We've all heard the admonitions of nutrition experts warning against eating too much red meat. But what exactly gives red meat its dark hue, while chicken and fish stay relatively white? You may think of a super-raw steak as "bloody," but gore has nothing to do with it. It's all chemistry. 

Cuts of beef consist of slow-twitch muscles, which contract slowly and release energy over longer periods of time for endurance activities. Fast-twitch muscles, by contrast, contract quickly to support rapid bursts of movement. 

Slow-twitch muscles rely on the presence of a red protein called myoglobin, which stores oxygen to keep the muscles going. Fast-twitch muscles use the lighter glycogen for energy instead. 

The more time an animal spends standing and walking around, as opposed to energy-intensive activities like flying, the more slow-twitch muscles it will have, and the darker its meat will be. That’s why plodding cows have a lot of red, myoglobin-rich meat, and frantic chickens have more white meat. Pork, despite its reputation as "the other white meat," is definitely red, though it has less myoglobin than beef. And in case you're wondering, human meat would be considered red meat. (Cannibals, watch out for heart disease!)

The American Chemical Society’s video series Reactions explains the science of grilling red meat: 

Meat starts blooming a vibrant red color when oxygen binds to the iron atom of the myoglobin. After a few days, the myoglobin loses its ability to bind, and the meat becomes brown. That’s why the redness of meat is typically a sign of freshness. But sometimes even fresh meat looks brown in the package, because of a lack of oxygen within the sealed plastic.  It’s also why meat producers often add carbon monoxide to packaged cuts of meat, to keep it looking red and juicy regardless of the packaging (or the expiration date). 

When the meat goes on the grill and the myoglobin heats up above 140 degrees Fahrenheit, it begins turning brown, turning your rare cut into a grey-brown hunk of well-done steak. Happy grilling, and remember: Meat is for eating, not for black eyes