The gut-liver axis describes the physiological links between the gut and the liver and how one affects the other. In this regard the health of the gut has a direct influence on the health of the liver and vice versa. Non-alcoholic fatty liver disease (NAFLD) is a condition that pathologically similar to alcoholic fatty liver disease, but while the former is caused by over consumption of fructose, the latter is caused by overconsumption of ethanol. The link between the gut and the liver as defined by the gut-liver axis suggest that the health of the gut has a direct influence on the development of non-alcoholic fatty liver disease. The gut contains a large number of microorganisms of a wide diversity and the health of the individual is now thought to be influenced by the ratio of beneficial Gram positive to detrimental Gram negative microorganisms that inhabit the gut. In this regard the colonies of bacteria in the gut may be able to influence the development of non-alcoholic fatty liver disease through the control of systemic inflammation.
In a recent editorial in the American Journal of Clinical nutrition the benefits of a healthy microorganisms balance in the gut on the health of the liver and the development of non-alcoholic fatty liver were highlighted1. It has been suggested that overgrowth of particular pathogenic strains of bacteria can increase the risk of liver disorders and increase the degree of inflammation of the liver for example by raising levels of circulating cytokines. Increased systemic cytokine production has been observed through an increase in the permeability of the tight junctions of the gut which can result from the effects of raised levels of Gram negative derived lipopolysaccharide (LPS) concentrations in the gut. These LPS fragments may affect the proteins that regulate tight junction formation such as occluding and zonulin, through activation of second messenger systems in the enterocytes. Influx of undigested food components and toxins from the gut to the circulation may then increase cytokine production.
Modulation of the types of bacteria within the gut through the use of pro- and prebiotics has been shown to be effective at combating the development of non-alcoholic fatty liver in humans and animals. While probiotics refers to the use of actual microorganisms in tablet or food form such as the beneficial bifidobacteria, Lactobacillus acidophilus and Lactobacillus casei, prebiotics refers to the use of energy sources that favour the growth of particular bacteria including fructo- and galactooligosaccharides. Administration of fructooligosaccharides for example can increase the growth of bifidobacteria and this may in turn be beneficial at improving the health of the liver. For example, it has been shown that increases in bifidobacteria from the consumption of fructooligosaccharides can improve liver histology and decrease aminotransferase values. This likely occurs through a reduction in the inflammatory burden experienced by the liver through a promotion of healthy anti-inflammatory Gram positive bacteria that modulate the gut-liver axis.