We've all been hungry before, but exactly how does hunger work? It is obvious to most that you feel hungry when you need food. It is a simple biological mechanism to make sure that we don't unwittingly starve to death. However, this explanation still doesn't really answer the question of what mechanism controls hunger.
One way that our body can tell that we need food is if our stomach contracts. When a balloon is inflated in someones stomach, they cease to feel hungry, and when it is deflated, they may feel hungry again. This is a simple way to tell us that there is something in our stomachs, so we don't need to eat any more, but it is certainly not the only mechanism that tells us to be hungry. Another reason that we can feel hungry is because of low blood sugar levels. Our body must tell us when we need to eat for energy, and blood sugar is a very good indication of energy levels. There are various other chemicals that can influence if we're hungry, like insulin, but they aren't as powerful.
How is the feeling of hunger achieved? Through hormones, of course. Two of the main hormones that influence our feeling of hunger are leptin and ghrelin. When we eat, adipocytes (a type of fat cell) begin to release leptin. Leptin is a hormone that reduces the feeling of hunger, so the more you eat, the more leptin there is, and the less hungry you are. Leptin levels drop with time, causing us to be hungry again. Leptin is such an interesting hormone that it warrants an article all to itself. Ghrelin is like the oposite of leptin, high levels of ghrelin increase a persons appetite. Levels of ghrelin are high when you're hungry, and then fall as you eat and become satiated. Levels of ghrelin can also increase with stress, which partially explains why some people eat more when they're stressed. Both leptin and ghrelin act on the hypothalamus, the hormone control center of the brain which releases hormones that act on the liver and cause the sensation of hunger.
Hunger can feel unpleasant, especially extreme hunger, but it does much more than that. It has been observed that starving animals are much more active than satiated animals and will gain more weight when they finally do get fed. The increase of activity could help the animals to scavenge or hunt more and thus increase their chances of survival. The same goes for the increase in weight gain. When a creature reaches a point where it is starving, the body believes that there is a scarcity of food, and so stores more of the food energy in fat cells so that the creature can last longer to find more food. If we go for too long without food, we will start to experience "hunger pangs." Hunger pangs are contractions in the stomach that will start after 12 to 24 hours without food, and are quite unpleasant. These contractions will last for only about 30 seconds, but will continue to happen for around 30 to 45 minutes. These contractions will start up again after a while if you still haven't eaten, but emotional states, such as anger or happiness, can reduce hunger pangs.
Hunger and starvation are still serious problems in our world today. There are a total of 925 million people suffering from malnutrition in the world, however this doesn't need to happen. There are nearly 7 billion people on the planet right now, but the FAO estimates that the earth could sustain 12 billion people. The problem is not lack of food, but more the globalized economy. Each country should be able to feed its own people, however, other countries often buy agricultural land or food from these countries for much more than the people of these countries could pay, thus sending all of the food away from poorer nations into richer nations such as the US. Ultimately rich nations take advantage of poor nations, causing their people to starve.