Trim them more often, get them removed, or try different shoes (or a different size).

My question is.......is this caused by shoes that are too small or too large? My shoes feel great.

They're Clifton 2's. I'm reading the 3's have a bigger toe box, but I honestly haven't had issues on shorter runs (up to 8 mi.). My longer runs are brutal on my toes. I'm assuming I'm not alone.

My shoes were actually "squishing" when I got home on Monday from a run outside (saturated). I know this is a blister recipe (I got a couple small blisters), but I wouldn't "think" it had any bearing on the toenail issue. But again, I'm the one looking for answers.

(and, FTR, I can't trim them any shorter)

^^^ this. If you repeatedly apply pressure to the toe nail during a run maybe about 80-95 times a minute, and do that for 1-1/2 to 3-1/2 hour, it causes damage. Imagine that.

I've lost I think 3 toe nails in the last 10 months. Shoe fit and not being diligent enough trimming toe nails explained them. In hot weather in particular in longer event,s your feet tend ot retain fluids and swell, or at least mine do. So a shoe that was snug, is now a 1/2 size too small.

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TrainingBible Coaching
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Your shoes are too small. I have never done anything over 32 mi, but I have never lost a toenail. Even in races with a vertical mile of descending--10 toenails at all times. Even when I practice downhill repeats on the -17% grade street in my neighborhood, I don't damage my toenails.

For me, LockLaces have helped this. Helps make the shoe remain firmly attached but still stretches and moves with my foot.

Your long runs shouldn't be brutal on your toes. Remember that your running shoe size is at least a half and possibly a full size larger than your everyday shoe size. Also, if you are too tight in the toe box, there are different ways to try lacing your shoes to give yourself a bit of space.

(Says the person who has lost the same toenail three times using at least two different shoe brands and also keeping toenails as short as possible, so what do I know.)

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http://mediocremultisport.blogspot.com

I appreciate everyone's input. It was 95 deg. (over 100 deg. "feels-like" when I did my long(ish) run. I did 7 indoors and by mi. 4 outdoors my feet were squishing (saturated). I don't have these issues on short runs.

I'm going to go try on the Clifton 3's (supposed to have a larger toe box) and see if a 1/2 size larger feels better. I already wear 1/2 size larger (sometime a full size) than my "other" shoes. Hopefully I'll get this figured out. Again, thanks for your insights.