Hoodia Gordonii a cactus native to the Kalhari Desert of South
Africa, contains an all natural appetite suppressant.  Hoodia contains a molecule, called P57 that has similar effects on nerve cells as glucose. It works to tricks the brain into feeling full.  Results of Hoodia human clinical trials in Britain suggest that hoodia may reduce the appetite by up to 1000 calories a day or more when consumed in effectious and large enough quantities.

In a BBC interview, Phytopharm's Dr. Richard Dixey explained how P57 works:

There is a part of your brain, the hypothalamus. Within that mid-brain there are nerve cells that sense glucose sugar. When you eat, blood sugar goes up because of the food, these cells start firing and now you are full. What the Hoodia seems to contain is a molecule that is about 10,000 times as active as glucose. It goes to the mid-brain and actually makes those nerve cells fire as if you were full. But you have not eaten. Nor do you want to.

References:

1. Jan. 31, 2005 - Natural Products Industry INSIDER "The Supreme Qualities of Hoodia Gordonii" References

2. Tulp OL et al. "Effect of Hoodia plant on food intake and body weight in lean and obese LA/Ntul//-cp rats." FASEB J. 15, 4:A404, 2001.

3. Tulp OL, Harbi NA, DerMarderosian A. "Effect of Hoodia plant on weight loss in congenic obese LA/Ntul//-cp rats." FASEB J. 16, 4, 2002.

4. Habeck M. "A succulent cure to end obesity." Drug Discovery Today. March 2002, pp 280-1.

5. MacLean DB, Lu-Guang L. "Increased ATP content/production in the hypothalamus may be a signal for energy-sensing of satiety: studies of the anorectic mechanism of a plant steroidal glycoside." Brain Res. 1020, 1-2:1-11, 2004.

6. Goodhart RS, Schils ME [eds]. Modern Nutrition and Disease, 6th Ed. Lea & Febiger, Philadelphia, pp 560-77"

In the brain, the hypothalamus and its nerve cells signal and control hunger.  When you eat, blood sugar goes up and these nerve cells signal your brain that you are full. Scientists discovered that Hoodia contains a previously unknown molecule called - Hoodia Steroidal Glycosides, or P57 that is estimated to be about 10,000 times more active than glucose. When it reaches your hypothalamus nerve cells, it actually makes those nerve cells react as if you were full.