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What Happens to Food? – Healthy Human Digestion

Jeremy Hendon | July 1

paleo digestion

Some people love building things. They probably had erector sets and legos when they were kids.

Some people love imagining things. I’m sure they read a lot as kids and daydreamed when they weren’t reading.

Me – I love knowing how things work. I’m incessantly drawn to books and articles that explain things to me.

I don’t even usually have a use for what I learn, but I can’t stand now knowing how something is working.

For better or worse, that’s led me down a deep rabbit-hole when it comes to nutrition and health, but it’s a super-interesting rabbit-hole.

I was giving a nutrition talk a few days ago, and I realized that I frequently needed to stop and explain how our bodies work before I could explain why a particular food had a certain effect. So I decided to write this article (and a few articles over the next couple weeks), with the hope that at least 3 or 4 people (maybe more) will find it helpful and entertaining.

Do You Need to Understand Digestion?

No. It’s not essential that you know how your food is digested. After all, humans lived for millions of years without knowing, and they seemed to do just fine.

However, it’s way more interesting than you might think, and if you’re really interested in knowing what certain foods will do to your body, then it’s 100% necessary that you start by understanding how foods get digested and absorbed by your body.

Digestion is – broadly speaking – the process of breaking down your food into very tiny pieces and then moving those tiny pieces from your digestive tract into your bloodstream.

It sounds simple, but your body is really AMAZING in terms of how it accomplishes this feat. Just the process of breaking down food is not easy.

In this article, I’m going to cover only the processes of digestion and absorption, and only the basics at that.

Obviously, there are a huge number of things that happen when you eat. In particular, your body sends a LOT of hormonal signals (involving insulin, leptin, peptide YY, ghrelin, etc.). Also, once in your bloodstream, nutrients need to be processed and transported around your body to be used.

That’s all very fascinating and important, but it’s beyond the scope of this particular article.

Why Knowing About Digestion is Healthy

I hinted at this above, but knowing about digestion is a must if you’re interested in health and nutrition, or if you’re just interested in improving your own well-being.

Most importantly, digestion can and often does go wrong. Foods don’t get digested for one reason or another, and then they either don’t get absorbed or else they get absorbed without first being broken down (which is potentially worse). Thing is, if you don’t know what’s supposed to happen, then it’s hard to figure out what’s going wrong.

In the following video, I talk very briefly about digestion and a couple of the things that can go wrong. It’s short, so watch it quickly, because I think it provides a good backdrop to why it’s so important to understand how digestion should work:

The Big Picture of Digestion

Until food makes it into your blood stream, you can pretty much consider it to be “outside” your body. In other words, there’s not a huge difference between food being in your hand versus in your stomach. Only when that food makes it into your blood can it start nourishing your body or else causing a lot of problems.

paleo digestion

The route that food takes through your body is pretty basic:




Small Intestines (composed of 3 parts – Duodenum, Jejunum, and Ileum)

Large Intestines


Every single step of this process is important, but it’s in the small intestines (and specifically in the very upper part of your small intestines) where most digestion and most absorption occurs.

I’ve seen videos and articles on the internet that try to explain what happens in each location along your digestive tract. Those videos and articles are interesting, but it’s actually easier and better to understand what happens to the different parts of your food (fats, proteins, carbs, vitamins, minerals, and fiber).

The Parts of Your Food

In general, every bit of food you eat will break down into 7 things:

  1. Water;
  2. Fat;
  3. Carbohydrate;
  4. Protein;
  5. Vitamin;
  6. Mineral; and
  7. Fiber.

I don’t mean to indicate that these are the only things in your food (for instance, polyphenols are another component found in most plant foods). However, the 7 items above are 99.9% of your food and are the most important parts for your body to digest, absorb, and/or expel.

Below, I explain how digestion and absorption of each of these parts occurs.


paleo digestion

Water does not really need to be broken down and is absorbed predominantly in the small intestines. Water digestion is comparatively uninteresting.


Proteins digestion occurs in the stomach and the upper part of the small intestines (the duodenum). In the stomach, 2 different things are released: (a) gastric acid and (b) pepsinogen.

Gastric acid is partially HCL (hydrochloric acid) and mostly KCl (Potassium Chloride) and NaCl (Sodium Chloride). Gastric acid does 2 things: (1) First, it “denatures” proteins, which is a fancy way of saying that it breaks the proteins down just a little bit and gets them ready to be broken down further; and (2) Secondly, Gastric acid turns pepsinogen into pepsin.

Once the proteins have been denatured by Gastric acid, they are further broken down by Pepsin, which is a protease. A protease is an enzyme that breaks proteins down into amino acids or else very short chains of amino acids.

