Part 6

Food and Digestion of Nutrients


We need to start this chapter by defining terms whose meanings we usually take for granted. If we don't do this, the discussion will become confusing later.

What is Food?

The obvious definition is that food is the stuff we eat.

But what is "eating"? It must include drinking since we can all agree that liquid like milk is indeed food. But we can also agree that pure water is not food; and that is because pure water has no nutrients.

So the best accurate definition is that food is anything we eat (or drink) that contains nutrients.


Nutrients are substances needed to maintain life. Generally nutrients enter the body from the digestive system; we swallow and digest them. This isn't universally true however. Some nutrients like vitamin D can come from our skin when it is exposed to sunlight and sometimes, when people have special health concerns, nutrients can come from injection (vitamin or iron shots). Likewise, we can get nutrients from intravenous (I.V.) infusion; and in extreme circumstances, when we are sick and unable to digest at all, we can receive all of our nutrients this way through what is called "total parenteral nutrition (TPN)". But most commonly, we swallow our nutrients.


What happens after we swallow food? How does a potato or piece of meat actually nourish us?

Humans have long-understood that food enters the mouth and later, in a vastly altered form, leaves through the anus as feces. For most of human history, what happened between eating and defecation was a mystery.

By the middle ages, people understood that the stomach and intestines were the conduit ("pipe") through which food passed on its journey and that these structures were responsible for somehow extracting vital "humors" (nutrients in modern terms) while expelling waste. Because this process involves a "sorting-out" of vital humors from waste, the process was called "digestion", taken from the Latin term "digestus" meaning "to separate".

In the modern era, we have learned a great deal about how this separation of nutrient and waste occurs. Today, digestion is a very well-understood process.

In order to use nutrients to sustain life, we must absorb them from the outside of our bodies to the inside. Swallowing is NOT absorption. Technically, the stomach and the intestines are part of the outside of the body, not the inside. The interior of the body, including the blood stream is a highly protected space that must be kept absolutely sterile (free of germs), yet we can only utilize nutrients once they have entered this space. Somehow, the body must convert big pieces of "dirty" food into sterile nutrients. THIS then is digestion.

Digestion, as medieval doctors understood, in indeed a "sorting-out" of nutrient and waste, but above all, it is a process of breakdown. This breakdown usually happens in many steps. Usually it begins with food preparation.

Food Preparation:

We can't swallow whole cows. They might not let us and they are too big to go down. This is true of most food. Before we can eat it, we must prepare it. Preparation includes several steps illustrated below.

1.  Killing

a.     Hunting

b.    Picking

c.     Gathering

2.  Separation

a.     Gutting, skinning, dressing, deboning

b.    Milling, sorting, juicing

3.  Chopping, grinding

4.  Heating

a.     Grilling

b.    Boiling

Usually, at least in primitive times, all or at least most of these steps had to happen before people could actually put food into their mouths. These were the steps of pre-digestion and just like digestion itself, pre-digestion involves breakdown; involves turning "big" tough things into "little" tender portions of food. Only then can people eat it and only then does true digestion start.

Mechanical Digestion:

By definition, digestion begins when food enters the mouth and ends when feces exits the anus. Let's start at the beginning.

Tearing & Chewing:

We have teeth in order to tear-off and chew bites of food. Tearing converts big food into little pieces and chewing grinds those little pieces into much tinier and softer shards.


Saliva lubricates food to prepare it for swallowing and saliva also contains one enzyme that chemically digests starch.


After food has been torn, chewed and lubricated, the mouth has done all that it can do. For digestion to proceed, food must now get to the stomach. It does so by being swallowed "down" into the esophagus and into the stomach where the next stage of digestion occurs.

Chemical Digestion

When it encounters most swallowed food, the stomach begins to secrete strong acid and enzymes. Acids and enzymes begin the phase of chemical digestion where the mechanically torn and chewed shards of food gradually become liquefied. As this occurs, the stomach gradually pushes this liquefying mass into the upper end of the small intestine (called the duodenum) where even more enzymes (from the pancreas) go to work liquefying the food mass. As this happens, food is transformed into a soup of chemicals. Some of those chemicals are nutrients that, at long last, the body can actually absorb and then use.

Nutrient Absorption:

AS the soup of nutrients moves away from the stomach, it passes through nearly thirty feet of coiled-up gut tubing called the "small intestine". The small intestine is the body's organ of absorption; it is where nutrients pass from "outside" to "inside". The small intestine is very long because nutrient absorption is a slow process, and above all, because nutrient absorption is an absolutely critical process. Without nutrients we die and the small intestine is long in order insure that we absorb every last "drop" of nutrient in food.

What We Can Absorb:

The small intestine can only absorb a few very specific types of chemicals. These chemicals are "allowed" to pass into the sterile and protected "inner world" of blood and lymph, while everything else in the small intestine is locked out. Generally, absorbable nutrients come in only a few forms. These are listed below:

1.  Simple sugars

2.  Amino acids

3.  Free fatty acids & cholesterol

4.  Glycerol

5.  Vitamins

6.  Minerals and salts

7.  Nucleic acids and purines


Once nutrients have entered the body's interior, they are transported, stored and/or metabolized. Each of these processes is a subject at least as vast and complicated as digestion and we will refer to them later, from time-to time as needed.

Beyond the Small Intestine: The Colon

By the time "food" leaves the small intestine, nearly all nutrients have been removed. Even the acids and enzymes that were used in digestion have either been reabsorbed or have themselves been digested and absorbed. What remains is a lot of water, cellulose (solid fiber), soluble fiber (fiber that forms gels) and a few indigestible starches and sugars. These things now enter the colon.

The colon or large intestine has a very different role to play in human health than does the small intestine.

Water Re-absorption:

Material leaving the small intestine is very watery. One of the colon's functions is to return this water to the bloodstream. This is why normal stool is solid.

Bacterial Fermentation

The small intestine is nearly sterile. On the other hand, the colon contains trillions of bacteria and is very "dirty". But the colon is dirty "by design", because its trillions of bacteria generally help rather than harm us. Because these bacteria may have a big role to play in how body weight is regulated, let's examine these colonic bacteria more closely.

Gut Bacteria, Immunity and Inflammation

Although the notion is slightly shocking or even revolting to some people, the trillions of bacteria in our gut are, in a sense, part of us and we cannot live very well without them. It turns out that these bacteria help regulate our immune system and even help us fight-off infections from parasites. Moreover, there is strong evidence that they affect body weight.

Firmicutes and Bacteroidetes

The human gut contains two main classes of bacteria called firmicutes and bacteroidetes. (to be continued)