What are Xanthones?
Xanthones are a set of chemical compounds that can be synthetically produced, but are more often found and more potent in their natural settings. They are powerful antioxidants in a family of phytonutrients. Xanthones are a natural occurring molecule found in the Mangosteen fruit's pericarp (rind), with the molecular formula 13H8O2. There are over 210 different types of Xanthones, with approximately 43 occuring in VEMMA. Mangostin Alpha, Mangostin Beta and Garcinone B have been among those studied more closely that appear to have health benefits. 
Mangostin Alpha is a potent antioxidant, which assists in preventing free radical damage to cells. Mangostin Beta appears to have an anti-inflammatory activity, inhibiting the COX2 cascade; Garcinone E seems to have an effect on inhibiting cancer cell growth. 
Xanthones provide the antioxidant support of VEMMA. Some scientists believe that free radicals in the body are the root cause of all illnesses and that a good supply of antioxidants can reduce the effect free radicals have on healthy cells in the body.
references:
Ho CK, Huang YL, Chen CC. Garcinone E, a xanthone derivative, has potent cytotoxic effect against hepatocellular carcinoma cell lines. Planta Med. 2002 Nov;68(11):975-9.
Matsumoto K, Akao Y, Kobayashi E, Ohguchi K, Ito T, Tanaka T, Iinuma M, Nozawa Y. Induction of apoptosis by xanthones from mangosteen in human leukemia cell lines. J Nat Prod. 2003 Aug;66(8):1124-7.
Moongkarndi P, Kosem N, Kaslungka S, Luanratana O, Pongpan N, Neungton N. Antiproliferation, antioxidation and induction of apoptosis by Garcinia mangostana (mangosteen) on SKBR3 human breast cancer cell line. J Ethnopharmacol. 2004 Jan;90(1):161-6.
Nakatani K, Nakahata N, Arakawa T, Yasuda H, Ohizumi Y. Inhibition of cyclooxygenase and prostaglandin E2 synthesis by gamma-mangostin, a xanthone derivative in mangosteen, in C6 rat glioma cells. Biochem Pharmacol. 2002 Jan 1;63(1):73-9.
http://altmedicine.about.com/od/completeazindex/a/mangosteen1.htm
Chen IJ et al. "Effects of xanthone glycoside on ephedrine-induced biting behavior and motor activity." American Journal of Chinese Medicine. 21.1 (1993):79-84.
Jiang DJ et al. "Pharmacological effects of xanthones as cardiovascular protective agents." Cardiovascular Drug Reviews. 22.2 (2004):91-102.
Matsumoto K et al. "Preferential target is mitochondria in alpha-mangostin-induced apoptosis in human leukemia HL60 cells." Bioorganic & Medicinal Chemistry. 12.22 (2004):5799-806.
Matsumoto K, Akao Y, Kobayashi E, Ohguchi K, Ito T, Tanaka T, Iinuma M, Nozawa Y. Induction of apoptosis by xanthones from mangosteen in human leukemia cell lines. J Nat Prod. 2003 Aug;66(8):1124-7.
Moongkarndi P, Kosem N, Kaslungka S, Luanratana O, Pongpan N, Neungton N. Antiproliferation, antioxidation and induction of apoptosis by Garcinia mangostana (mangosteen) on SKBR3 human breast cancer cell line. J Ethnopharmacol. 2004 Jan;90(1):161-6.
Nakatani K, Nakahata N, Arakawa T, Yasuda H, Ohizumi Y. Inhibition of cyclooxygenase and prostaglandin E2 synthesis by gamma-mangostin, a xanthone derivative in mangosteen, in C6 rat glioma cells. Biochem Pharmacol. 2002 Jan 1;63(1):73-9.
http://altmedicine.about.com/od/completeazindex/a/mangosteen1.htm
Chen IJ et al. "Effects of xanthone glycoside on ephedrine-induced biting behavior and motor activity." American Journal of Chinese Medicine. 21.1 (1993):79-84.
Jiang DJ et al. "Pharmacological effects of xanthones as cardiovascular protective agents." Cardiovascular Drug Reviews. 22.2 (2004):91-102.
Matsumoto K et al. "Preferential target is mitochondria in alpha-mangostin-induced apoptosis in human leukemia HL60 cells." Bioorganic & Medicinal Chemistry. 12.22 (2004):5799-806.