Purple sweet potato
The same tuber with a different pigment. Purple is anthocyanin, the same compound as in a blueberry, and there is a lot of it.
Glycemic index
No separate reliable value for the purple variety appears in the international tables. Reckon on the neighbourhood of the orange one, and lower if you let it cool first.
How to use it
As the starch on your plate, or roasted as a side where the colour does the work. It is firmer and less sweet than the orange one.
The best version
Deep purple right through and not only at the skin. A tuber that runs white inside carries little of the pigment you are paying for.
How to prepare it
Boil or steam it and then let it cool. The pigment dissolves in water, so boiling it in plenty of water pours part of it down the sink; steaming or roasting keeps more of it.
What it does
The pigment has been tested in people. Forty healthy adults drank three bottles a day of a purple sweet potato beverage carrying 177 milligrams of anthocyanins per bottle, or a placebo with 1.3 milligrams, for eight weeks. The liver markers GGT, AST and ALT came out lower in the purple group than in the placebo group.
Let cooked starch cool and part of it crystallises into resistant starch: it reaches your large intestine undigested and the bacteria there turn it into butyrate, the fuel your colon wall runs on. In a randomised trial in 42 older and 42 mid-age adults, butyrate in the stool rose in the older group, and Bifidobacteria increased in both groups.
Watch out
That was a drink and not a tuber, in forty people, and the researchers call it a pilot trial themselves. Three bottles a day is also an amount of anthocyanin you will not reach by eating.
Purple does not mean provitamin A. If you want the pigment from the orange kind, you need the orange kind: these are two different compounds that both give colour.