Pathways of Energy, Part 2: Fat

Tracking the path of fat and fatty acids through the body helps give a better understanding of how the body deals with energy.

The human body has evolved a sturdy, complex, and ingenious solution to what is perhaps the most fundamental problem that any life form has to solve—acquiring and managing energy. In this series of three Insights, we’re describing some key aspects of that solution by following several fuel molecules as they move through the body. 

In a previous Insight we tracked glucose molecules. In this one we will track fats, or rather, a molecule that is a component of fat and a form of fuel in its own right, called a fatty acid (see Insight: What Is Fat Made Of?). 

  The fat molecule at left contains three similar chains, each of which can exist separately as a fatty acid (shown at right.)  

This fatty acid came into the body in some salad dressing. A tiny droplet of that fat survived intact all the way to the small intestine before bile from the liver and some chemicals from the pancreas worked together to break up the droplet, and then break apart its fat molecules into fatty acids and a chemical called glycerol.

Our fatty acid molecule was moved into a cell of the small intestine (again with the help of bile), and there joined to two other fatty acids and glycerol to make a new fat molecule. Fat molecules in the body are generally referred to by their chemical name: triglycerides (see Insight: Mixing Oil And Water—The Transportation Of Fat In The Body, Part 2). This triglyceride molecule traveled around in the body until it ended up just outside a heart muscle cell, where it is again broken apart into three fatty acids and a glycerol molecule, so that the pieces can be moved through a cell membrane. Once in the cell, they are broken down to release energy. Something similar is happening with the 2 or 3 billion other heart muscle cells, providing the energy that they need to contract in synchronization. And so your heart keeps beating.

The next fat molecule we will track goes through a similar digestion process, but the resulting fatty acids aren’t needed at that point for energy, so they end up as parts of a fat molecule that is tucked away in a fat cell that is part of a deposit of fat in the abdominal area (see Insight: Why You Should Pay More Attention To Fat You Can’t See). 

The third fat molecule we’re going to track gets transported to a tiny blood vessel in the brain, but unlike the glucose molecule we tracked in Part 1, it’s prevented from traveling into the brain because nerve cells do not use fat or fatty acids for energy. So it keeps circulating until it gets to the liver, where it is broken down to make a ketone molecule (see Insight: Keto). That ketone molecule ends up back in the brain, where it is now allowed in, and used to produce energy for a nerve cell in the region of the brain responsible for vision.

Our fourth fatty acid, once it gets through the wall of the small intestine, gets used as a component of cholesterol, a substance that has many uses in the body. 

And our final fatty acid is used to make a piece of the membrane that surrounds a cell.

The tales of these travels tell us that fatty acids are multitaskers—they provide energy, get stored for the future, are components of vital body substances, or become parts of cell membranes. Because of just one of these potential destinies, fat gets a bad name, but in truth it’s as vital to your body’s operation as oxygen. 

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