, norepinephrine also helps control blood pressure. Drugs that affect the level of these neurotransmitters interfere with other body processes and produce negative side effects. A drug that increases norepinephrine will decrease appetite, but

Figure 3.1 Appetite is controlled by many body processes, as shown here. The arrows indicate things that increase and decrease hunger. All of these processes work by sending signals to the brain to indicate a feeling of hunger or satiety (fullness). Certain diet pills called appetite suppressants may work in the same way as some of these body processes, by sending signals to the brain that indicate satiety and say “stop eating!”

will also increase blood pressure. Other common side effects of drugs that increase norepinephrine are dry mouth, insomnia, and constipation. Examples of diet pills that affect these neurotransmitters are sibutramine (Meridia®) and phentermine

Figure 3.2 Normally, when the stomach is full it sends a signal to the brain telling the body to stop eating. This message is carried by neurotransmitters (chemical messengers in the brain) to the hunger center in the hypothalamus (a part of the brain). Examples of neurotransmitters that carry this message are norepinephrine, serotonin, and dopamine. Many diet pills increase these same neurotransmitters that signal the brain that the stomach is full. These diet pills, called appetite suppressants, trick the brain into thinking the stomach is full and therefore decrease hunger.

(Ionamin®). Sibutramine increases norepinephrine and serotonin. Phentermine increases norepinephrine only. Other hormones and proteins in the body may also affect appetite.Many of these have been discovered recently and may be useful in the development of new diet pills. As previously discussed, the most promising protein so far is leptin, which is produced by fat stores and tells the body it is full. Several other known proteins and hormones affect appetite and may in the future be formulated into drugs for weight loss.

Understanding how hormones work in the body and how they affect satiety (the feeling of being full) is crucial to the development of new products for weight loss. Scientists continue to learn of different processes in the body, even at the cellular level, that can lead to weight gain. Amid the explosion of obesity, scientists continue to identify chemicals with the potential to be turned into drugs that can be used to assist with weight loss, although diet and exercise will always be the best, first-line strategies for losing weight.

Reference:

1. Diet Pills (Drugs: the Straight Facts) by Debra Henn and Deborah DeEugenio (Hardcover – Oct 2004)