My nutrition research has found me this lately:
Did you know? Women need twice as much iron as men, while pregnant women need three times as much as men! (Men: 8mg/day, women 18mg/day (after menopause women's goes down to 8mg/day), pregnant women 27mg/day)
A really nutrition-focused store-bought bread like whole wheat Dave's Killer Bread can get you 1mg/slice (I would assume homemade 100% ww could get you there too or maybe more, if it's more dense and thickly sliced).
It's hard to find good research on 100% whole wheat sourdough bread, but a quick search suggested it may even have 5mg/slice. Add to this the ability of sourdough to increase iron absorption!
Finally, according to Jovial Foods, Einkorn wheat has 22% more iron than modern wheat. Einkorn bread is also denser than other breads, meaning one slice may be even more nutrient-compact. Assuming that the 5mg/slice in 100% modern whole wheat sourdough bread is roughly accurate, that could put the iron content of one slice of 100% whole wheat Einkorn sourdough bread at over 6.1mg! That is a whopper! Comparing that to other well-known sources of iron, that's over 4 times as much iron as your daily 1/4 cup of nuts, and over twice as much as 100g of beef. It's almost as much as 100g of liver, too, which is great if, like me, you like liver in moderation, but get sick of eating 100g/day, especially as your fresh batch dries out more and more in the fridge every week.
That iron in wheat has low bioavailability. It's common to read that sourdough fermentation fixes that, but unless there's a very long fermentation (longer than most of us use), not much of the iron gets released. The optimum pH for wheat phytase to work is between 5 and 5.5 but most sourdough doughs get below 5.0 fairly quickly. In addition, the best temperature for the phytase reactions is higher than room temperature, somewhere around 40C. There's still activity at room temperatures but it's lower. One paper I read showed an activity of around 80% of optimal
In addition, the iron from plant sources is the "non-heme" form, and its effectiveness for nutritional purposes is rather lower than for the heme form, which basically only comes from animals. So wheat bread isn't usually an effective source of iron (and other nutrients that are also bound up as phytates). Eating it will vitamin C will improve its absorption.
The form of phytase in rye bran works effectively at lower pH values, the values most sourdough doughs would spend most of their time at during fermentation. The best temperature for rye phytase to work is higher at room temperature, but at least it will have more effect at practical fermenting temperatures than wheat phytase.
I have been experimenting with soaking bran sifted from whole grain flour. Soaking it in mildly acidified water for a long time, and possibly at elevated temperature, ought to release the iron and other nutrients in the bran, and the soaked bran and its soaking water could be added back to the dough. It's not so easy to get this right, in part because the buffering action of the bran causes the pH to rise too high if you don't keep an eye on it. If anything comes of these experiments, I'll post here on TFL about it.
Ironically, yeasted wheat dough (meaning no sourdough) does typically stay around pH 5.5, so it keeps in the right range for its phytase do its work. So if one used so little yeast that fermentation would take a very long time, possibly a day, there would be more released nutrients than with sourdough. (there are other mechanisms for nutrient release than only phytase activity, but that appears to be the main one).
Here's a somewhat technical paper on the subject of freeing up nutrients in grains by the use of phytase:
Reduction of phytic acid and enhancement of bioavailable micronutrients in food grains
TomP