- Prebiotics are types of fiber which enhance growth of beneficial gut bacteria
- Gut bacteria release metabolically important molecules such as short chain fatty acids
- Short chain fatty acids decrease small intestinal pH, therefore making the lower digestive tract less susceptible to disease causing microorganisms
What are prebiotics?
Prebiotics are types of fiber which escapes digestion in the upper gut and can affect the proliferation and ecosystem of bacteria mostly residing in the large bowel, which feed off our undigested carbohydrates. Generally carbohydrates are classified in terms of their chain length where the prefix is the number of molecules in the chain:
mono- meaning one
di- meaning two
oligo- meaning 3-6
poly- meaning more than six
and the suffix being -saccharide, from Latin saccharum meaning sugar.Our main source of prebiotics comes from oligosaccharides which are plant derived carbohydrates such as beta glucans found in oats. Archaeological analysis of preserved faeces of hunter gatherers has shown us a typical male would consume an average of 135g of inulin a day, a prebiotic found in several root plants such as chicory and garlic. 
Main bacteria involved:
The main bacteria involved in the fermentation of prebiotic fiber are Bifidobacterium and Lactobacillus, which contain several genes encoding enzymes that break down carbohydrates. In terms of digestion, these enzymes act as biological scissors, breaking the bonds between individual sugars in a chain so that they may be more easily absorbed.
In comparison to the microbiome the human body itself contains a miniscule amount of its own carbohydrate digesting enzymes: certain bacteria such as Bacteroides thetaiotaomicron can produce 260 alone! The bacteria benefit from this sugar fermenting process because it releases energy which they need for survival and proliferation, such that for every 100g of carbohydrate fermented 30g of bacteria is produced. Our interaction with the bacteria is certainly a two way contract because we too benefit from them releasing important biological molecules such as short chain fatty acids (SCFA).
Short chain fatty acids- fatty acids with less than 6 Carbon molecules produced by bacterial fermentation in the large intestine
Prebiotics and short chain fatty acids
One of the main functions of SCFA is promoting a healthy large intestine. Your large intestine is a tube absorbing water and storing faeces until they are ready to be released. An imbalanced microbiome can’t serve its protective function against inflammatory diseases such as IBS (irritable bowel syndrome) which is estimated to affect 2 in 10 people in the UK.
Bowel- section of the digestive tract including the small bowel (small intestine) and large bowel (large intestine and rectum)
Given their acidic nature, Short chain fatty acids decrease the pH of the colon reducing the risk of proliferation of dangerous bacteria. Arguably the most important SCFA for colon health is butyrate which is the preferred metabolic fuel for colonocytes.
Colonocyte- cell lining the large intestine/colon
In laboratory experiments butyrate has also shown to promote DNA repair in colonocytes, reducing the risk of colonic carcinogenesis, in other words cancer of the small intestine. 
Common prebiotic food examples
- Garlic-11% of its fiber content is prebiotic
- Barley-contains 3-8g of beta glucan per 100g serving
- Apples– 50% of their fibre contant comes from pectin
Just like with any food, prebiotics should be eaten in moderation. Introducing them to your diet may initially induce bloating and gas production as a result of carbon dioxide and hydrogen gases being produced from bacterial fermentation, but don’t let this put you off! It is only temporary and a sign your microbiome is flourishing but note that if you do feel unusually unwell consult your doctor as soon as possible.
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