Are your Firmicutes making you fat?

This blog is written by our clinicians and aims to keep patients informed with up to date information on medical conditions. The editor of the blog is Dr Cristina Romete.

How your gut microbiome may be affecting your weight.

The bacteria that live in our gut (more recently termed the “gut microbiome”) are becoming linked increasingly with a number of health conditions ranging from cardiovascular disease and autoimmune disorders to mental health conditions such as anxiety or depression.

Another condition emerging with possible links to our gut microbiome is that of obesity.

Studies as early as 2006 show that the types of bacteria living in our gut may have an effect on our weight. These studies showed that when gut bacteria were transplanted into mice living in germ-free conditions, the mice started gaining weight despite no changes being made to their diet – suggesting that the bacteria have a role to play in extraction of calories during the digestive process.

The majority of our gut bacteria can be divided into two separate communities – the Firmicutes and the Bacteroidetes – both considered beneficial types. However, researchers showed that harbouring a lower proportion of bacteroidetes and corresponding higher proportion of firmicutes results in mice that are fatter. They hypothesised that the firmicute population were more effective at releasing calories from the food, hence leading to fatter mice.

Similar studies have been undertaken in humans. One study used a number of twins to compare microbial diversity (the number of different species within the gut community) – with one of the twins being overweight. Interestingly, this showed a significant reduction in gut microbial diversity in the overweight twin compared with the leaner counterpart.. The study showed again that certain communities of bacteria tended to associate with the overweight individuals,
Such as Actinobacteria and Firmicutes, whereas Bacteroidetes were more metabolically active in lean individuals.

Researchers also showed that in the obese individuals, the bacteria seemed particularly active in their ability to process carbohydrates. This may explain why more calories are extracted from food in mice living in a normal environment compared with a germ-free environment.

The Gut-Brain Axis

Another mechanism by which our gut microbiome may affect our weight is by a communication system now called the Gut-Brain Axis (GBA). This system is comprised of the central nervous system (the brain and the spinal cord) and the gut’s own nervous system (the enteric system). This enteric system is able to function relatively independently from the brain.
It is now thought that substances produced by the microbes in our gut can act as signalling molecules to parts of the brain like the hypothalamus that is involved in energy balance.
Similarly, hormones produced naturally by our body can be affected by our gut microbiome. For example, leptin is a hormone that is produced by our fat cells that signals to the hypothalamus in the brain how much fat is stored. When leptin levels are high, this is meant to signal to our brain that we do not need to eat as much, and our appetite should reduce. Leptin resistance occurs when this fails to happen and it thought to be a major contributor to obesity. Studies are now suggesting that the gut microbiome may contribute to leptin resistance by as yet unknown mechanisms.

Studies such as these pose interesting an interesting question of whether modification of the gut microbiome can help in weight loss in obese individuals. Modification could occur by antibiotic regime, or perhaps even by faecal transplantation-whereby the inhabitants of a lean person’s gut microbiome are transplanted into that of an obese individual.

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