Gluttony is not a common vice for me, but it is an annual obligation.
If I don't stuff myself on Thanksgiving to rival the turkey, I am sure to hurt my mother's feelings. We can't have that! So stuff I do -- until able to stuff no more.
And once I have bravely soldiered up to that line, I lay down my fork. And then I, or one of my comparably sated tablemates, is the first to say something like: "Whoa! I am so full, I couldn't eat another bite!"
The aroma of pumpkin pie, pecan pie and chocolate brownies wafts in from the kitchen, so we are quick to append: "What's for dessert?"
There is usually some savant among the holiday guests who notes that of course there's room for dessert -- there is a hollow leg or extra stomach set aside for just that purpose.
But as you likely know, we take anatomy in medical school, and explore all the nooks and crannies of the human body. I have it on good authority that the proverbial extra stomach, or hollow leg, has never been found. My cadaver certainly had neither.
But the fact is, we do have room for dessert when stuffed to the gills (which anatomy lab has also failed to find, by the way). How can that be?
It is courtesy not of the stomach or the leg, but of the brain, and specifically the appetite center housed in the hypothalamus. The explanation is sensory specific satiety.
Translated into English, sensory specific satiety means, basically, feeling full of one kind of food but not necessarily another. The appetite center is stimulated by several different flavor categories, among them salty, savory and sweet.
When you eat enough of something that is salty and crunchy, salty and crunchy loses its appeal. But the appetite response to luscious and sweet is just waiting to get switched on. Go from salty and crunchy to sweet and luscious, and you turn on a whole new appetite response. When satiety, or fullness, is reached for one taste category, appetite remains for the others.
This is why we can be full at the end of a meal and still have room for dessert. We are, indeed, full -- of salty, and savory and meaty. But the appetite center for sweet hasn't even gotten into the game yet.
And research suggests that the "satiety threshold" for sweet -- how many calories in that category it takes to register full -- is set higher than for other flavors. So it's no coincidence that almost every culture ends a meal with dessert, rather than starting with it.
You've experienced sensory specific satiety whenever you felt full at the end of a large meal, but still had room for dessert. You've also run into it when, despite your vow not to do so, you overate at an all-you-can-eat buffet.
Variety is the spice of life, we say, and it certainly spices up the appetite center.
But more problematic than the obvious variety built into the occasional feast or buffet is the hidden variety engineered into processed foods.
Most processed foods have long ingredient lists that include various forms of sugar, notably high fructose corn syrup; various forms of sodium, such as monosodium glutamate; and artificial flavorants as well. And the flavors hidden in foods can be quite surprising.