One of the most basic functions of human physiology is digestion, yet how many of us really understand what this process entails and how it affects what we should eat?

So often, we hear about foods and ways of eating that are “healthy”, and as a whole, we have held many erroneous beliefs about what should constitute “healthy”. The science behind what makes food “healthy” or not is simplified too much and the truth is hidden behind corporate slogans rather than actual science. Once we can understand the building blocks of food, how we digest them into individual molecules, and how we metabolize those molecules to make energy, the term “healthy eating” shouldn’t be so debatable or vague.

I have been reading about this information for years in an attempt to search for and serve my family the healthiest foods possible. Reading Sally Fallon’s book Nourishing Traditions was the first thing that changed my view about food and made me realize how misguided we have been about fat and the food pyramid. (Check out the Weston A. Price website to learn more!)

In writing this article, I started with Sally Fallon’s ideas as a framework in my brain, then used a college textbook called Understanding Nutrition by Whitney and Rolfes to really delve into the scientific explanations of carbohydrates, fats, and proteins and how we digest them (throughout this article, my descriptions come from this textbook unless otherwise linked). But even this seemingly benign textbook was not written without bias and made many leaps about what we “should eat” based solely on government recommendations rather than actual science.

So after reading this textbook and using it to explain the facts, I continued to ask my own questions, do my own research, and am now presenting an in depth synthesis of what I’ve learned about the foods we eat and how they are broken down during digestion.

In a Nutshell

When we eat any food, from cookies and cakes to burgers and fries to salad and dressing, it is all broken down into single molecules before being absorbed through the small intestine and sent into the bloodstream to be used as energy, for building, or stored for later use. All food can be categorized as and broken down into:

• Carbohydrates –> Monosachharides
• Fat (Triglycerides) –> Monoglycerides and Free Fatty Acids
• Protein –> Amino Acids

Carbohydrates, Fats, Proteins, and the Smallest Molecules They are Broken Down Into

*It takes about 6-8 hours for food to pass through the stomach and small intestine. In a study with 21 participants, it took  men an average of 33 hours for the food to be eliminated from the large intestine. It took an average of 33 hours for children too, but an average of 47 hours for women. Interesting!

Digestive System

Here is a picture of the entire digestive system just to give you a visual reference for where we are going. In my drawings, you’ll notice that I have included only what is necessary and exaggerated certain things for the purposes of clarity.

Digestive System (Photo Credit: Wikimedia Commons, LadyofHats, 2006)

Carbohydrates

Carbohydrates are primarily an energy source for plants, some animals, and humans.

From Polysaccharides to Monosaccharides

Foods with Carbohyrdates

There is a misconception (probably due to that silly food pyramid!) that carbohydrates only refer to things like breads and pastas and not things like fruits and vegetables. But the truth is that lettuce is a carbohydrate, apples are carbohydrates, grain is a carbohydrate, and sugar beets are carbohydrates.

When you look at the nutritional profile of these carbohydrates, however, you’ll notice that the amount of carbohydrates differs greatly among different food sources. Also, keep in mind that carbohydrates (which all come from plants) can be an excellent source of vitamins and minerals if grown properly and not overly processed.

• Lettuce, which is 96% water (3 g of carbohydrates per 100 g)
• Apples, which are 87% water (14 g of carbohydrates per 100 g)
• Whole wheat flour (73 g of carbohydrates per 100 g)
• Table sugar (100 g of carbohydrates per 100 g)

The total carbohydrates measured on nutrition labels include both simple sugars (monosaccharides and disaccharides) and soluble and insoluble fiber (polysaccharides).

Types of Carbohydrates 

Types of Carbohydrates (The numbers refer to the number of molecules.)

• Monosaccharides: Monosaccharides are one molecule of sugar. Some foods contain monosaccharides and others are created when disaccharides are broken down during digestion. They are small enough to pass through the walls of the small intestine.
  • Glucose: The primary product of photosynthesis, found in all fruits and plants, most carbohydrates that we eat are converted to glucose during digestion
  • Fructose: Found in fruits, some root vegetables, cane sugar, and honey
  • Galactose: Combines with glucose to make lactose (milk sugar), not found on its own

• Disaccharides: Disaccharides are two molecules of sugar. Some foods contain disaccharides and others are created when polysaccharides are broken down during digestion.
  • Sucrose: Made up of one molecule of fructose and one molecule of glucose, found in the stems of sugar cane and roots of the sugar beet, occurs naturally in some fruits and vegetables alongside glucose and fructose (especially in certain fruits and carrots), table sugar
  • Maltose: Made up of two molecules of glucose, formed during the germination of certain grains, mostly barley which is converted into malt, found in beer
  • Lactose: Made up of one molecule of glucose and one molecule of galactose, a naturally occurring sugar found in milk

• Oligosaccharides: 3-10 monosaccharides connected together.  They are not digested or absorbed in the small intestine (so they give us no calories yet give us that full feeling). Instead, they pass directly to the large intestine where they are consumed by microflora thus increasing the amount of healthy bacteria. Examples include: artichoke, burdock, chicory, leeks, onions, and asparagus.

