It All Goes Straight to the Hips

With all the talk about sugars and obesity, what's the direct relationship? We know the basics: eat more calories than you burn, and you gain weight. And we know how to burn calories. But how does the food get turned into fuel?

The First Steps
The digestion process begins in the mouth. Saliva breaks down starch into maltose sugars using the enzyme amylase, and the enzyme lipsase breaks down fats into the constituent fatty acids and glycerol. The food travels from the mouth to the stomach, where 4 main gastric enzymes break down the food into smaller molecules.

The first set of gastric enzymes is similar to those found in saliva: these are the lipases and amylases, that break down fats and starches. However, the stomach also contains proteases which break down proteins and gelatins/collagens into amino acids and peptides.

From the Outside to the Inside
Because the digestive tract (mouth, stomach, intestine, anus) is connected by a sphincters, it can be seen as a long pipeline, with an opening at either end. So how do nutrients get to the organs and muscles, and how does the excess get stored as fuel?

The key here is the actual lining of the pipes. The entire digestive canal has an interior membrane, but it has different properties in each organ. In the mouth, and throat, the membranes are for protection, and don't have any digestive properties. They act as a shunt to take the food to the major digestive sites: the stomach and the intestine.

The lining of these organs take a more active role in food digestion. The stomach's epithelial layer contains glands that secrete proteases, bacteria-killing gastric acids, and a thick mucous that protects the stomach from the acids. Underneath this lining is a sheet of smooth muscle, which allows the stomach to churn it's contents, exposing your food to the different enzymes. When the food finally exits the stomach for the intestine, it's been broken down to peptides, amino acids, fatty acids and glycerols.

Later, we'll discuss how the intestine takes these small molecules and uses it for the body's needs.

Image: Wikipedia