Dissertation: Digestion of lignin in the gut


Lignin, part of the cell wall in plants, is a fascinating research area, for which VTT is developing new practical applications. One interesting possibility would be to use lignin in food applications. Based on a doctoral dissertation by VTT's Research Scientist Piritta Niemi colon microbiota can partially metabolise lignin. Her research also suggests that even a high lignin content in dietary fibre does not prevent colon microbiota from playing its important role of converting carbohydrates into short chain fatty acids.

Very little research has been done on the importance of lignin in digestion. It has been generally assumed that lignin is not metabolised during digestion and that it has no significant benefits or disadvantages in this regard. However, there are also studies suggesting that colon microbiota can metabolise at least part of lignin into various kinds of metabolites, which may be bioactive. These include intestinal enterolignans, for example.

In Piritta Niemi's doctoral dissertation, lignin-rich fractions were enzymatically fractionated from BSG (Brewers' Spent Grain), and the fractions were used to investigate the interactions between lignin and colon microbiota in an in vitro metabolic colon model. The microbiota formed several phenolic metabolites from the lignin-rich fractions and these metabolites were most likely derived from lignin. According to these results colon microbiota can at least partially metabolise lignin.

The dissertation also examined whether high lignin content suppresses the microbial conversion of carbohydrates, or the growth of Lactobacillus and Bifidobacterium bacteria which are generally considered to be beneficial. Colon microbiota can metabolise carbohydrates in dietary fibre into sugars, which is used as an energy source by fermenting them to short chain fatty acids. Short chain fatty acids are beneficial to humans, which is why it is important that lignin does not suppress their formation. In the colon model study no signs of suppression of microbial carbohydrate conversion by a high lignin-content (20–40%) in fibre were detected. Neither did a lignin-rich growth substrate inhibit the growth of Lactobacillus and Bifidobacterium bacteria.

More than 30 million tons of BSG are produced in the world every year as a by-product of the brewery sector. BSG, which is food grade material, is mainly used as a feed additive for ruminants. In addition to dietary fibre, it contains around 20% protein, which means that it could be put to more valuable use than in feed – for example in food applications.

With respect to lignin, in the future it will also be important to study the bioactivities of the metabolites observed in Ms Niemi's dissertation and whether they have potentially health-promoting effects. It is likely that lignin has more importance as a constituent of dietary fibre than has so far been realised.

VTT's Research Scientist Piritta Niemi (MScTech) will defend her dissertation at 12 noon on 15 April 2016 at Aalto University (Kemistintie 1, entrance from the Biologinkuja side) in Espoo, Finland.

The dissertation is available online at https://aaltodoc.aalto.fi/handle/123456789/51

MEDIA MATERIAL: Piritta Niemi (photo: VTT)


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