What is the observation of the mere smell or sight of food stimulates hunger
Answers
Answer:
sponses to chew & spit - ghrelin and obestatin

This is my second guest post for Science of Eating Disorders blog. Tetyana has a lovely piece up looking at Deep Brain Stimulation as a potential therapy for intractable AN. If you haven't seen it yet, please go check it out and join the discussion!
Your body responds to food long before it reaches your stomach. The taste, smell, even the mere sight of food all act to trigger a physiological response, “priming” the gut by stimulating various enzymes required for proper digestion and absorption of nutrients. This is called the “cephalic response”, and it is mediated by a part of the nervous system that’s generally not under conscious control (the autonomous nervous system). Keep in mind, the actual consumption of food is NOT necessary to trigger this reflex.
As you may have already guessed, the act of chewing and spitting out food activates this response, increasing the secretion of stomach acid, digestive enzymes and insulin as well as changing one’s metabolic rate. Which leads me to wonder – what are the changes that occur in the body during the cephalic response? Are these responses altered in patients with eating disorders who also chew and spit? If so, how? And are there any long-term consequences for engaging in chew and spit?
These are obviously very broad questions. To narrow it down, in this post I’d like to look specifically at two hormones involved in appetite control: ghrelin and obestatin. A little background first. Ghrelin and obestatin are both gut-brain hormones that are secreted (mainly) from the stomach and act on the brain. While ghrelin has many neurological functions (such as buffering against stress-induced depression), it was first identified as a hunger-promoting hormone. Ghrelin is secreted during the cephalic phase, and can promote feeding in multiple ways.
First, ghrelin travels through the bloodstream to areas of the brain involved in appetite regulation. There it acts directly on specific receptors designed to trigger hunger and drive food-seeking behaviors. In fact, injecting humans with synthetic ghrelin was enough to produce feelings of intense hunger. Ghrelin can also make food seem more desirable. It does this by activating the reward system, causing dopamine release at the sight, taste and smell of yummy foods. Hence, it may play a central role in conveying the pleasure and reinforcing aspects of high-calorie “rewarding” foods. Finally, ghrelin can directly act on the GI tract, blunting the stomach’s “fullness” signal and promoting overeating.
There is much less research on the recently discovered obestatin. However, it seems to directly oppose ghrelin’s effect on food intake, acting as a “brake” for the desire to feed (note, this is a MASSIVE oversimplification).
In the current paper, researchers wanted to know if patients with anorexia nervosa (AN) secret ghrelin and/or obestatin differently than control subjects when allowed to chew, taste and spit out food. To test this, they recruited 8 women with AN (both binge-purging and restrictiving) and 8 age-matched healthy female subjects. Following an overnight fast and a standardized breakfast, researchers served the women a lunch made up of 67% carbohydrates, 13%protein and 20%fat. To chew and spit in a controlled setting (in this case, called “modified sham feeding”), the women were allowed to see and smell the food for 5 minutes before chewing and spitting each bite into a napkin. Blood samples were taken before and after feeding (or chew and spit) for analysis, and the subjects were asked to fill out the Three Factor Eating Questionnare (TFEQ) to assess their eating behavior.