54 | New Scientist | 14 September 2019
Food fatigue
I often feel tired after a large
meal and I am told it is because
blood “rushes to the stomach to
help with digestion”. Is this actually
what happens? How can the body
regulate that?Joy Cummings
Leeds, West Yorkshire, UK
When food enters the digestive
system, enterogastrone hormones
are released. These increase blood
flow through the dilation of
vessels supplying the digestive
system. They also reduce blood
flow in other parts of the body,
leading to tiredness.
Tiredness is particularly
noticeable after large meals or
ones rich in carbohydrates, due
to their high glycaemic index,
meaning that glucose is released
quickly into the bloodstream.
This increases insulin production,
which helps tryptophan cross the
blood-brain barrier. Tryptophan
can cause drowsiness and also
helps the body produce serotonin,
which plays a role in sleep
regulation.Sam Buckton
Chipperfield, Hertfordshire, UK
Tiredness following a meal is often
called a “food coma”. Its causes are
complex and still debated, as is its
evolutionary relevance.
Blood does indeed rush to the
stomach and intestines following
a meal, providing relevant systems
with oxygen and transporting
the products of digestion. To
maintain overall blood pressure,
our heart rate increases and
blood vessels constrict.
However, up to a third of older
people experience postprandial
hypotension, where this process
doesn’t work properly, leading
to dizziness and drowsiness.
You might also feel unusually
tired if you have an allergic
reaction to food, as your body
uses energy to combat the
resulting inflammation.
In most people, the rush of
blood to the digestive system
probably isn’t the major causeof food coma. Eating releases
enterogastrone hormones, which
can increase levels of serotonin
and melatonin, both of which
promote sleepiness. And an
excess of glucose inhibits a
hormone called orexin, which
promotes wakefulness.
Food coma could be adaptive
in evolutionary terms. It has been
suggested that we evolved to feel
alert when hungry so we could
locate food, then following a
meal, we could afford to rest.Human traces
In the near future, a mystery
virus wipes out the entire human
race almost overnight. Later on,
a group of extraterrestrials lands
on Earth. There are now no visible
signs that human civilisation
ever existed. How many years
in the future would this be?Graham Perkins
Milton Keynes, UK
This exact question is addressed
by Alan Weisman in The World
Without Us. Megastructures
like the Panama Canal would
disappear quickly because theyare dependent on continuous
maintenance. But the answer
depends on how closely the
aliens look. After 500 years,
the only things visible from
space would be plants, but surface
exploration would uncover signs
of us for another few thousand
years, just as we find ancient
civilisations today.
Unsurprisingly, plastics and
nuclear waste will be around the
longest. Perhaps the ocean floor
will contain a layer of polymer
sediment to give us away, much
like the 65-million-year-old
iridium layer that reveals the
dinosaur-killing asteroid impact.
Radiation hotspots could
be detected even later than this,
but might be considered natural
unless some of our technology
was fossilised. Ultimately, tectonic
plate subduction will melt all our
remains into the mantle, leaving
aliens to make up stories about
what might have been.Tony Holkham
Boncath, Pembrokeshire, UK
Humans have made an almost
indelible impression on this
planet, so we will be detectable
for many millions of years. There
will be fossils of us and of our
domesticated animals, just as
we find fossils of dinosaurs and
their distant ancestors. There will
be clues in the chemistry of Earth
that are indicative of a civilisation
that has changed the climate,
extracted raw materials and
manufactured things.
There will also be signs of
our visits to other bodies in
the solar system, particularly the
moon and Mars. But perhaps the
most telling evidence will be our
electromagnetic communications
signature, which is spreading out
into space at the speed of light in
an ever-expanding envelope. It
may even include the details of
our demise, if we have time to
broadcast these before perishing.Hillary Shaw
Newport, Shropshire, UK
Traces of human activity
could linger on to infinity.
Vegetation, storms, fires, frost,
rust, earthquakes and burrowing
animal activity would erase most
of our visible traces within a
thousand years, but the ruins of
some massive concrete structures
might remain for millennia. Our
pollution record might be found
after millions of years, if aliens
knew where to look and could
interpret the geochemical results.
But our fossil tunnels may
endure for much longer still.
Today, in Australia, there is crust
surviving from 4.4 billion years
ago. Any tunnels built in crust
that isn’t subducted could
endure for at least as long. ❚This week’s new questions
Different minds Why do some people become interested in
science and some don’t? Students of year 7A, Mirboo North
Secondary College, Victoria, AustraliaGurgling guts What causes the sound in a “rumbling”
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