Gut bacteria play an essential role in the digestive process of most carbohydrates since the human body only produces the needed enzymes to digest fats, proteins, starch, and a few simple sugars. The remaining food components are either passed through undigested or digested by microorganisms in the colon. Prebiotics are classified as food ingredients that stimulate the growth and/or activity of bacteria in the digestive system in ways claimed to be beneficial to health. In that regard, it could be argued that most of the fermentable substrates found in vegetables and fruits act as prebiotics since they seem to influence the composition of gut bacteria and health in a positive way.
Polysaccharides are complex carbohydrates making up the cell walls of plants. With the exception of starch, humans don’t produce the enzymes needed to digest polysaccharides found in plants, and we depend on the hundreds of species of microorganisms in the gut to provide the enzymes to break these complex carbohydrates into simple sugars.
Less complex carbohydrates such as lactose can also be digested by gut bacteria. Lactose intolerance occurs because the body produces insufficient levels of lactase, the enzyme needed to break down lactose into glucose and galactose.
However, gut flora can also provide the needed enzymes to break down milk sugars, and this is the reason why people with lactose intolerance tolerate lactose if they get lactose digesting bacteria into their colon.
Gut bacteria also play a role in protein and fat digestion, but it’s unclear if the food components involved in these processes have any significant prebiotic value.
A lot of fermentable substrates work as prebiotics
If the aggregate of all the micrpobes in the gut posess the necessary genes to break down the food we eat, fermentable carbohydrates found in plant matter seem to work as prebiotics since they stimulate the growth and/or activity of bacteria in the digestive system in ways claimed to be beneficial to health. The consumption of the right types of fermentable substrates isn’t only important because we want to feed the “good” bacteria in the gut, but it’s also vital because gut bacteria in return provide us with short-chain fatty acids (SCFA’s) that help maintain a healthy intestinal lining (no “leaky gut”).
SCFA’s produced in the colon are also especially important in regards to maintaining a healthy immune system and lowering chronic low-level inflammation in the body. Consumption of fermentable substrates from vegetables, fruits and other whole foods is associated with increased absorption of certain minerals, proper bowel pH, and intestinal regularity.
Although a lot of the fermentable substrates we get through diet seem to influence the growth and/or activity of gut bacteria in a positive way, only the most well-studied carbohydrates that stimulate the growth of beneficial bacteria like bifidobacteria are usually labeled as prebiotics. These prebiotics are again classified according to which bacteria they stimulate and where in the colon they are primarily used.
Although there is plenty of research to suggest that a lot of fermentable substrates work as prebiotics, most of the studies have investigated the prebiotic effect of inulin, oligofructose, and resistant starch.
- Inulin and various types of oligosaccharides
– Food sources include onions, leeks, jerusalem artichoke and chicory root.
– Studies show that inulin and oligofructose stimulate the growth of bifidobacteria in the colon (1,2,3).
- Resistant starch
– Food sources include green bananas, beans, and legumes.
– Studies show that resistant starch acts as a prebiotic and increase levels of butyrate (SCFA). Different forms of resistant starch have different effects on the community structure and population dynamics (1,2,3).