The gut is actually an external surface of the body, and food within is still considered outside the body. Foods that do not cross the gut lining and enter circulation have therefore never entered the body. Many people assume that for foods to be beneficial they must be absorbed and circulate in the blood. However this is not true, as research has identified a number of foods that show beneficial effect within the lumen and structures of the gut itself.
Fibre is classified as the indigestible parts of the plant. Cellulose and lignins are common dietary fibres, and humans do not possess the enzymes necessary to digest them. However, bacteria do possess such enzymes and as such fibre is a source of food for colonic bacteria. These bacteria ferment fibre to short chain fatty acids. These fatty acids then lower the pH of the gut which inhibits the growth of pathogenic strains of Gram negative bacteria.
Most polyphenols found in plants are great antioxidants. However, most never make it into the body. This is because they are rapidly metabolised by enterocytes during absorption to phase II conjugates. However, evidence suggests that polyphenols in tea (catechins) and red wine (resveratrol) may not need absorbing to have benefits. In fact, they may inhibit oxidation of lipids in the gut thus reducing the absorption of toxic lipid peroxides.
Processed meat is associated with gut cancer. This is likely because processed meat contains nitrates and nitrites. Both nitrites and nitrates can be converted to nitrosamine in the gut in the acidic environment of the stomach. However, vitamin C can inhibit this process because it is able to react chemically with the precursors of nitrosamine formation at a rate faster than allows the formation of nitrosamines. Vitamin C is therefore protective of gut cancer.
Live yoghurt contains Gram positive bacteria such as Lactobacillus acidophilus and Bifidobacterium. When consumed in yoghurt, these organisms can survive the journey through the stomach to the intestines. Here they grow and colonise the colon, where they outcompete Gram negative bacteria for food. Gram negative bacteria are damaging to to the health because they contain lipopolysaccharides on their outer coats which can be absorbed and act as toxins.
Glutamine is an amino acid that has some interesting gut properties. Glutamine is very poorly absorbed to the circulation because most of it is utilised as an energy supply by the gut cells during absorption. This is beneficial because the glutamine increases the energy available to the cells and increase growth rates. This means that glutamine can be used to increase the healing speed of the gut, and is used medicinally for this purpose.