In the mid 1980s, when I was pursuing my degree, the discipline of Chemical Engineering felt itself to be a cross roads.  The perception at the time was that traditional jobs for chemical engineers involving the design and optimization of manufacturing facilities were going to fade away.  In an effort to try to remain viable, the scope of the Chemical Engineering curriculum was broadened to include Biochemical Engineering.  Why am I telling you about trends in Chemical Engineering edjucation from several decades ago in a column about the stomach?  Let me explain.

In the mid 1980s, cholesterol in general and eggs in particular had fallen into disrepute due to a study which showed increased rates of heart disease in rabbits that had been fed a high cholesterol diet.  At the time, there were many scientists who pointed out that rabbits, as vegetarians, are not physiologically equipped to handle a high load of cholesterol in their diet, rendering the results of the study moot.  Unfortunately, eggs causing heart attacks made for much better headlines than explaining the shortcomings of the testing methodology, and cholesterol-containing foods endured a public thrashing.
One of the skeptics was Dr. John Gainer, a Chemical Engineering professor at the University of Virginia, who, as part of Chemical Engineering’s new focus on biochemistry, was studying the mechanism of digestion and absorption of cholesterol in humans.  To paraphrase Dr. Gainer, “The suggestion that eating eggs or, for that matter, other high-cholesterol foods, are a primary cause of a person having high cholesterol in their blood stream is simply wrong.”  At the time, Dr. Gainer’s opinions were controversial; today they are mainstream and well documented.  Over the years as I have read news reports on different alleged diet-health connections – “eat margarine,” no, “eat butter,” no, “eat margarine” – I have tried to maintain some of Dr. Gainer’s healthy skepticism.
To understand how our food impacts our health, you need to understand the primary interface between your meals and the rest of your body, the stomach.  (Well really, the entire gastrointestinal system, but “The Science of the Stomach” was such a catchy title.) The dictionary definition of digestion is “The mechanical and chemical breakdown of food into smaller components that are more easily absorbed into the blood stream.” 
Digestion begins in your mouth when your teeth grind up food and enzymes in your salvia begin the process of converting carbohydrates into sugars. Once the food enters your stomach, the breakdown of carbohydrates continues and your stomach acid and additional enzymes break proteins in their constituent amino acids.  There is an important point here that I think warrants some emphasis.  The carbohydrates and proteins in your food do not enter your blood stream intact, but are disassembled into smaller pieces.  Keep this point in mind, as I will refer back to it later.
About two hours after eating, your stomach will have turned your recent meal into a thick, viscous liquid called chyme, which then enters your small intestine. Generally speaking, the fat, cholesterol, vitamins, and fiber in your food, as opposed to the carbohydrates and protein, are still intact when they enter your small intestine.  A lot goes on in the small intestine, so I’ll just try to hit the high points.

  • 95% of the absorption of your food into your blood stream occurs here.
  • Fats are broken down into monoglycerides and fatty acids, which then enter the blood stream. Again the fats you eat do not enter your bloodstream intact, but are broken into their smaller, constituent pieces first.
  • Enzymes from your pancreas continue the breakdown of starches and complex sugars to simpler sugars, most of which now enter the blood stream.
  • Vitamins are absorbed, mostly in their intact form. 
  • Fiber for the most part just cruises on through.
  • Our supposed nemesis, cholesterol, is encapsulated by bile from the gall bladder and transported to the liver.  Depending on what’s occurring in your body at the time, your liver has a number of options in which it can utilize the cholesterol, including: diverting it to feces, making hormones, making bile, or using it to send signals to your cells to stop making cholesterol. 

The job of the large intestine is a bit less exciting than that small intestine. Key functions include:

  • Final breakdown of complex sugars into simple sugars via bacterial fermentation.
  • Extraction of water and minerals like iron from the chime, with the left over solids constituting feces.

As we consider what happens to your food on the way through your stomach and intestines, the key points to understand are that carbohydrates, proteins, and fats are broken down into smaller pieces, vitamins are absorbed intact, and cholesterol is sent to the liver, which can deploy the cholesterol as needed depending on conditions.
Your bloodstream carries the material from your intestines to the cells in your body which then use the sugars, amino acids, fats, vitamins, and minerals for both energy production and as building blocks to construct a bewildering number incredibly complex molecules that sustain your body.  Among the molecules that your cells construct are cholesterols.  As a result, when you have the cholesterol in your blood measured, the predominant source is cholesterol that your cells have produced rather than cholesterol from a recent cheeseburger. This was Dr. Gainer’s point from the mid 1980s.
Our job as eaters is to have a diet which provides the proper array of building blocks to our bodies such that we have all of the necessary raw materials to maintain our health.  As we’ll discuss in upcoming columns, this task is ironically becoming more challenging over time, due to changes in the agro-industrial complex that now supplies the bulk of our foods.
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