• Polysaccharides: Polysaccharides consist of many monosaccharides connected together.
  • Starch: Starches consist of tens to hundreds to thousands of monosaccharides connected together. They are how plants store glucose for future use. About 70% of a plant’s structure is typically made up of the starch amylopectin (which is highly branched making it easy for the plant, and for humans to hydrolyze, or break down in the presence of water) and the other 30% is typically made up of the starch amylose (which has a more linear structure that makes it easy to store, but can’t be broken down without the enzyme amylase).
  • Resistant Starch: Resistant starches cannot be broken down during digestion, and so they are sent to the large intestine where they feed the healthy bacteria residing there. Examples include: green bananas, rolled oats, green peas, white beans, lentils, pearl barley, cold potato and cold pasta (occurs due to retrogradation).
  • Fermentable Fiber: We can’t digest the cell walls of plants, but some of them can be fermented in our large intestine like fructans (that occur in agave, artichokes, asparagus, leeks, garlic, onions, and wheat), inulin (occurs mainly in chicory), pectins (occurs mainly in the skins of citrus fruits and in apples, oranges, plum, guavas, and gooseberries), and raffinose (found in beans, cabbage, brussel sprouts, broccoli, asparagus, and whole grains). This fiber is soluble, meaning that it can mix with water, which creates a viscous gel that slows down digestion as it passes to the large intestine to be fermented by the microflora that resides there.
  • Nonfermentable Fiber: Humans do not possess the enzymes to digest some components of cell walls like cellulose, hemicellulose, and lignin (which provide plants with the stiffness they need to stand upright), nor do we possess the bacteria to break them down either. (Ruminants and termintes possess symbiotic bacteria that help them to break these elements down.) These types of polysaccharides will pass through us unused. They are what is referred to as insoluble fiber (not soluble in water and NOT digestible or fermentable) and what gives bulk to our stool. Examples include pretty much any part of the plant that is hard to chew such as cucumber skins, the outer hull of grains, the hull of popcorn kernels, potato skins, grape skins, 80% of lettuce, and more.

How we Digest Carbohydrates

The digestion of carbohydrates occurs mostly in the small intestine.

How We Digest Carbohydrates

1. In the mouth: The breakdown of carbohydrates begins in the mouth with the salivary enzyme amylase. Amylase works to break up the starch amylose, and hydrolysis begins breaking down the starch amylopectin. Very little digestion actually takes place here, however.
2. In the stomach: Carbohydrates are churned into a paste in the stomach, but no chemical breakdown occurs during this process. The stomach actually neutralizes any amylase that was swallowed.
3. In the duodonom: When carbohydrates enter the duodonem (which is the beginning part of the small intestine) the pancreas releases the enzyme amylase which breaks down polysaccharides into shorter glucose chains and maltose. (Babies produce very little amylase until over the age of one. Human breast milk contains a significant amount of amylase that helps babies digest breastmilk.)
4. Throughout the small intestine: The brush boarder that lines the small intestine performs the final breakdown of carbohydrates by releasing the enzymes sucrase, maltase, and lactase that break down the disaccharides sucrose, maltose, and lactose into the monosaccharides glucose, fructose, and galactose.
5. Absorption: The monosaccharides of glucose, fructose, and galactose are now small enough to pass through the walls of the small intestine and enter the bloodstream. Glucose and galactose need to hitch a ride on a sodium-dependent hexose transporter which will only transport them with a sodium ion. Fructose hitches a ride on another hexose transporter and doesn’t need sodium. As the blood circulates the liver, cells there take up fructose and galactose and covert them to other compounds, mainly glucose. This is why we say that most carbohydrates are converted to glucose in the blood!
6. In the large intestine: Within one to four hours after a meal, all of the sugars and most of the starches have been digested. What passes into the large intestine are things that could not be digested or absorbed. This includes resistant starch (such as asparagus), fermentable fiber (such as the peel of an apple), and nonfermentable fiber (which includes cellulose, one of the components of cell walls). Resistant starches and fermentable fibers are water soluble and attract water which softens the stool. They are also able to be fermented by the good bacteria that (hopefully) resides in the large intestine releasing water, gas, and short chain fatty acids.
7. Elimination: The nonfermentable fiber merely “bulks up the stool” and passes through unchanged. (Ever notice whole kernels of corn or popcorn hulls in your poop?)

Fats

Fats are the most efficient source of long term energy storage in both animals and humans.

From Triglycerides to Monoglycerides and Free Fatty Acids

Types of Fatty Acids

(*The following description of fats is adapted from Sally Fallon’s book Nourishing Traditions.)

