What Happens to the Fat We Eat?

(The illustration is a simplification of a figure in the 2007 Encyclopedia Brittanica. If it seems too fuzzy to decipher, click on it to see a clearer version.)

For the low-carber, fat is an important macronutrient. What happens when we eat fat?

One of the important aspects of fat is that it is not water-soluble. In order to begin the digestion process, the liver makes bile, which in collected in the gallbladder and is secreted into the small intestine. The bile acts as a detergent. The bile salts in it have a lipophilic side, which binds to the fat droplets, and a hydrophilic side, which suspends the droplets in the watery mixture of the food we have just eaten.

The triglycerides or fats in the suspended droplets cannot be absorbed by the intestine. To accomplish absorption. the pancreas secretes an enzyme called pancreatic lipase into the small intestine, Pancreatic lipase breaks down each triglyceride molecule into two free fatty acids plus a monoglyceride. "Mono" means one, and in this case it means that one of the fatty acids remains attached to the original glycerol backbone. When the triglyceride is broken down into subunits, it is able to pass into the absorptive cells of the intestinal mucosa. After the three subunits have transited the wall of the intestine, the fatty acids are added back to the glycerol backbone and they form a triglyceride once more.

Inside the cells of the intestine, triglycerides are packaged into chylomicrons. Chylomicrons are large diameter (75-1200 nanometer) particles that contain a bit of protein, a bit of cholesterol and lots of triglycerides. The chylomicrons are not secreted directly into the blood but into the lymphatic system. They eventually arrive at the thoracic duct and then are deposited into the blood at the left subclavian vein. Once they enter the blood, they are transported into capillaries and are able to reach the entire body.

One of the proteins in a chylomicron is called apo C-II. This protein has the ability to activate an enzyme called lipoprotein lipase or LPL. Lipoprotein lipase resides on the capillary walls of tissues that have a high requirement for triglycerides, such as cardiac muscle cells, skeletal muscle cells and fat (adipose) cells. The activated lipoprotein lipase acts on the triglyceride molecules (called triacylglycerols in the illustration above) stored inside the chylomicron. It hydrolyzes or breaks down the triglycerides into two fatty acids plus a monoglyceride. Just as we saw in the intestine, intact triglycerides cannot pass through the cell walls, but when they are hydrolyzed into subunits, they can be absorbed into the cells. Once inside, they can be used for energy in the muscle cells or reassembled into triglycerides and stored in the adipose cells.

When we eat a piece of bacon, we start with fat and end with fat (or for the low-carber--energy from the fat). But, as you can see, there are may steps involved in getting from the beginning of the process to the end.

What Is Fat?

Low-carbers spend lots of their time thinking about fat, both in terms of the excess girth on their bellies and hips, and as an important macronutrient. Although fat is a perfectly natural substance, it may surprise you to know that fat is a chemical.


In chemical terms, fats are referred to as triglycerides. They are composed of two types of subunits. The first subunit, glycerol, is shown above. Although the glycerol portion is not the most important subunit of a fat, it is the root of its chemical name, triGLYCERide. Glycerol has a three-carbon backbone, depicted by the vertical line of C's in the figure above. The active groups in glycerol are the -OH or hydroxyl groups.

The other subunits are called fatty acids. There are three of them in each fat molecule; hence the prefix TRIglyceride. In the example above, each fatty acid contains a chain of carbons represented by a horizontal row of nine C's. In real life, the length of the chains can be from four C's to twenty eight C's. The chains do not have to be the same length within the triglyceride--any assortment is possible. The active group in the fatty acids is known as a carboxylic acid and is also shown above.

The active groups of the three fatty acids are joined to the active groups of the glycerol backbone though a process called esterification. For those who are interested in the enzymatic reactions involved, they are described here. At any rate, three fatty acids attached to one glycerol backbone produces a TRI-GLYCER-ide, a triglyceride, which is one molecule of fat.

That's probably enough biochemistry for this time. There will be more fascinating facts about fats in later posts. :-)

Be Encouraged!

As most of my readers know, low-carbing is a lifestyle, not a quick weight-loss diet. Early in the process of low-carbing, weight is often lost rapidly, and some of the health improvements come right away. But as weeks move into months move into years, changes come more slowly and more gradually. Reading a diary or meeting an old friend will be a reminder that the low-carb life is better, but day-to-day excitement gradually morphs into an overall feeling of wellbeing.

As low-carbing becomes a way of life, what used to be a black-and-white eating plan begins to become shades of gray. What about eating a slice of Smart Carb bread instead of using a lettuce wrap on my sandwich? I miss bread, and this bread even contains exta (incomplete) protein. Could I substitute one or two low-carb Monster Energy drinks for a couple of bottles of water? They sure taste good and give me a mental and physical boost after all.

There are all sorts of low-carb substitutes for high carb foods. There are many vendors ready to sell them to us, and lots of cookbooks to show us how to make them ourselves. We see low-carb forums with large areas devoted to recipes. And if we try low-carb substitutes, in the short term it very often does not hurt. But what happens in the long term?

In April 2009 there was a Nutrition & Metabolism Society conference in Charleston, South Carolina. Jimmy Moore attended and posted pictures of some prominent low-carbers on his menus blog. Please check out the pictures of low-carb experts Laura Dolson and Dr. Mary Vernon. Another low-carb expert, Dana Carpender, also seems to be having weight issues. Jimmy Moore himself has recently reported that he weighs 246 pounds (an obese-level BMI of 30.7) with a body fat percentage (measured on a bathroom scale) of 31.5.

How could this be? These are prominent low-carbers. Please click on and scroll through the websites of Laura Dolson, Dr. Mary Vernon--note the array of fruit across the top, Dana Carpender and Jimmy Moore's menus blog for a clue.

Does this mean that low-carbers are doomed--doomed to gain weight in the long run? No. It does mean that the basic low-carb formula of complete protein, healthy fat and a few low-carb vegetables is hard to maintain over time. Dr. Michael Eades recently had a blogpost that graphically demonstrated that two groups of people living under similar circumstances could have drastically different outcomes for health and longevity. The hunter-gatherers had periods of starvation and fairly short lifespans, but were healthy in most respects. The agriculturalists had access to the same array of animal proteins, but they preferred to eat carbs. They were willing to suffer from increased infant mortality, painful defects in bone formation, dental cavities and bone infections in order to get a high percentage of calories from carbs rather than animal sources.

The pull of carbs and carb-replacements is strong. For one thing, they taste good. For another, the culture we live in encourages high-carb eating. But for those who are hanging in there and eating complete protein, healthy fat and low-carb vegetables, keep up the good work! In the long run, you're doing what is best for your body and in the long run, you will reap the rewards.