To understand why you want to eat what you do, you first have to understand the way your brain works.

By looking at the role the brain plays in determining what we eat, Dr. Michael Roizen, co-author of "You: On a diet" demystifies the science behind hunger.

Chemical hunger

The hypothalamus, which is located in the centre of the brain, regulates hunger based on two counterbalancing chemicals.

Ghrelin is the hormone responsible for making you want to eat more.

"It comes from our stomach, so as we grow our stomachs, if we eat big portions, we make more of this and it stimulates that brain centre in the hypothalamus to cause you to eat more," Roizen told Canada AM Wednesday.

Leptin, on the other hand, is a protein secreted by stored fat and it tells the body when to stop eating.

"They (ghrelin and leptin) work against each other," Roizen said.

So, in other words, the battle over eating isn't really about willpower, but about finding a balance between these two competing hormones.

The third factor controlling hunger is a digestive stop sign that tells your body you're full.

"It comes out of your intestinal wall and comes out of your stomach wall and it's called CCK," Roizen said

Cholecystokinin, or CCK, is a peptide hormone that is released by gastrointestinal system and helps the digestion of fat and protein.

"It stimulates the vagus nerve and says 'stop eating' and it's a great thing because it turns off the ghrelin," he said.

Since the CCK is released when the body senses fat, the best way to stop hunger is to eat healthy fat.

"You turn on CCK by having healthy nuts, such as these walnuts -- which are the best of them, but all nuts are good."

The good fat from the nuts causes the intestinal wall to secrete CCK and eight to 20 minutes later, you're won't be hungry.

To battle overeating at dinner, Roizen suggests having have some nuts and a drink beforehand.

It is also important not to misinterpret your brain's signals.

The hypothalamus has four things that it has appetites for and only one of them is food.

It also tells the body when it needs sleep, liquid and sex.

"So if you feel hungry, it may be water or it may be sex," Roizen said.

"And any of them can substitute for one another."

Thursday: Canada AM will look what we must and must not eat.