How Diet Pills Work

In the quest for the perfect diet pill, scientists, doctors, and those who are going to use these drugs need to understand how diet pills work. Knowing what actions a drug takes and how it affects the body helps scientists and doctors understand what side effects the drug may cause. Recognizing side effects and understanding what causes them can, in turn, help researchers formulate drugs with fewer or less serious side effects.

HOW DO DIET PILLS CAUSE WEIGHT LOSS?

Diet pills work by affecting the body processes that cause weight gain. Diet pills work to cause weight loss in one of three main ways:

1. Increasing energy expenditure (calories burned).
2. Decreasing the number of calories that are absorbed
3. during digestion.
4. Suppressing appetite to decrease how much food
5. is eaten.

Increasing Energy Expenditure (Calories Burned) Calories consumed in food are used by the body as fuel. The body’s use of calories as its source of energy is called thermogenesis. Literally, thermogenesis means “the production of heat” because when people burn calories, heat is produced. The body needs a constant supply of fuel to maintain normal functions that people don’t usually think about, like breathing.

Increasing Energy Expenditure (Calories Burned)

Calories consumed in food are used by the body as fuel. The body’s use of calories as its source of energy is called thermogenesis. Literally, thermogenesis means “the production of heat” because when people burn calories, heat is produced. The body needs a constant supply of fuel to maintain normal functions that people don’t usually think about, like breathing.

Certain drugs cause the body to burn more calories as it performs its normal daily functions. By burning more calories, people lose weight even though they are eating the same amount. These drugs increase the body’s fuel requirements, ideally to a point where the body needs more calories than the person takes in from food. When this happens, the body begins breaking down its fat stores as fuel. As these fat stores are used, people lose weight. Side effects associated with this type of diet pill often occur because, in causing the body to burn more calories, the pills speed up many body processes—possibly to a level that becomes dangerous. Examples are increased blood pressure and heart rate, which can lead to heart problems over time. Other side effects include nervousness and insomnia. Many diet pills that work by increasing the number of calories burned have been developed. Most of these contain one or more of the three ingredients: caffeine, phenylpropanolamine (PPA), and ephedrine. In the United States, PPA and ephedrine have been withdrawn from the market
because they produce serious side effects, including heart attack and stroke.

Decreasing Calorie Absorption

After food is eaten, it is digested in the stomach. The digested nutrients from food are absorbed, mainly in the intestines, into the bloodstream and are used as fuel for the body.When people eat more food than they need, the body stores the extra nutrients as fat, which accounts for weight gain. Some diet drugs are able to prevent the body from absorbing some of the calories contained in food. People who take these drugs can eat the same amount of food and take in the same number of calories and still lose weight, because the drug causes their bodies to absorb and use fewer calories. The excess calories are eliminated in the stool. Drugs that preventabsorption of nutrients in the intestine may also prevent other drugs a person may be taking from being absorbed. For this reason, it is important not to take these diet pills at the same time as other medications, and to separate them by a few hours. Since these drugs affect the digestive system, including the stomach and intestines, they may also lead to side effects like diarrhea and flatulence (passing gas).

Currently, the only drug on the market that works by blocking the body’s absorption of calories is orlistat, which is sold under the brand name Xenical®. Orlistat specifically
decreases the absorption of fats from food consumed.

Appetite Suppression

Appetite is controlled by many factors. 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. 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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. 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!”