Plants, food and fibre: an introduction to food structure

How does the structure of food and plants affect the digestion of carbohydrates and what our gut bacteria feed on?

Let’s find out.

How do plants become part of human nutrition?

Plants are a major part of the human diet. Plants, like humans, need nutrients to survive and thrive, and accumulate/synthesise these nutrients from their surroundings.

Some of these components provide us nutrition as part of our diet. The composition of our food is clearly important for our health, as we need to consume the necessary balance of macro- and micro-nutrients.

Macronutrients

These are essential for healthy living and include fats, proteins, carbohydrates.

Micronutrients

These are also needed by the body in smaller amounts, which include the minerals and vitamins.

However, the structure of our food is critical for determining what impact those nutrients have on our health and how they interact with our gut microbiota.

Where does fibre come from?

Plants, like all living organisms are made up of individual cells. Unlike most animal cells, plant cells possess an outer cell wall which gives the cell added protection and adds rigidity to structures such as stems and stalks.

The cell wall is made out of complex carbohydrates such as cellulose, pectin, β-glucan, etc. depending on the plant.

These carbohydrates are indigestible by our own digestive enzymes and so are the main source of dietary fibre in plants. In fact, many seeds and nuts have evolved to pass through the digestive system of an animal unchanged, in order to be dispersed and grow.

Structures within the cells

In addition to cell walls, structures within the cells are also important when considering types of fibre. As you learned in the previous step, starch is a complex carbohydrate made in plants to store energy and the main carbohydrate in the human diet.

Depending on the types of glucose chains, starch is made up of amylose (straight glucose chains), amylopectin (branched glucose chains), or a combination of both.

Plants have developed highly organised structures to act as nutrient and starch storage reserves. Some examples include:

• Potatoes, for instance, are a storage reserve for starch produced by photosynthesis.
• In cereals such as wheat, maize and rice, the starch granules are packed into the endosperm tissue, which is the main component of the cereal grain.
• Beans, peas and lentils (collectively called pulses) also accumulate starch as they mature into dry seeds.

Depending on the molecular structure of the starch molecule, whether the starch is physically entrapped by fibre, and the amount of processing (see next section), some of the starch will be broken down by the enzyme amylase and absorbed in the small intestine.

The remaining starch will reach the gut microbiota in the large intestine.

If you’d like to learn more about the human microbiome, check out the full online course, from EIT, below.