This is confusing. Not sure what quite you are looking for.
If I want to preserve all the goodness including vitamins in meat - I would stew/braise, slow and low cooking.
As to a high heat solution, talking good steak or veal or lamb here, get to room temperature, smear with oil and S&P it, onto hot pan to sear. Quick sear, only to point of rare. Let rest for as long as you cooked it, letting it relax (put it under some foil if you like, but loosely). Juices relax into the fibres of the meat.
As it rests, make a sauce from any pan juices nice crunchy brown bits - doesn't have to be butter added. Can use low salt beef stock or red or white wine, any combination of these that suits you. whichever you like. Reduce it while the meat rests. Really doesn't take long on medium heat. Pour any juices from the meat back into the reduction...and....Yum! (I add cream at the end sometimes, but its not a diet option. A good Tbsp of greek yoghurt does just as well).
And please - warm your plate Many forget this, and the food is cold before they are done.
Or...rather than worry about the extra milligram of protein lost in cooking meat, besides meat get your proteins by mixing grains and legumes, grains and seeds, seeds and legumes, or adding milk, eggs, cheese to any of the above, or having milk, eggs and cheese on their own. There are proteins all around us! Proteins are protean! I think you'd have a harder time trying to limit them than to increase them. If it's for your health, eat a wide range of foods - we need it all. And the better you make it taste, the more protein you'll end up eating. If you steam your meat or something to maintain the protein, it will taste lousy and you won't be inclined to eat it.
I had no idea that meat loses protein when you cook it. I mean, aren't the muscle fibers made up of strands of protein? I didn't know that in denaturing, they dissolve and can leach out. Myoglobin -- also protein -- definitely does leach out, that I know for sure (it's the juice people often incorrectly refer to as "blood" ), but the best way to prevent that is to not freeze the meat/fish/poultry because that breaks cell walls and allows the myoglobin to leak out.
But if any of the protein is lost from the muscle fibers, my guess would be micrograms, not milligrams. (I could be wrong, and if anyone finds other evidence, great.)
(First, been reading this place for a while but just now registering and posting. Hi!)
I'm going to build on what someone said before... saving "proteins" isn't what matters here. You mentioned that you're on a new diet. From a biochemical perspective, then, what you're interested in is actually amino acids. Amino acids are the monomer building blocks of proteins and serve two basic dietary functions -- they can be oxidized for energy or used to support or promote lean mass (especially muscle). Carbohydrate and fat are better at the first, so even if you are following an absurdly low-carb diet like Atkins, the calories from protein for energetic metabolism are going a non-factor because you'll get it from fat. That means you're looking at muscle, and muscle tissue actually absorbs amino acids rather than (large) proteins.
If I went into the laboratory right now and wanted to break up a protein, I'd put it in a low pH (highly acid) fluid at an elevated temperature. That sounds a lot like a stomach. If I wanted to degrade it further, I might throw a lot of enzymes at it to break even more bonds, and this sounds a lot like the pancreas. By the time the food is absorbed by your intestine, it's mostly in very short chained peptides or even individual amino acids.
So, unless you're burning the **** out of your food so that all of it is a black, sooty mass, then don't worry about cooking degrading protein. Your body knows how to deal with it.