Skip to content

Your cart is empty

Have an account? Log in to check out faster.

Continue shopping

Gut health, explained

Your gut does far more than digest.

It is home to trillions of microbes. They feed on what you eat, and they are in constant conversation with your immune system. Here is what the research says, and what it does not.

What you are looking at Choose one to find it.

  • 1
  • 1

This page is general education about diet and the gut, with every source listed at the end. It is not medical advice, and it is not a claim about any Quas product.

Gut microbes and plant fibre, illustrated as if under a microscope. Not to scale.
A field of ripening rye and barley in low evening sun, with a dry stone dyke and rounded hills beyond.

Where fibre comes from

Most plant foods carry fibre your own body cannot digest. 1

Wholegrains, pulses, vegetables, fruit, nuts and seeds all contain it. What happens to it next is where your gut microbes come in.

What lives in your gut

Trillions of microbes, fed by what you eat.

Your gut is home to trillions of bacteria and other microbes. What you eat, and especially the parts your own body cannot digest, becomes their food. 1

1 2 3 4

StomachFibre arrives largely intact.

Small intestineMost of your meal is absorbed here.

Large intestineHome to most of your gut microbes.

Gut wall, magnifiedFatty acids feed the lining cells.

Illustrated, not to scale. The numbers match the steps.

  1. Step 1 · Stomach

    You eat fibre

    It passes through your stomach largely intact. Your own digestive enzymes cannot break most fibre down. 1

  2. Step 2 · Small intestine

    It passes through untouched

    Your small intestine absorbs most of the rest of your meal. Most fibre travels on, largely undigested. 1

  3. Step 3 · Large intestine

    Microbes ferment it

    Most of your gut microbes live in the large intestine. There they break fibre down by fermenting it, using enzymes the human body does not make. 14

  4. Step 4 · Gut wall, magnified

    They make short chain fatty acids

    Fermentation produces butyrate, propionate and acetate, a major energy source for the cells lining your gut. 1

In short

Fibre is food for your gut microbes. 1

Your own body cannot digest most fibre. Your gut microbes can, and what they make from it helps feed the cells lining your gut.

Your gut and your immune system

Your gut is a major centre of immune activity.

Every day, the gut meets food and microbes. Its immune cells help maintain this boundary, including plasma cells that make antibodies. 37

≈70%

of the body's antibody-producing plasma cells are found in the gut. 7

How your gut and immune system are connected

  • Where the body meets the world

    Every day the gut meets food, resident microbes and sometimes harmful ones, so it needs constant immune watch. 3

  • Trained by microbes

    Resident gut microbes help the immune system learn which bacteria to tolerate and which to fight. 2

  • Felt beyond the gut

    Researchers increasingly recognise that gut microbes also affect immunity elsewhere in the body. 2

A 200,000 year diet, changed in 100

200,000 years of foraging. About 100 years of change.

For more than 200,000 years, most humans lived as foragers, eating large amounts of plant material. Processed food, antibiotics, modern sanitation and low fibre diets are recent arrivals by comparison. 4

A stone larder stocked with sacks of oats and barley, strings of onions, root vegetables, jars of dried beans and a trug of brambles.
Diets rich in plant fibre
The industrial era Roughly the last 100 years, about 0.05% of the timeline. Drawn wider than true scale so you can see it.
How gut microbes differ between people living traditional and industrialised lifestyles today. 4
Traditional lifestylesSuch as the Hadza foragers of Tanzania Industrialised lifestylesSuch as Western countries
Diet Plenty of fibre rich plant foods More processed food, far less fibre
Variety of gut microbes Higher Lower
Fibre digesting enzymes A wider range A narrower range
Seasonal change in gut microbes Present Absent

These are comparisons between groups of people alive today, not with ancient humans. In one, people in the United States had about 75% of the bacterial diversity found in Amerindians living in Venezuela. 4

A hypothesisNot yet settled

It is our opinion that aspects of our microbial identity have gone extinct and that this extinction results in a mismatch between our recently adapted microbiota and our more slowly adapting human genome.

Erica D. and Justin L. Sonnenburg, Stanford University. Opinion article, Nature Reviews Microbiology, 2019. 4

The fibre gap

Only 4% of UK adults eat enough fibre.

Fibre is what gut microbes feed on, yet national survey data show that very few people in the UK reach the recommended amount. 5

  • 30g

    Recommended fibre a day for adults in the UK 5

  • 14%

    of children aged 4 to 10 eat enough fibre for their age 5

4 in 100 UK adults eat the recommended 30g of fibre a day

British Nutrition Foundation, 2026, using National Diet and Nutrition Survey data from 2019 to 2023. 5

The Stanford diet study

What changed with more fibre and fermented foods.

