I have the same problem, really bad, I tried a vegan diet once and lost it in gobs. Then over the past few months while my budget has been really tight I went a lot of weeks without meat and just eating pasta and frozen dinners (fridge was broke but I have a little chest freezer), so it started falling again, very noticeable in the brush, but worse in the shower.
Anyway, back during the vegan thing I went to a naturopathic doctor who was also and m.d., he did muscle testing and blood tests then told me that in addition to the PCOS androgen imbalance, my body was not processing the amino acids in the vegetables correctly to make protein, so it was using my hair follicles and ova for protein. He said that a lot of people who are of northern decent need a diet that includes at least 6 ounces of red meat per week. (BTW for all you die hard vegans who figure I was doing it wrong, I was very health conscious and combined my vegetable matter to get full advantage of the different aminos in each, thus the naturopathic doctor.)
I just started the Atkins induction diet this past week, so I don't notice a halt to the shedding yet, but my hair is quite long, so sometimes a little loss looks bigger because of the length. Sometimes, I use Nioxin, but after several months of use it started giving me dandruff so now I use it a couple times a month, hopefully the low carb diet will make it unnecessary.