I am somewhat confused about the difference between masa harina flour vs. cornmeal. Wikipedia says the process of nixtamalization needs to be done in order for the corn to be nutritional, kill toxins and make it more palatable. Basically they use burnt ash or lye to break up the hull of the corn when they are making masa. I always use cornmeal and I don't die from toxins and I assume it is just as nutritional as eating tortillas.
Nixtamalization is a culinary process that begins with dried corn kernels (maize), and uses alkalinity to alter the chemistry of the corn. The process is thousands of years old, first recorded by the the Aztecs, but probably older and more wide spread.
This guide is very informative. Here the author describes the result of the nixtimalization process:
This practice accomplishes several things: soaking dried maize kernels in an alkaline solution makes it easier to remove the thin outer seed coat, or pericarp, that encases the kernel, either through grinding or by rubbing. Next, both alkalinity and heat treatment help to not only soften the kernel, but also chemically alter the endosperm and germ of cooked maize [...] (Bressani et al. 1958; Bressani and Scrimshaw 1958). Finally, nixtamalization flavors maize, altering the taste profile of the kernels, giving them a slightly bitter and earthy flavor (Briggs 2015).
...and of course, the added bonus is that it makes corn (masa) delicious!