My mother said that her diabetic friend told her that soaking potatoes in water removes starch. I questioned whether this was a fact or not. My research turned up this:
"The potato is in such universal use, and is so highly nutritious, that above all other vegetables it would seem that it should be cooked in perfection. However, it may be in reality that no other vegetable is so carelessly cooked. The proper cooking of the potato has much to do with its nutritive value. It is a starchy food; the microscopic starch grains are intermingled with the watery juice, which contains the albumen, gelatine, minerals, sugar, and acids. All of these elements are highly valuable as food or stimulants, but by careless and unscientific methods in preparing and cooking they may be partially lost and wasted. How often do we hear epicures criticising our best hotel cooking, remarking the impossibility of being served with mashed potatoes of a good quality? Where food must be prepared in such large quantities the potatoes are pared and soaked in water for from twelve to eighteen hours before being boiled, thus losing a high percentage of the agreeable flavors,
the nutritious starch and albumen and valuable mineral matter."
http://chestofbooks.com/food/ingredients/Potato-Compilation/Notes-On-Cooking-Potatoes.html