OUR guts are a David Attenborough-esque wonderland for researchers who continue to discover the incredible influence of our microbiome on our mood, our weight, our brains and our health.
They know that what we put in our mouths changes our gut bacteria.
Now, a new study published in the journal PLOS Biology has found, for the first time, how our gut bacteria can communicate with our brains to change our food choices.
Specifically, they found that when lacking in certain nutrients, animals would later choose foods high in those nutrients. They also found that the right balance of bacteria in their guts could tide them over when they were deficient, suppressing those cravings.
The researchers from the Champalimaud Centre in Portugal and Australia's MonashUniversity knew that nutrients and our balance of gut bacteria (our microbiome) impact health. However they did not know what causes us to choose certain nutrients or whether manipulating diet and microbes could determine those choices.
To find out, they conducted a series of experiments with fruit flies looking at how they chose to eat dietary protein, an essential nutrient for muscle growth, reproduction and longevity.
While we need protein, we can't store it in the body, so too much of it is dangerous. This means that controlling that balance is important for animals and humans alike but it is unclear how we do this.
In one experiment, they deprived the flies of amino acids, in the form of protein. Later, when the flies were given a choice of a sugary solution or a protein-rich yeast solution, they went straight for the yeast.
"The question is how finescaled is this ability to detect and sense protein-deficiency?" said MonashUniversity's Matthew Piper. "At what level does this occur in the body?"
At a finely-scaled level, it turned out.
"Some amino acids are more effective than others in eliciting a protein appetite," