Anabiomics

Bioavailability and Nutrient Absorption: What Happens After Digestion?

Breaking food down is only part of nutrition. Nutrients must also be released from the food matrix, cross the intestinal lining and enter circulation in a form the body can use. Bioavailability describes this broader journey from digestion to absorption and utilisation.

Science explainer

Bioavailability and Nutrient Absorption: What Happens After Digestion?

Content type

Science explainer

Focus

Digestion, nutrient release, intestinal absorption and bioavailability

The big question

Digestion does not end when a nutrient has been chemically broken down.

For a nutrient to become useful to the body, several things have to happen.

It must be released from the food matrix.

It must be present in a chemical form that can be absorbed.

It must cross the intestinal lining.

And, in many cases, it must then be transported, converted or stored before the body can use it.

This broader concept is known as bioavailability.

The big question is:

What determines whether a nutrient that is present in food actually becomes available to the body?

How do scientists study it?

Scientists study nutrient bioavailability by looking at several linked stages.

Digestion

Large food molecules are broken into smaller components.

Proteins become peptides and amino acids.

Starches become smaller carbohydrates.

Dietary fats are broken into absorbable lipid components.

Release from the food matrix

Nutrients are physically and chemically embedded within food.

Processing, chewing, gastric mixing and digestive enzymes can all affect how easily those nutrients are released.

Solubility

Some nutrients must dissolve or become incorporated into structures that allow them to move through the intestinal environment.

Fat-soluble nutrients, for example, depend partly on bile and lipid digestion.

Transport across the intestinal lining

The small intestine contains specialised transport systems that move nutrients from the gut lumen into intestinal cells and then into blood or lymph.

Post-absorptive metabolism

Absorption is not always the end of the process.

Some nutrients are modified by intestinal cells or the liver before reaching the rest of the body.

Bioavailability therefore reflects more than just how much of a nutrient is present in the diet.

What can we measure?

Several concepts are useful when thinking about nutrient availability.

Digestibility

Digestibility describes how much of a nutrient disappears from the gastrointestinal tract rather than being excreted.

It is closely related to absorption, but it is not exactly the same as bioavailability.

Absorption

Absorption is the movement of nutrients across the intestinal lining into the body.

Bioaccessibility

Bioaccessibility describes the fraction of a nutrient that is released from food and becomes available for absorption.

Bioavailability

Bioavailability goes a step further.

It considers how much of the nutrient becomes available for normal physiological use after digestion and absorption.

Food matrix effects

The structure of a food can influence nutrient release.

The same nutrient can behave differently depending on whether it is present in a raw ingredient, processed food, fibre-rich matrix or liquid preparation.

Nutrient interactions

Some nutrients affect the absorption of others.

Minerals can compete for transport or bind to compounds such as phytate.

Fats can influence the absorption of fat-soluble vitamins.

This means nutrient availability depends on the whole meal, not just the amount printed on an ingredient list.

What have we learned?

Several important principles emerge from nutrition research.

More in the food does not always mean more absorbed

A nutrient can be present in large amounts but still have limited availability if it remains trapped within the food matrix or is poorly absorbed.

Digestion can influence bioavailability

Breaking food into smaller components is often a necessary step before absorption can occur.

The efficiency of digestion can therefore influence the amount of absorbable material presented to the intestinal lining.

The small intestine is central

Most nutrient absorption takes place in the small intestine.

Its large surface area, created by folds, villi and microvilli, is specialised for transferring nutrients from the gut into the body.

Chemical form matters

Different forms of the same nutrient may have different absorption characteristics.

The food matrix matters

Fibre, phytate, fat content, processing and other components can influence how easily nutrients are released and absorbed.

Absorption and microbiome substrate are connected

Material that is not absorbed remains within the gastrointestinal tract.

Some of it may continue into the large intestine, where it can become substrate for microbial metabolism.

Why does this matter?

Bioavailability extends the Science Hub's digestion framework one step further.

It is not enough to ask:

What did the dog eat?

We can also ask:

What was digested?

What became absorbable?

What was actually absorbed?

What remained available further down the gut?

That creates a broader sequence:

Diet → Digestion → Bioaccessibility → Absorption → Residual substrate → Microbiome

This is relevant to anabiomics because upstream nutrient handling helps shape the nutritional environment encountered by the microbiome.

More effective digestion and absorption can change the quantity and type of material reaching the large intestine.

Conversely, nutrients that escape absorption can become substrates for microbial fermentation.

The relationship between nutrition and the microbiome therefore begins well before the colon.

What should we keep in mind?

Bioavailability is a useful concept, but it is also complex.

Digestibility is not the same as bioavailability

A nutrient can disappear from the gastrointestinal tract without necessarily being used efficiently by the body.

Different nutrients behave differently

Protein, carbohydrate, fat, vitamins and minerals use different digestive and absorptive pathways.

There is no single measure of bioavailability that applies to everything.

Individual dogs vary

Age, health, diet, gastrointestinal function and other biological factors can influence nutrient absorption.

Food processing can have mixed effects

Processing may improve the accessibility of some nutrients while reducing or altering others.

Higher absorption is not automatically better

Some dietary components are intended to remain within the gut and interact with microbial communities rather than be completely absorbed.

The aim is therefore not simply to maximise absorption of every component.

It is to understand how digestion, absorption and residual substrate interact as part of the whole gastrointestinal system.

Sources and further reading

References

Further reading:

Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ. Small Animal Clinical Nutrition. 5th edition. Mark Morris Institute.

Wernimont SM, Radosevich J, Jackson MI, et al. The Effects of Nutrition on the Gastrointestinal Microbiome of Cats and Dogs: Impact on Health and Disease. Frontiers in Microbiology. 2020;11:1266.

Pilla R, Suchodolski JS. The Gut Microbiome of Dogs and Cats, and the Influence of Diet. Veterinary Clinics of North America: Small Animal Practice. 2021;51(3):605–621.

National Research Council. Nutrient Requirements of Dogs and Cats. National Academies Press. 2006.

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