Graham55 said:
My mum asked what is happening when i get that realy hungry feeling.
That is a really interesting question that exercises the minds of many nutritional researchers. I am afraid the answer is not at all clearcut - hunger is probably the result of a mixture of different cues - physiological, neurological, physical and emotional. I'm not an expert in this area, and it is a bit complicated, but I will try to explain what I do know of it.
The physiological parts of this are that hunger is - to a great extent - controlled by the interactions of a number of different hormones. Insulin is one of these, and although it is true that increased levels of insulin can be associated with hunger it is simplistic to say that this causes hunger. After all, if that were the case then untreated Type 1 diabetics (who have low and falling insulin levels) would not feel hungry - whereas in fact extreme hunger is sometimes a symptom of diabetes. The thing about hormones is that they often interact with each other in quite complex ways to regulate the body - kind of like the way that cruise control in a car links various sensors to the throttle to maintain a constant speed.
The current view of the physiology of hunger is that there are at least four hormones involved in this: leptin, ghrelin, insulin and cortisol (there are probably more, this whole area is still not very well understood). Leptin is produced by fatty tissues and monitors the overall nutritional status of the body. When you eat, an increase in blood glucose causes fat cells to release leptin, which acts directly upon the brain to make you less hungry. Leptin also reduces the production of ghrelin. Ghrelin is produced by cells lining the stomach, and it is thought that the main purpose of this is to directly regulate the hunger response. Ghrelin acts upon the brain to make you hungry. Since leptin is released after eating that reduces hunger by inhibiting ghrelin. As the leptin effect fades away several hours after eating, then more ghrelin is produced and that initiates hunger. That is a neat story, but it is unfortunately a simplification. Insulin has various effects over and above the glucose regulation that we are all familiar with. One of these effects is that it modifies the effect that ghrelin has upon the brain (as does leptin). Cortisol is another hormone that is involved with this. Cortisol is produced by the adrenal gland and is heavily involved in the stress response, and in the regulation of sleep patterns. Leptin turns on a set of events that lead to cortisol production, and ghrelin turns it off. This is why both the time of day and stress can also effect the hunger response. Cortisol also has an effect on glucose regulation - roughly the opposite effect that insulin has (it increases blood glucose levels), and it is also involved in various brain functions - which means that the brain not only responds to hunger, but it has a role in regulating it. It is quite well known that people on low carbohydrate diets tend to feel less hungry than people on high carbohydrate diets, and there has recently been some research published that high protein low carbohydrate diets cause a reduced ghrelin production, which probably explains this.
So much for the physiology. There is also a simple physical aspect to hunger in that there are nerves that sense when the stomach is empty, and they cause a sensation of hunger. In a famous experiment hungry volunteers were persuaded to swallow an inflatable balloon. When this was inflated in their stomachs they stopped feeling hungry. However, this is only a part of the explanation for hunger (people who don't have a stomach at all can still feel hungry).
Because of the involvement of the brain with all this, there it is also quite likely that there is a psychological aspect to hunger. It may well be that some components of the hunger response is "learned", which is why psychological techniques like hypnosis and neuro-linguistic programming can help some people to curb hunger and loose weight.
Sorry if this seems a bit complicated, it is because biology is like that! A short version of this answer might be as follows... When you get hungry it is because your brain has concluded that you are hungry as a result of various signals that come from a complex dance of interacting hormones, as well as the fact that your stomach is physically empty. This is all subject to various other issues like the time of day and your overall level of stress. Because this is all being filtered by the brain, there is also the scope for psychological factors fouling it all up.
If you want to know more then Google Scholar is your friend! Believe it or not, I find this sort of stuff fun 8). These days I am mostly a computer geek, but the
real geeks are biologists!
