Its holiday dinner time. Will you be having the white meat or dark?
Have you ever sat down for Thanksgiving or Christmas dinner and found yourself wondering why turkeys have some dark meat and some white meat? Well, you were not the first.
A scientist named Ranvier reported differences in muscle color within and among animal species back in 1873. The explanation for the color differences is pretty simple and has a basis in physiology.
The dark meat of your holiday bird is "red" or slow-twitch muscle. The white meat is "white" or fast-twitch muscle, and most animals have some combination of these two fiber types.
Humans also have dark and white meat.
Some of our muscles, like those used for maintaining posture in the back, have a higher percentage of slow twitch fibers than fast. Our calf muscle, the soleus, is made up of around 90% slow twitch fibers. Others such as those controlling eye movements are made up of only fast twitch fibers. Function dictates form in these highly specialized muscles.
From an evolutionary standpoint this makes sense. Our not so very distant ancestors' daily survival sometimes dictated a long walk or jog in search of food. Other times, a fast sprint or jump may have kept one out of harm's way. The exact composition of each muscle is genetically determined, but, on average, humans have a mixture of about 50% slow and 50% fast fiber types.
Why are they colored differently?
The red (slow) muscle fibers are more efficient at using oxygen to generate fuel known as ATP (Adenosine triphosphate) produced by mitochondria (the powerhouse of the muscle cell that converts energy into usable forms). Slow muscles have more mitochondria, and are packed with more myoglobin (an iron and oxygen binding protein) giving them a darker, reddish color. The more myoglobin there is in the cells, the redder, or darker, the meat.
Red (slow twitch) fibers are suitable for maintaining sustained contractions necessary for posture, and are considered fatigue resistant. This is because they can harvest more energy from cellular respiration to produce ATP, and are activated by small diameter (slow conducting) motor neurons.
White (fast twitch) fibers are dominant in muscles used for rapid movement and are capable of generating large amounts of tension over a short period of time. They have fewer mitochondria and less myoglobin (hence their “white” appearance) and fatigue much more easily than red fibers. They depend on glycolysis for ATP production, and are activated by large diameter (fast conducting) motor neurons.
Athletes can have a higher percentage of one or the other type. For instance, Olympic sprinters may have as much as 80% fast-twitch fibers and long-distance runners may have as much as 80% slow-twitch. Weight-lifters need fast-twitch fibers for quick bursts of strength, and long-distance swimmers need the constant movement provided by slow-twitch fibers.
So, the next time someone calls you a turkey, they may be more right than they know! Happy holidays!