Everything above occurs in the stomach, but the proteins are still very very long chains even after being broken down slightly by pepsin.

In the upper part of the small intestines (the duodenum), proteins get further broken down by 2 other proteases (trypsin and chymotrypsin), both of which are released into the small intestines by the pancreas. Finally, the walls of the small intestines break down the proteins into single amino acids, which are then absorbed into the bloodstream.


Fats are not digested at all until they reach the small intestines. (Neither saliva nor gastric acid has any effect on fats.)

Fats have the most complicated and interesting digestion process, largely because fats are not soluble in water. Think about pouring oil or grease into water and how it won’t really mix together. That’s essentially the case for fats in your digestive tract.

Once in the small intestines, the gall bladder releases bile salts (which are produced by the liver but stored in the gall bladder). These bile salts interact with fats and start to break them apart. The real digestion occurs after the fats are partially broken apart, when the enzyme lipase (produced by your pancreas) starts to break the fats into free fatty acids and glycerol (the most basic parts of fat).

Once that process is complete, the free fatty acids and glycerol then interact again with bile salts so that they can be transported through your intestines and into your bloodstream.


Digestion of carbohydrates actually begins in the mouth. While in the mouth, the salivary glands excrete saliva, which contains salivary amylase. Salivary amylase breaks down starches into maltose and dextrin.

In the stomach, carbohydrates are not broken down any further.

Like proteins and fat, carbohydrates are broken down primarily in the small intestines. The pancreas secretes pancreatic amylase into the small intestines, which further breaks down starches into “disaccharides,” which means that they’re 2 molecules long.

Finally, the small intestines themselves produces enzymes called lactase, maltase, and sucrase to break the carbohydrates into single glucose and fructose molecules, which are absorbed into the bloodstream.

Vitamins and Minerals

paleo digestionUnlike Proteins, Fats, and Carbohydrates, vitamins and minerals do not require much digestion prior to being absorbed by your body. In general, vitamins and minerals, once separated from proteins, fats, and carbohydrates are already small enough for your body to absorb.

A very small amount of absorption can actually occur in your mouth for certain vitamins and minerals (B6, B12, C, Folate, and Zinc).

Pretty much no absorption of vitamins and minerals occurs in the stomach.

Almost all absorption occurs in the small intestines. Different vitamins and minerals are absorbed in different ways and in different parts of your small intestines.

Fat-Soluble vitamins (such as A, D, E, and K), are absorbed along with fats in the upper part of your small intestines.

All other vitamins are absorbed throughout your intestines, where there are special proteins in the walls of your intestines that attach to and transport these vitamins into your bloodstream.

Minerals are absorbed primarily in the duodenum and the jejunum (the upper and middle parts of your intestines).

There are 2 special cases: (a) Vitamin B12 is absorbed primarily in the ileum (the bottom part of the small intestines), and (b) Vitamin K, Biotin, Sodium, Chloride, and Potassium are all absorbed primarily in your large intestines.

Fiber and Undigested Food

Anything that is not digested and absorbed in your small intestines eventually makes its way to your large intestines. Here, you have hundreds of trillions of bacteria that feed on and digest much of the food that your body didn’t otherwise digest.

Among other things, the bacteria in your large intestines start to ferment undigested carbohydrates and fiber, turning these into fatty acids, which your body can then absorb.

In addition, these bacteria produce vitamin K and Biotin for your body to absorb.


Anything that’s left over and not absorbed at all becomes…poop. (For a detailed article on why your poop is green, check out this.)

That’s All There Is To It, Folks

I thought about just ending this article with the poop section (a rather poetic end), but I’m hoping that there are a few of you who want to know more about what happens when you eat.

If that’s the case, then let me first say that I’ve got a few articles and videos lined up that I’ll publish over the next month.

Also, go buy The Paleo Solution if you haven’t already. Robb does as good a job as anybody at actually explaining what happens to food in your body. In addition, he goes into great detail about all the bad things that can happen when you eat food that you shouldn’t (e.g., non-Paleo food).

Finally, let me know what you want to know more about! I write mostly about what interests me, but I take suggestions as well! Let me know in the comments below, and also answer this question – what did it take to get you interested in food and digestion? Was there a single occurrence?

Images (in order): Melissa Youngern Lori Greig via photopin cc.

Nigel Unwin - August 12

Thank-you…Ilook forward to your further articles.

Roseann - August 15

I’ve had IBS for about 30 yrs. first 20 – 25 yrs. it was intermittent. Since about 2008 it is almost a daily occurrence. Been to several Dr. Many medications nothing is working any more. And the pain is unbelievable! I’m very interested in the digestive process. Any help or advice you can give me I’ll take. I did take your 7 day challenge but was not as prepared as I should of been. I’m still trying.

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