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Why do chickens and turkeys have white meat and dark meat? And, like, gross, but .. do humans have the same thing? It's all about our muscles: what they're made of, and what they're made for. Quick Questions has the answers!
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Sources:
http://ptjournal.apta.org/content/81/11/1810.long
http://college.holycross.edu/faculty/kprestwi/exphys/lecture/ExPhysEx1Lect_pdf/ExPhys_03_L09_Fiber_Type.pdf
http://www.loc.gov/rr/scitech/mysteries/turkeymeat.html
http://www.wisegeek.com/what-is-the-difference-between-white-meat-and-dark-meat.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/atp.html
http://www.elmhurst.edu/~chm/vchembook/547glycogen.html
http://www.bbc.co.uk/science/humanbody/body/factfiles/fastandslowtwitch/soleus.shtml
If you’ve ever been in charge of serving turkey at Thanksgiving, you may know that birds have both white and dark meat. 
 
That’s because they have two main types of muscle fibers: slow twitch and fast twitch.
 
As you might expect, slow twitch muscle fibers contract more slowly. They’re built for endurance activities, like long-distance flying, and they’re full of the protein myoglobin, which is a special kind of nutritious, salty slime that your muscles have to carry oxygen around.
 
The cells of slow-twitch muscle fibers are packed with mitochondria, these little organelles that act like power plants. The mitochondria take in the oxygen from the myoglobin and metabolize it into a really important molecule called Adenosine Triphosphate, or ATP. 
 
ATP is like fuel. It’s the molecule that stores the energy for living things to do just about everything.
 
And the process of turning oxygen into ATP is pretty efficient; as long as there’s oxygen available, the mitochondria can keep at it all day. But it isn’t fast.
 
So birds also have fast twitch muscles, which are usually at rest, but can burst into action when needed. Fast twitch muscles don’t contain myoglobin; in fact, they don’t even have mitochondria which means they can’t use oxygen at all.
 
Instead, those muscle fibers produce glycogen, which is a type of sugar that can be burned in an emergency.
 
Now both myoglobin and mitochondria are pigmented: myoglobin is red, and mitochondria are brown. So slow twitch muscles look darker, redder, and browner. Glycogen, on the other hand, is pretty much colorless.
 
So nearly-flightless birds that only ever fly short distances, like turkeys and chickens, have light-colored, fast twitch muscles in their breasts and wings. 
 
But long distance fliers like ducks or geese pretty much only have slow twitch muscles. That’s why their meat is dark and gamey.
 
Now the really important question: do YOU have both dark meat and white meat?
 
The answer is no, sort of. But also kind of yes, sort of.
 
Most animals have muscles that are either dark or white. In fish, for example, you’ll usually find white meat because they float; they don’t really need a lot of muscle power to get around. But some fish like tuna and salmon have darker meat, because they’re constantly swimming through a current.
 
Mammals, however, are different. Instead of having some muscles that are slow twitch and some that are fast twitch, almost all of our muscles contain both kinds of fibers. 
 
The concentrations of each kind can vary depending on where the muscle is and what it does -- the muscles in your eyes, for instance, have more fast-twitch fibers, whereas ones in your back that maintain your posture are primarily slow-twitch. 
 
But the combination of the two fibers throughout your body is why, instead of having brownish muscles and white muscles, you’re nice and pink all the way through.
 
That’s what we call red meat. I could really go for a steak right now.
 
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