Hunger - psychological or physical?

The causes of hunger are many, not just the state of the upper intestine.

Hunger is triggered by regular meal times and the presentation of food cues. For instance show fast food adds, present cooking smells or even just talk about food and people will feel hungry.

Regular meal times can be so ingrained that even after an afternoon banquet a person may feel hungry again at the regular mealtime.

Clearly these cues can be averted. They develop through conditioning and they can be extinguished the same way eg by thinking about gaining weight and being ugly (negative reward).
 
On purely physically triggered hunger, most people do not experience it as they eat too regularly for such cues to trigger. I estimate than more 95% hunger feelings are psychologically triggered in the average person.

Once hunger is triggered, the stomach is checked for status which is why we associate hunger feelings with the stomach: this is a secondary, not primary hunger feeling. People who have no stomachs also get hungry, indicating that the stomach plays a secondary, not primary role in hunger.

Putting this together it is clear that when psychological hunger is extinguished the physical hunger is all that is left and this is relatively mild, at least initially.

Note that people who experience voluntary fasting report a loss of appetite after a period of time (includes those on hunger strikes). This does not occur when fasting is imposed or unwanted in which case hunger can lead to desperate behaviour.

How can voluntary starvation, as in fasting for health, lead to loss of appetite when imposed starvation, such as those lost during hiking or boating at sea, leads to desperation over similar periods without food?? The answer is that most of the hunger we experience is psychological, not physical.

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"Thus, this study found that women who have recovered from anorexia nervosa show two related patterns of changes in brain circuit function that may contribute to their capacity to sustain their avoidance of food.
First, hunger does not increase the engagement of reward and motivation circuits in the brain. This may protect people with anorexia from hunger-related urges. Second, they showed increased activation of executive 'self-control' circuits in the brain, perhaps making them more effective in resisting temptations."

How do anorexics control their appetite?

Many adults, regardless of their weight, resolve to avoid fatty foods and unhealthy desserts. But despite one's best intentions, when the moment for decision comes, that chocolate lava cake is often too enticing and self-control vanishes.

This behavior is normal because hunger increases the intensity of food rewards. Yet, individuals with anorexia nervosa (AN), despite their state of starvation, are able to ignore such food-related rewards.
A new study by Dr. Christina Wierenga, Dr. Walter Kaye, and colleagues, published in the current issue of Biological Psychiatry, sheds new light on the brain mechanisms that may contribute to the disturbed eating patterns of anorexia.
They examined reward responding in relation to metabolic state (hungry or satiated) in 23 women recovered from AN and 17 healthy women without eating disorder histories (e.g., the comparison group). Women with active AN weren't studied to reduce potential confounds related to starvation.
The healthy women, when in a state of hunger, showed increased activity in the part of the brain that motivates the seeking of reward, but the women recovered from AN did not. The recovered women also exhibited increased activation of cognitive control circuitry regardless of metabolic state.
Thus, this study found that women who have recovered from anorexia nervosa show two related patterns of changes in brain circuit function that may contribute to their capacity to sustain their avoidance of food.
First, hunger does not increase the engagement of reward and motivation circuits in the brain. This may protect people with anorexia from hunger-related urges. Second, they showed increased activation of executive 'self-control' circuits in the brain, perhaps making them more effective in resisting temptations.
"This study supports the idea that anorexia nervosa is a neurobiologically-based disorder. We've long been puzzled by the fact that individuals with AN can restrict food even when starved. Hunger is a motivating drive and makes rewards more enticing," said Wierenga, an Associate Professor of Psychiatry at the University of California, San Diego. "These findings suggest that AN individuals, even after recovery, are less sensitive to reward and the motivational drive of hunger. In other words, hunger does not motivate them to eat."
"This study offers new insights about the brain in AN, which we are using to guide treatment development efforts, and reduce stigma associated with this life-threatening disorder," added Kaye, who is a Professor of Psychiatry and Director of the Eating Disorder Program at UCSD.
"Anorexia nervosa is a devastating illness and this study sheds new light on brain mechanisms that may enable people to starve themselves. In identifying these mechanisms, this work may provide circuit-based targets for therapeutics," commented Dr. John Krystal, Editor of Biological Psychiatry. "But these same circuits and processes seem to be engaged 'in reverse' for obesity. Thus, this study may have broad implications for the country's obesity epidemic as well."