Source from Department of Chemistry, National University of Singapore, Republic of Singapore. 
PMID:17960880 [PubMed - indexed for MEDLINE]

mangosteenAlpha-amylase inhibitor (alpha-AI) activity of Garcinia mangostana, commonly known as mangosteen, pericarp extracts was studied by assay guided fractionations from lipophilic to hydrophilic using combined solvent extraction and Amberlite XAD2 adsorption chromatography. Neither the lipophilic, xanthone containing fraction, nor the highly polar fraction, which has no affinity on Amberlite XAD2, showed any alpha-AI. 

The fraction that shows very high inhibitory activity contains primarily polyphenols and can be adsorbed on Amberlite XAD2. The IC50 of 5.4 microg/mL of this fraction is comparable to that of acarbose, a prescribed alpha-AI used in the control of type II diabetes, at 5.2 microg/mL.

Total phenolic content (TPC) of each fraction was measured and the TPC has no correlation with the alpha-AI activity.

The lipophilic fraction contains mainly xanthones as revealed by HPLC-MS analysis. Colorimetric analysis coupled with UV-vis and IR spectroscopic analysis demonstrated that the fractions with high alpha-AI activity are primarily oligomeric proanthocyanidins (OPCs) with little gallate moiety.

There is also evidence to show that the alpha-AI by these OPCs is not purely by nonspecific protein complexation. Both tannic acid and G. mangostana OPCs precipitate BSA equally well but G. mangostana OPCs are 56 times more effective in inhibiting alpha-amylase.
The Xanthones from the mangosteen have been shown in laboratory studies to be powerful Antioxidants, Anti-Inflammatories and to have other special properties. However, laboratory results do not guarantee that the same will happen in the human body. Mangosteen is a nutritious fruit not a drug, and no therapeutic claims are made for this product.