The mangosteen is not related to the mango. Part of its mystique seems to be due to the fact that it is fairly rare and not that easy to grow. An attempt to start a mangosteen orchard in the US, resulted in one tree that bore one fruit and then promptly died. It has become widely touted as a superfruit cure all that has potent cancer-prevention powers.
In fact, it seems that the most remarkable thing about the mangosteen is its lack of macro- or micro-nutrients. All mangosteen's micronutrients fall below 5% of their respective Dietary Reference Intake (US Academy of Sciences). The exceptions are carbohydrate and dietary fiber, and some relatively puny amounts of vitamin C, manganese and folic acid.
Mangosteen's cheerleaders claim that it is a terrific source of antioxidants. There is absolutely no evidence of this. No independent ORAC studies can be found and, typically, white-fleshed fruit have a very low ORAC rating.
The thing that may redeem mangosteen is the presence of xanthones. These are anti-inflammatory agents that can inhibit carcinogens. The problem is that the jury is very much still out. Scientific understanding of xanthones is very much at the baby-step stage and published research is sparse. In PubMed, the online database of medical abstracts from the US National Library of Medicine, there are about 45 citations on xanthones, compared to 10,000 on polyphenols (the antioxidant in colorful plants).
On that note, I shall go back to goji berries. They taste a whole lot better too.