A Stanford team asked healthy adults to change their diet for ten weeks. One group ate more fermented foods, the other more fibre, and the researchers tracked their gut microbes and immune markers throughout. 6

The study at a glance

Who took part
36 healthy adults
Diet period
10 weeks
Design
Randomised into two groups
Where
Stanford University, 2021
  • Jars and bowls of sauerkraut, kimchi, kefir, yoghurt, cottage cheese and pickled vegetables on a wooden counter.

    Group one

    More fermented foods

    Yoghurt, kefir, fermented cottage cheese, fermented vegetables, vegetable brine drinks and kombucha.

    Servings of fermented food a day, on average
    Before
    0.4
    Week 10
    6.3

    What changed

    • The variety of gut microbes rose steadily over the diet.
    • 19 of 93 blood proteins in an inflammation panel went down, including one called IL-6. 6
  • A kitchen table of fibre rich foods: rye bread, oats, lentils, beans, leeks, carrots, red cabbage, apples, pears and nuts.

    Group two

    More fibre

    Fruit, vegetables, legumes, whole grains, nuts and seeds.

    Grams of fibre a day, on average
    Before
    21.5
    Week 10
    45.1

    What changed

    • Genes for enzymes that break down plant carbohydrates became more abundant in the gut microbiome.
    • Immune responses varied between people, with different patterns linked to their starting gut microbes.
    • The variety of gut microbes stayed about the same over the ten weeks. 6

  • What it shows

    • Changing your diet can shift your gut microbes and some immune markers within weeks.
    • Each diet produced a different pattern of microbial and immune changes.

  • What it does not show

    • The main planned measure, immune-cell responses to lab stimulation, did not significantly change in either group.
    • It was small and short: 36 people over 10 weeks. Larger studies are needed.
    • It tested whole diets, not any single food or drink, and that includes Quas.
A can of Quas Original beside a tulip glass of cloudy, dark Quas on a dinner table with rye bread and roast vegetables, and an Edinburgh skyline through the window.

Where Quas fits

A fermented drink that is high in fibre.

Nothing above is a claim about Quas. The research looks at whole diets, not any single food or drink. What we can say is simple: only 4% of UK adults meet the recommended 30g of fibre a day, and Quas is high in fibre, with 6.7g per can. 5

LIVE Keep in the fridge

Shop Live QuasThe science of kvass

  • High in fibre

    6.7g of fibre in every can, from chicory inulin.

  • Low in sugar

    2.5g of sugar or less per 100ml.

  • Sourdough starter

    Every batch is fermented with a sourdough starter kept alive for 180 years.

  • Billion live cultures

    In every can of Live Quas, lab verified.

Quas is made from rye and barley and contains gluten. Live Quas contains live cultures and is kept in the fridge.

Sources

References.

Every numbered citation on this page links here. We chose peer reviewed papers and national survey data, and we say what kind of evidence each one is.

This page is general education, not medical advice. If you have questions about your diet or digestion, speak to your GP or a registered dietitian.

  1. Review

    Influence of diet on the gut microbiome and implications for human health

    Singh RK and colleagues. Journal of Translational Medicine, 2017.

    How gut microbes ferment fibre and make short chain fatty acids.

    Read the paper

  2. Review

    The interplay between the gut microbiome and the immune system in the context of infectious diseases throughout life and the role of nutrition in optimizing treatment strategies

    Wiertsema SP and colleagues. Nutrients, 2021. The authors work for Danone Nutricia Research.

    How gut microbes interact with the local and wider immune system.

    Read the paper

  3. Review

    Allergy and the gastrointestinal system

    Vighi G and colleagues. Clinical and Experimental Immunology, 2008.

    The gut as the main contact point with the outside world, and the size of its immune tissue.

    Read the paper

  4. Opinion article

    The ancestral and industrialized gut microbiota and implications for human health

    Sonnenburg ED and Sonnenburg JL. Nature Reviews Microbiology, 2019.

    The foraging timeline, the lifestyle comparison and the mismatch hypothesis.

    Read the paper

  5. Report

    Dietary Fibre in 2026: An Update on Barriers and Opportunities to Fill the UK Fibre Gap

    British Nutrition Foundation, 2026, using National Diet and Nutrition Survey data from 2019 to 2023.

    The 30g recommendation and the share of UK adults and children who meet it.

  6. Randomised study

    Gut-microbiota-targeted diets modulate human immune status

    Wastyk HC and colleagues. Cell, 2021.

    The ten week fermented food and fibre study on this page.

    Read the paper

  7. Quantitative analysis

    The total mass, number, and distribution of immune cells in the human body

    Sender R and colleagues. Proceedings of the National Academy of Sciences, 2023.

    The estimated share of antibody-producing plasma cells found in the gut.

    Read the paper