The fate of food: Introduction

We consume food to allow our body to both grow and maintain itself. Solid and liquid forms are consumed and, after some delay, both forms are excreted. Even a child intuitively understands the connection between eating and drinking, and then 'having to go to the toilet' some time later. What happens to the stuff we eat and drink? How long does it take to pass through our digestive system? What processes are involved in order to extract the necessary nutrients required before the unused portion is expelled?

Under normal circumstances, the consumption of about 2 L of food and liquid results in the absorption of over 90% of material, mostly by the small intestine (Figure 1). But in order to optimally extract the nutrients, there is an exchange of about 9 L of fluids in secretion, absorption and excretion steps. This includes inputs of 1.5 L of saliva, 2 L of gastric juices, 1.5 L of pancreatic secretions and another 0.5 L of bile; absorption of 8.9 L in the small and large intestines; and finally defecation of ~150 mL of waste (Sensoy2021).

Figure 1. Transit times (indicated by squiggly arrow) and digestion volumes for food traversing the human digestive tract (from Sensoy2021).

Food transit times vary significantly, depending on the type of food consumed and the site of digestion. For example, the median retention time for
the whole gut is about 60 hours in UK subjects (Cummings1991), but any time between 2-5 days is not atypical (Mayo Clinic).

Transition from the mouth to stomach is rapid, but then the process slows down. The stomach acts as a reservior for food, generates acid to kill pathogens and aid in digestion, releases digestive enzymes and facilitates gastrointestinal mobility (OConnor2014). Food retention by the stomach can be brief (~15 min) or lengthy (>4 hours).

Most human digestion, nutrient absorption and fluid recycling occurs in the small intestine. For a 2 L meal, the small intestine will process about 9 L of digesta, absorbing about 8.5 L (~94%) of it. This part of the digestive tract has a large surface area to aid in the absorption process and transit velocities are relatively quick. Average retention times are in the order of 2 hours.

The caecum section of the large intestine receives material from the terminus of the small intestine (ileum). Over the course of the next 6-12 hours, the digesta undergoes further breakdown by gut bacteria and electrolytes are absorbed (Cummings1991). As the content transitions to the left colon, the volume is reduced almost 10-fold, the concentration of gut microbes skyrockets and retention times also increase. For a discussion on the makeup of faeces, see Composition of a Stool.

Building blocks of the human body

A balanced diet contains seven major groups of nutrients: carbohydrates, fibre, protein, fats, minerals, vitamins and a fair bit of water.  Knowing what structural and functional building blocks our body contains will give us a reasonable understanding of the food types we need to eat.

Firstly, we are mostly (65%) water: a lean adult human weighing 60 kg would have about 40 kg of it.  Virtually all of the other 20 kg is in the form of insoluble solids.  Bone, which is composed primary of hydroxyapetite (a form of hydroxylated calcium phosphate), accounts for about 15% of a body's mass, or ~ 8.5 kg.  Recyclable organic polymers of various types, such as carbohydrates (0.1 kg of glycogen), nucleic acid molecules (0.5 g of RNA) and proteins (5 kg) account for about 6 kg, while insoluble, compartmentalised molecules like fats (5-6 kg) make up for much of the remainder.

Simple solutes, like free glucose and salts (sodium and potassium) only account for < 1/2 kg.  Glucose - the main energy source for cells - is maintained in the blood stream at about 5 g total at any given time.

Breaking it down (digestion)

Lets do a rather gruesome thought experiment and imagine our 60 kg human body being virtually 'hydrolysed' down to its basic (monomeric) components.  What type - and how much - of each component would we expect to see?

Amino acids come from protein. Our 5 kg of protein from the 60 kg human should provide 20 amino acids (plus traces of selenocysteine) ranging from the most common, leucine (0.46 kg), to the least common, tryptophan (0.06 kg)(Kozlowski2017).  Nine of these amino acids are essential in that they are not made by our bodies and must be obtained through a balanced diet.

Glucose comes from carbohydrates and glycerol from glycerides.  The hydrolysis experiment would yield some glucose and glycerol, the former coming from stored glycogen, while the latter being derived from hydrolysis of mono-, di- and tri-glycerides.  Ribose would come from the breakdown of RNA and small amounts of D-galactose, L-fucose, sialic acid and fructose would also be detected.

Insoluble fatty acids (FA) from glycerides and phospholipids: these FA typically have sidechains ranging from C8 (caprylate) up to C22 and beyond.  The bulk of saturated FA's are palmitic (C16) and stearic (C18) and we would expect around 1.5 kg of each in our hydrolysis experiment.

This thought experiment might seem abstract, but it is precisely what occurs to the food we ingest everyday.  Humans have enzymes and simple catalysts (e.g. HCl) that break down digestible carbohydrates, proteins and glyceride fats into the individual components described above. 

In fact, our digestive tract has quite a bit more variety of compounds to deal with than is needed by our body.  Unwanted compounds absorbed by the gut are either altered by the liver, filtered through the kidneys or both. Food remnants not absorbed make their way to the large intestine where hundreds of species of resident bacteria begin to digest the remaining solids. In a healthy gut microbiome, complex carbohydrates, undigested proteins and poorly absorbed fats are broken down further, producing a number of useful byproducts that can be utilised by the body (butyric acid, vitamin B12, etc.).

For further details about the fate of carbohydrate-containing food items, see the section on carbohydrates, including the Fate of Digestible Carbohydrates, Digestible and Resistant Starches, and the Fate of Indigestible Carbohydrates.

Information about the digestion of proteins can be found in Fate of Food: Protein.

Finally, information about the digestion of fats can be found in Fate of Food: Fats.