Cereal grains become more bioavailable when treated with an alkali solution like soaking in water with baking soda overnight (IOM et al., 2006). Use 1/2 – 1 t baking soda per liter of water. The reason for the lower bioavailability in grains is that the niacin molecules are bound to niacytins and niacinogens. These can be removed by different processes, such as nixtamilization with corn where it is treated with lime. You can check for this on the packaging. Lime should be the only other ingredient besides maize/corn for your tortillas. If you are purchasing things like stone ground cornmeal or polenta, you should be aware that much of the niacin in these products is not available because it is bound to niacytins.
If you are interested in other factors that affect niacin bioavailability, see this post: https://thecarmencooks.com/staging/3924/niacin-and-panic-attacks-why-you-should-take-a-niacin-supplement-if-you-suffer-from-chronic-panic-attacks/
