Amira - Philadelphia, Alberta, writes
Have you ever felt so hungry that you thought you could eat a pot of mac and cheese, but after a few bites you were too full to finish eating your portion? Why do we feel hungry anyway?
The food we eat contains nutrients such as vitamins, minerals, carbohydrates and proteins. These nutrients keep our bodies functioning, give us energy and provide the essential building blocks that make every cell in the body. Hunger and satiety are the body's way to tell us whether we need to eat or not to eat.
Did you know? Calorie is the most common unit of measurement for the energy we get from food. Scientifically, calorie is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celsius.
Most of the food we eat is broken down and converted into glucose (blood sugar). When the levels of glucose in the blood are low, the body is signaled that it needs food.
What controls our eating behavior?
The desire to eat is controlled mainly by the hypothalamus, a part of the brain that senses glucose levels and signals coming from digestive organs and body fat. These signals are chemical messengers called hormones. In response to these hormones, the hypothalamus regulates the awareness of hunger and satiety in the brain.
There are many hormones that modify the activity of the hypothalamus. Let's take leptin for example. Leptin signals the brain that the body has had enough to eat. It is produced by fat cells and is released into the blood in proportion to the amount of fat that we have. So the more we eat the more leptin we release.
Did you know? Some people are obese because they are resistant to the effect of leptin. Although these people usually have high levels of leptin (because of the many fat cells making it), they never feel full so they tend to eat more than they should.
Ghrelin, on the other hand, is another hormone produced by the stomach to trigger appetite. Ghrelin is released when the stomach is empty. When we eat, the food that enters the stomach causes it to stretch and ghrelin levels in the blood drop. Leptin and ghrelin are considered to be complementary in their influence on appetite.
Why is it that we enjoy food so much?
Hunger is not entirely a metabolic process. As human beings, we cannot ignore our cognitive part. External signals like smell, taste, or texture of food can greatly affect our appetite. For instance, if you like pizza, the smell of melting cheese is most likely to trigger your appetite.
Did you know? Colors can also affect hunger. Whereas, a red or a green apple may trigger your appetite, a blue apple won't. In fact, blue food is rare in nature, and therefore is thought to be an appetite suppressant.
The interaction between sensory signals produced by food and internal signals from the hypothalamus occurs in the orbitofrontal cortex, a brain region that is involved in decision-making. This interaction is very important. If food wasn't so enjoyable we might not eat and disappear altogether.
The hunger system is complex, so even if you knock out part of it, it will still work quite well. In fact, this is often the case for evolutionary conserved systems. Because hunger is so critical for our survival, it can't be switched off. Our hunger system is remarkable as it has many compensatory mechanisms to meet this challenge.
Did you know? After we've finished growing, our body weight is held to a stable value called the set point. Everyone has a genetically determined weight that the body tries to maintain. That's why some people can eat whatever they want and stay thin. But don't lose hope! The set point can be changed with exercise and healthy diet.
Hunger and Eating
Hunger and Eating Disorders
Broberger C. Brain regulation of food intake and appetite: molecules and networks (2005) J Intern Med 258:301-27
Druce M, Bloom SR. The regulation of appetite (2006) Arch Dis Child 91:183-7
This answer was researched and written by Moran Weinberg. Moran is a PhD student in the Department of Physiology at the University of Toronto. She is studying the activity in the brain of Parkinson's disease patients. Apart from research, she likes to practice yoga and to read interesting books while drinking tea.