It isn't "lore" but I have a fair amount of biology in my background, so...
Kajiit- these are meat eaters- just look at the teeth. Those teeth aren't designed for munching carrots but for slicing through flesh and snapping neck bones. That doesn't mean they can't get nutrition from fruits and veggies its just that meat would probably be much more nutritious for them. Remember the "Famous Potato Bread" lady? Kajiit. So they aren't *exactly* cat-a-likes. If you want to make something not very nutritious at all I'd make it dairy. (Actually I'd make it grains... but can't because of the potato bread.)
Orcs- Same thing: what are those teeth for? These are primarily meat eating creatures, but like bears orcs have grinding teeth as well. So they are omnivores with a preference towards meat. A less nutritios item for orcs might be grains. They probably also need to eat a LOT.
Argonians- These guys make less sense. Those are sharp teeth they have but not really for killing. So meat is NOT the primary food for these guys. Fish definately, but the way the scripts you're working with go I don't think there's any way to separate fish from other meats. So I'd make veggies (think seaweed and swamp roots) better for them and just leave meats normal or slightly less nutritious. Grains might be something they get less value from, or maybe dairy? C'mon- milk in Blackmarsh?! It would go bad in a heartbeat.
Dunmer- These guys eat anything. You might make it so they get a little extra value out of everything (i.e they have to eat less) to simulate a lean, survival metabolism.
Altmer- Less emphasis on meat, more on fruit?
Bosmer- Well, I'd go with the Green Pact. If they've lived that way for a while then they'll be tuned to get more nutrition out of meat than anything else. Diary, as well. Lowest would be veggies. Fruits are DESIGNED to be eaten and doing so doesn't harm the plant at all so I think they'd still eat them. Regardless, they should get nutrition from non-meats- just less.
The human races... harder to separate out. Going by the descriptions of their homelands...
Nords- Probably less fruits and veggies and more meat. More grains, as well- they do better with a short growing season and store well. Dairy is good for them, too. These folks also need to eat a lot.
Bretons- High Rock probably has more veggies and grains than fruit. Meat would have slightly less importance than veggies and dairy. ??
Imperials- like everywhere in the game, flat across the board.
Redguards- Hammerfell is an urban and maritime province with "poor farms" - so maybe less grains? Less fruits?
Just ideas. Looking forward to the mod.

-C