Ever feel a bit sluggish after a big meal?
Maybe even a little sea sluggish? (Okay, that's stretching it. Sea slugs probably don't celebrate Thanksgiving — American, Canadian or otherwise.)
But they may, surprisingly, have a lot to tell us about that peculiar condition we know as a "food coma" — you know when you really gorge and then claim that you can't do dishes because you're incapable of movement and just need to close your eyes for a moment?
According to a study published this week in the journal Scientific Reports, sea slugs fall into a kind of food coma, too. And rather than making them completely useless, this food coma serves an important biological function.
"The sensation of a 'food coma' after a hearty meal is well known to anyone who has ever experienced a Thanksgiving dinner," senior author Thomas Carew of New York University's Center for Neural Science notes in a release. "In fact, most animals tend to slow down and rest after a large intake of calories, suggesting that there is a biological function to this reaction."
"Our new study proposes that such 'rest-and-digest' responses to feeding may have been shaped by evolution to promote the formation of long-term memories."
Specifically, researchers looked at the California sea hare, also known as Aplysia californica. The team noted that after having their fill of seaweed, the creature would slow down and become, well, extra sluggish.
Sound familiar? In humans, a food coma often follows a big meal. As our bodies labor to digest it, blood is rerouted from the rest of the body to work overtime in the stomach and gut. That rush of blood to the digestive end of the body can leave people feeling weak. Some people may even nod off — embarrassingly — at the Thanksgiving table while Uncle Ned is telling one of his always-riveting tales from back in the day.
It's important to keep in mind that a food coma isn't caused strictly by how much you cram into your belly — but also the quality and combinations in that food.
Now, let's get back to those sea slugs. Thanks to their oversized neurons and relatively simple physical structure, the researchers could get a glimpse of what was happening when they fell into a food coma.
"In humans, food intake promotes the release of the hormone insulin, which prompts the cells of the body to absorb nutrients from the bloodstream and turn them into fat for long-term storage," notes Nikolay Kukushkin, a postdoctoral student who worked on the study.
"However, insulin is thought to have little effect on the brain. By contrast, a related hormone, insulin-like growth factor II, has been shown to be critical for proper brain function, including long-term memory formation. However, its release does not depend on calorie intake."
Essentially, the human body makes use of two types of insulin. One is tied to calorie intake, helping us pack away food's energy. The other — insulin-like growth factor II, or IGF2 — arises independently of digestion and bolsters the brain.
In fact, IGF2 is known to fortify contacts between neurons and lock long-term memories into place.
Sea slugs produce both insulin and IGF2 at the same time. Their bellies grow, along with their brains.
"Thus, Aplysia's 'food coma' is controlled by their insulin-like system, which acts by redistributing the animal's energy away from active behavior and towards storage of both nutrients and memory," Carew explains.
The insulin system in humans, on the other hand, isn't unified, but rather pumps each protein out at different times. It's possible though that they interact with each other much like those in a sea slug. We may have even once shared the sea slug's all-in-one system.
"It remains to be established whether the human 'food coma' is a vestige of our evolutionary past, or still an important part of memory formation," Kukushkin says.
We know that for many animals, humans included, sleep is crucial for locking in memories of what we experienced during the day.
"Perhaps the drowsiness experienced after a meal is a similar way to preserve a memory about that meal, so as to come back to it in the future," says Carew. "Whether seaweed or Thanksgiving turkey, a good dinner is always worth revisiting."
Maybe a food coma is our way of remembering Auntie Sue's delicious holiday feast — in addition to being a way to get out of dish duty.