Get Psyched: The gut-brain axis

HOW DOES THE BAC­TE­RIA IN YOUR GUT ACT LIKE A ‘SEC­OND BRAIN’?

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[ SHARMISHTA SARKAR ]

WE ALL KNOW that our mood and be­hav­iour is con­trolled by the brain, but are we over­look­ing an­other im­por­tant as­pect of our neu­ro­bi­ol­ogy?

There is an in­creas­ing amount of ev­i­dence to sug­gest that the ac­tiv­ity of bac­te­ria in your gut can sig­nif­i­cantly af­fect your brain. This re­la­tion­ship is called the gut-brain axis, and while its ex­act mech­a­nisms and sig­nif­i­cance haven’t been fully fig­ured out, it is thought that the mi­crobes colonis­ing your di­ges­tive tract are re­spon­si­ble for com­plex in­ter­ac­tions be­tween your di­ges­tive sys­tem and the ner­vous, en­docrine and im­mune sys­tems.

Your in­testines are filled with bac­te­ria. When you think of bac­te­ria you prob­a­bly think of the germs that make you sick, but we ac­tu­ally have a lot to thank these tiny mi­croor­gan­isms for. We rely on ‘good’ bac­te­ria to help break down food, pro­duce vi­tal nu­tri­ents and de­fend us against harm­ful bac­te­ria. But this could just be the tip of the ice­berg. Sci­en­tists spec­u­late that gut mi­crobes can send sig­nals to the brain via three dif­fer­ent meth­ods.

The first in­volves bac­te­ria re­leas­ing neu­ro­trans­mit­ters (chem­i­cals that help to trans­mit nerve im­pulses) to trig­ger the neu­rons in your di­ges­tive tract, which in turn send sig­nals to your brain via the va­gus nerve. Some stud­ies have shown that cer­tain species of gut bac­te­ria can pro­duce sero­tonin, an im­por­tant neu­ro­trans­mit­ter that plays a role in reg­u­lat­ing your ap­petite and mood.

A sec­ond pro­posed method is that mi­crobes in the gut pro­duce mol­e­cules called me­tab­o­lites as by-prod­ucts when they break down our food. These me­tab­o­lites can stim­u­late an in­crease in the pro­duc­tion of neu­ro­trans­mit­ters by cells that line the gut (ep­ithe­lial cells), which ac­ti­vate the va­gus nerve. For ex­am­ple, a re­cent study found that some gut mi­crobes can pro­duce the fatty acids bu­tyrate and tyra­mine, which pro­mote the pro­duc­tion of sero­tonin by cer­tain cells.

The third hy­poth­e­sis is that gut bac­te­ria can in­flu­ence the brain in­di­rectly by trig­ger­ing the im­mune sys­tem. Gut bac­te­ria can stim­u­late im­mune cells to pro­duce small pro­teins called cy­tokines, which travel through the blood­stream to the brain. It is thought that these pro­teins can in­flu­ence the de­vel­op­ment and ac­tiv­ity of mi­croglia (the brain’s im­mune cells), which are re­spon­si­ble for re­mov­ing dam­aged cells at an in­jury site. Re­searchers be­lieve mi­croglia also play a role in the reg­u­la­tion of ap­petite and me­tab­o­lism.

Although there are few hu­man stud­ies at the mo­ment, an­i­mal stud­ies have linked the ac­tiv­ity of gut bac­te­ria to a va­ri­ety of con­di­tions, in­clud­ing Parkin­son’s dis­ease, obe­sity, de­pres­sion, anx­i­ety, schizophre­nia and car­dio­vas­cu­lar dis­ease, and they may also cause cer­tain types of strokes.

While much more re­search will be needed to fur­ther in­ves­ti­gate these ini­tial find­ings, if links are con­firmed it could rev­o­lu­tionise how we treat cer­tain neu­ro­log­i­cal dis­or­ders. Per­haps in the fu­ture doc­tors will be pre­scrib­ing pro­bi­otic di­ets to sup­ple­ment treat­ments.

Your in­testines con­tain a vast net­work of nerves (shown above, in yel­low), pro­vid­ing com­mu­ni­ca­tion links be­tween your brain and gut.

Sci­en­tists are only just be­gin­ning to un­der­stand the im­pact your mi­cro­biome can have on your brain.

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