40 | New Scientist | 22 June 2019
We often say we make decisions on the basis
of gut feelings, and this may be truer than we
realise. Nausea, for instance, makes us judge
certain moral violations more harshly. This
is just one of many ways in which our gut
influences what goes on in our head.
It is easy to forget that the gut is a sense
organ, detecting incoming nutrients, toxins
and pathogens, and relaying that information
to our brains. It contains some 500 million
neurons that coordinate the process of digestion.
The gut is also home to about 2 kilograms
of bacteria: our microbiome, which influences
every organ in the body, including the brain.
A wealth of studies in mice show that changing
the bacteria in the gut can change behaviour –
in some cases, turning the animals into
antisocial loners.
The microbiome may be particularly
important in childhood, while the brain is
still developing. Mice that lack microbes called
Bifidobacteria in their gut during infancy seem
worse at learning new information, for example.
Evidence from humans is accumulating,
too. One imaging study found that consuming
a fermented milk drink containing various
strains of live bacteria had a profound effect
on people’s resting brain activity and their
responses to seeing faces showing emotion.
This year, a study of 1054 people in Belgium
found that certain types of gut bacteria are
less prevalent in people with depression.
There are also tantalising hints that certain
neurological conditions, such as autism, and
diseases, like Alzheimer’s, may originate
in the gut. In Parkinson’s, synuclein fibres,WHAT MAKES A BRAIN
a hallmark of the disease, seem to appear
first in the gut before spreading to the brain.
We don’t know what triggers it, but it could
be an unknown microbe or toxin. In epilepsy,
changes to the microbiome may explain
why the high-fat “keto” diet prevents seizures
in some people.
Research on the gut-brain connection
is in its infancy, but it has sparked the idea
of medicines that target the microbiome
to improve our mental health – dubbed
psychobiotics. John Cryan at University College
Cork, Ireland, believes it is an exciting prospect,
but a lot more work is needed to pinpoint
which bacteria are beneficial for which
conditions and how to deliver them to the gut.
How bacteria actually influence the brain
is also something of a mystery, but the picture
is becoming clearer. The tens of trillions of
bacteria in our gut are a hive of metabolic
activity, producing a wealth of chemicals
that we can absorb. Working out which ones
get to the brain and exert effects there is a
big focus of current research. Some bacteria
even feed on GABA, a brain chemical
implicated in depression.
It might seem odd that our brains are
influenced by what is in our guts, but it isn’t
so surprising when you consider that these
microbes have always been with us, says Cryan.
“I see them as friends with social benefits
because they really affect the social brain
in early life and development,” he says.
“This relationship is very important and
I think it’s been evolutionarily wired.”
Sam WongA few years ago, scientists took
human brain cells and injected
them into mice. A year later, the
cells had multiplied and the mice
had got smarter, learning more
effectively than mice with regular
brains. Perhaps that isn’t so
surprising – until you hear that
these brains cells weren’t neurons.
As building blocks of the brain go,
neurons hog the attention. There
are some 86 billion of these stringy
cells carrying electrical impulses
around the brain, helping us to
control our bodies and think
thoughts. But there are plenty
of cells in the brain that aren’telectrically active. These are
known as glial cells, and they are
at least as numerous as neurons.
A type of glia called an astrocyte
was injected into those mouse
brains, suggesting – not for the
first time – that they could be
important in learning.
Glia used to be considered mere
gap fillers. No longer. “There is
a lot of evidence to say they are
more than just glue,” says Anne
Cooke, chief executive of the British
Neuroscience Association. “They
are the unsung heroes of the brain.”
They come in different types. Small
ones called microglia, for instance,roam the brain gobbling up foreign
material to protect the neurons.
Astrocytes take care of the
neurons’ environment too,
controlling levels of chemical
messengers known as
neurotransmitters and helping
to repair damage. Evidence is
mounting that these cells also have
a role in the development of human
intelligence. We know that babies
start off with many connections
between their neurons and that
these are gradually pruned down to
create smaller numbers of stronger
signalling pathways. Astrocytes, it
seems, are involved in this pruning.“While neurons are still very
important, it seems that glial cells
are involved in setting the gain on
the system,” says Ed Lein at the Allen
Institute for Brain Science in Seattle.
It isn’t all about cells. Holes play
a part too. Deep inside your head
there are slim chambers called
ventricles that produce the fluid
that bathes the cells of your brain.
We make as much as 500 millilitres
of this cerebrospinal fluid daily,
which keeps everything in working
order by providing cushioning and
nutrients, and washing waste away.
There is no doubt plenty more
to be discovered. Just last year,Does the gut
influence the mind?“ The human
brain starts
to shrink
around the
age of 40”
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