if you heat up your chicken, rice, beef, potatoes, paste etc in a microwave just take them raw (not frozen) and they will be 100% fine. Do it all the time.
Precook any beef/poultry/pork and your carbs (potatoes/pasta/rice) and then reheat with raw veggies. Frozen veggies reheated may be a problem (never tried), always grab fresh.
Frozen produce is typically just as and often significantly more nutrient-dense than fresh produce, owing to the bagged stuff being flash-frozen much closer to proper ripeness than the "fresh," which needs to be harvested far enough in advance (weeks) to survive transport and display without going rotten before sale.
This is particularly evident with a lot of fruits, such as strawberries, which will taste and even look significantly different in frozen versus fresh forms -- non-local "fresh" strawberries will typically have a solid white and somewhat bitter center indicative of being picked way too early; locally-harvested, recently-picked strawberries will share a consistent red hue with their frozen counterparts indicative of much better ripening. If "fresh" grocery store fruit has ever tasted a little lackluster to you, this may be why.
I can't link the studies (another few years of posting, I'm sure), but frozen broccoli has been shown to be equal or superior to the store-bought fresh counterparts in several vitamins and folic acid. No real negatives nutritionally.
Where people often go wrong with frozen vegetables is the preparation. Throwing your vegetables into a big pot of boiling water and then cooking the crap out of them -- pretty much literally -- will definitely reduce the nutritional content, particularly of anything remotely water-soluble. Boiling's also one of the least flavorful ways to enjoy (or not enjoy) veggies, which leads to a downward spiral of people eating even less of the reduced-nutrition vegetables because "they taste bad," or using "generous" portions of butter to compensate -- story of my childhood, but I won't go there.
Buying big bags of frozen broccoli, cauliflower, or whatever else, then throwing a few hundred grams on a cookie sheet to roast at 370-400 Fahrenheit produces fantastic results, particularly if you spritz with a few grams of oil and cover in some tasty spices. Solid foundation to almost any meal. Plus, you can sometimes even get "lazy" and do combo recipes where you bake some meat--chicken or salmon filets work great--at the same time.
It's easy to defrost frozen veggies in the microwave and then add them to skillet recipes, too. All sorts of creative ways to incorporate.
Not that I'm saying there's anything bad about fresh vegetables -- they're still great! -- but I can often get better value and availability for some of the staples by keeping frozen forms on hand, and that's A-OK.
