Gut bacteria
Prebiotics, gut bacteria and the facts about ‘detox’ products
Prebiotics are fibres and other compounds in food that the body cannot digest but beneficial gut bacteria can. They are found in onion, garlic, bananas, pulses, oats and whole grains. Eating them regularly increases helpful bacteria and the useful substances they make. The body does not need special juices or kits to remove waste. The liver, kidneys and gut do that, and fibre is what supports them.
In short
- A prebiotic is food for beneficial gut bacteria.
- Ordinary Indian foods supply plenty, when the diet includes pulses, vegetables and whole grains.
- Increase them gradually. In irritable bowel syndrome, some prebiotic foods cause gas.
- Products sold to “cleanse” the gut or liver have no proven benefit, and some cause harm.
What is a prebiotic?
An international expert panel defines a prebiotic as a substance that is selectively used by the body’s own microbes and confers a health benefit. Most are types of fermentable fibre. They pass through the small intestine undigested and are fermented by bacteria in the colon.
All prebiotics are fibre, but not all fibre is prebiotic. Some fibres mainly add bulk and are fermented little.
Which foods contain prebiotics?
| Prebiotic | Indian food sources |
|---|---|
| Inulin and fructans | Onion, garlic, wheat, barley, slightly unripe banana |
| Galacto-oligosaccharides | Rajma, chana, moong, masoor and other pulses |
| Resistant starch | Pulses, cooked and cooled rice or potato, green banana |
| Beta-glucan | Oats, barley |
| Pectin | Guava, apple, citrus fruit, carrot |
Plant compounds in tea, spices, berries and cocoa are also used by gut bacteria and are being studied as possible prebiotics.
What do prebiotics do?
Increase beneficial bacteria
Particularly bifidobacteria and lactobacilli.
Raise short-chain fatty acids
Fermentation produces the acids that fuel the colon’s lining. See postbiotics.
Improve bowel regularity
Softer, more frequent stools.
Help with mineral absorption
Some prebiotic fibres increase calcium absorption.
How much do I need, and how do I start?
There is no separate daily requirement. A diet that reaches 25 to 30 grams of fibre from varied plant foods contains enough. See fibre.
- Add one prebiotic-rich food at a time
- Increase over two to four weeks
- Expect some extra gas at first; it settles as bacteria adapt
- Soak pulses overnight and cook them thoroughly
Who should go slowly?
Many prebiotic foods are also FODMAPs, the fermentable carbohydrates that trigger symptoms in irritable bowel syndrome. If onion, garlic, wheat or pulses bloat you, small portions and a structured approach work better than avoidance. See the low FODMAP diet. Prebiotic supplements in powder form can worsen bloating in a sensitive gut.
Does the gut need a “detox”?
No. Waste and unwanted substances are handled continuously by:
- The liver, which converts them into forms that can be removed
- The kidneys, which filter the blood
- The gut, which carries bile and waste out in the stool
- The lungs and skin, to a smaller extent
No juice, tea, powder or kit has been shown to improve on this. Some carry risks:
- Laxative teas cause loss of fluid and salts, and cramping
- Several herbal “liver” products are known causes of liver injury
- Prolonged juice-only fasts are low in protein and high in sugar
What supports the body’s own clearance?
Fibre
It binds bile acids and keeps stools regular, which is how the gut completes the liver’s work.
Water
Enough to keep the urine pale.
Less alcohol and less sugar
Both add to the liver’s load.
Sleep and movement
Both improve how the liver handles fat and sugar.
Care with medicines and supplements
Take only what you need, and tell your doctor about herbal products.
When should I see a doctor?
See a doctor for yellow eyes, dark urine, persistent tiredness, abdominal swelling, or bowel symptoms lasting more than a few weeks. These need tests, not a cleansing programme.
Questions patients ask me
Are prebiotics or probiotics more important?
For most people, prebiotics. Feeding the bacteria you already have, through fibre, has more consistent evidence than adding new ones.
Do I need a prebiotic supplement?
Rarely. Food supplies enough for most people. Supplements are sometimes used for constipation, starting at a low dose.
Why do pulses give me gas?
Their prebiotic fibres are fermented by gut bacteria. Soaking, thorough cooking, smaller portions and a gradual increase reduce it, and the gut adapts over a few weeks.
Is a juice fast good for the gut?
No. It removes fibre, which gut bacteria depend on, and delivers a large amount of sugar.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- International consensus statement on the definition of prebiotics. Gibson GR et al. Nat Rev Gastroenterol Hepatol. 2017. PubMed 28611480
- Systematic reviews of carbohydrate quality and human health. Reynolds A et al. Lancet. 2019. PubMed 30638909
- Trial of high-fibre and fermented-food diets and immune status. Wastyk HC et al. Cell. 2021. PubMed 34256014
- American College of Gastroenterology guideline on irritable bowel syndrome. Lacy BE et al. Am J Gastroenterol. 2021. doi:10.14309/ajg.0000000000001036