Fatty acids can be categorized by how saturated they are:

• Saturated: All available carbon bonds are occupied by a hydrogen atom
• Monounsaturated: Has one double bond in the form of two carbon atoms double-bonded to each other and therefore lacking two hydrogen atoms
• Polyunsaturated: Has two or more pairs of double bonds and therefore lack four or more hydrogen atoms

In addition, they are also categorized by how long they are:

• Short-Chain Fatty Acids: Has four to six carbon atoms (always saturated, found mostly in butterfat from cows and goats)
• Medium-Chain Fatty Acids: Has eight to twelve carbon atoms (found mostly in butterfat and tropical oils)
• Long-Chain Fatty Acids: Has fourteen to eighteen carbon atoms
• Very-Long-Chain Fatty Acids: Has twenty to twenty-four carbon atoms (DHA)

How We Digest Fats

In children and adults, fat digestion occurs mostly in the small intestine (although in infants, it occurs mostly in the mouth). Most of the fat in our bodies and the fat we eat is in the form of triglycerides (three fatty acid chains attached to a glycerol molecule).

How We Digest Fats

1. In the mouth: Fat digestion starts slowly in the mouth. Some hard fats begin to melt as they reach body temperature. A salivary gland at the base of the tongue releases an enzyme (lingual lipase) that plays a minor role in fat digestion in adults and an active one in infants. In infants, this enzyme efficiently digests the short and medium chain fatty acids found in milk.
2. In the stomach: Once fats hit the stomach, they would float if it were not for the muscle contractions that direct all contents towards the pyloric sphincter at the bottom of the stomach. This churning action emulsifies the fat by dispersing it into large droplets. The gastric lipase enzyme in the stomach (that performs best in an acidic environment) starts to work on breaking these droplets down. But very little fat digestion takes place in the stomach.
3. Bile in the small intestine: When the large fat droplets enter the duodonem (the beginning part of the small intestine), they are coated with bile. (Bile is made in the liver and stored in the gallbladder. When fat enters the small intestine, it triggers the release of the hormone cholecystokinin which signals the gallbladder to release its bile.) The bile emulsifies the large fat droplets into smaller droplets. This increases their overall surface area making it easier for the next step. (Bile acids in the bile often pair up with amino acids which have one end attracted to water and one to fat. This helps with the emulsification process.)
4. Lipases in the small intestine: The pancreas secretes a lipase enzyme that hydrolyzes (breaks down in the presence of water) the triglycerides into monoglycerides and free fatty acids. *Infants do not secrete much of this enzyme; this is why the lingual lipase excreted from under their tongues plays more of an active role.
5. Absorption: Monoglycerides and free fatty acids are now small enough to pass through the intestinal wall.
6. Elimination: If you are eliminating too much fat in your stool (white poop anyone?), it is a sign of poor health (i.e. your intestines don’t absorb food, your pancreas doesn’t make enough digestive enzymes, or your gallbladder isn’t passing on enough bile).

Protein

Protein is the building block of life.

From Protein to Amino Acids

Foods with Protein

There are both animal and plant based sources of protein. Animal based sources of protein have all of the essential amino acids that we need, including the ones that we can’t make and can only get from dietary sources. Here are some examples of different foods and the amount of protein they contain:

• Chicken (31 g of protein per 100 g)
• Hamburger (27 g of protein per 100 g)
• Salmon (25 g of protein per 100 g)
• Eggs (19 g of protein per 100 g)
• Milk (3 g of protein per 100 g)
• Kidney beans (9 g of protein per 100 g)
• Tofu (8 g of protein per 100 g)
• Broccoli (2.82 g of protein per 100 g)
• Barley (2 g of protein per 100 g)

How we Digest Protein

The majority of protein digestion occurs in the stomach. *Watch a cool video that explains the entire process here.

How We Digest Protein

1. In the mouth: Protein (basically a bunch of amino acids all connected and bunched together) are crushed and moistened in the mouth, but no chemical breakdown occurs during this part of the process.
2. In the stomach: Hydrocholoric acid in the stomach uncoils, or denatures, each protein’s tangled strands so that the digestive enzymes can attack the peptide bonds. Hydrocholic acid in the stomach also converts the inactive form of the enzyme pepsinogen to its active form, pepsin. Pepsin cleaves large polypeptides into smaller polypeptides and some amino acids.
3. In the Duodonem: When the smaller polypeptides enter the duodenum (the beginning part of the small intestine), proteases are released from the pancreas that hydrolyze them further (break them down in the presence of water) into short peptide chains, tripeptides, dipeptides, and amino acids.
4. In the Small Intestine: Then peptidase enzymes on the membrane surfaces of the intestinal cells split these tripeptides and dipeptides into single amino acids.
5. Absorption: These single amino acids are now small enough to be absorbed through the small intestine and enter the blood stream. Only a few peptides escape digestion and enter the bloodstream intact.
6. Elimination: Only a small amount of dietary protein is lost in the feces.

In Conclusion

I have been reading, learning, making big posters, drawing models, redrawing models, talking to anyone who will listen, synthesizing, and applying all that I have learned about digestion for years in an attempt to understand it as best as I can. I feel like I could keep drawing better diagrams or synthesizing the information better and further, but I have to just stop here and move on knowing that I am going to continue to dig deeper, learn more, write more, and draw more, and keep building on this with future posts.

I think that understanding digestion is one of the fundamental building blocks for understanding health, and I hope that my synthesis of this information can help you understand it better as it has helped me. I am excited to move on and keep learning! I hope you’ll join me!

See it in action! Watch a camera go inside the digestive system to see a 5 minute video of what the process looks like first hand!