but,at the time JA was writing ,it was considered to be particularly nourishing for invalids.

It was in fact, an edible starch ( which is how you and I know it ,Marianne!) which was produced from powdering and drying the pith of the maranta plant. This was imported to England from the West Indies and from India, and,as such, was a luxury item. Which is why Emma is so kind as to send some to Jane Fairfax. She knows they would not be able to afford it, and she knows , from her experience in looking after Mr Woodhouse, that it is exactly the type of food item recommended for invalids. Emmas resposnse is practical and generous. Note she sends some that is of "superior quality"

In her book A New System of Domestic Cookery (1815) Maria Rundell inlcudes recipes for arrow-root in her chapter entitled Cookery for the Sick and for the Poor

Included in the Sick Cookery section of that chapter, is her recipe for Arrow-root Jelly:

Of this beware of having the wrong sort, for it has been counterfeited with bad effect. If genuine, it is very nourishing,especially for weak bowels. Put into a saucepan half a pint of water, a glass of sherry or a spoonful of brandy, grated nutmeg and fine sugar; boil once up, then mix it by degrees into a dessert-spoonful of arrow-root, previously rubbed smooth with two spponfuls of cold water; then return the whole into the saucepan; stir and boil it three minutes
Page 279

Note that Mrs Rundell - who was writing her book for young ladies like her daughters who had little experience of runing a household-includes a warning against buying adulterated( and cheap) arrow-root, in her recipe.

Warnings about adulterated arrow root ( adulterated food before the introduction of food hyegeine laws and standards was rife in early 18th century England) appeared in the newspapers. Look at this one from The Times, Thursday 3rd July 1828:

Arrow-Root. The adulteration of this very valuable article of diet for children and invalids with fine flour of wheat or with what is termed potato starch(a common practise in this metropolis(London-JW)...) maybe easily detected-ten grains of the flour or of the potato starch, forming with two ounces of boiling water a pretty strong jelly while the same proportion of genuine arrow root forms a very thin jelly. The jelly of the true arrow root will retain its solidity three or four days while that of the potato starch becomes nearly as thin as water in the course of two days,a fact that strongly points out the superiority of arrow-root over the potato starch as a nourishing article of diet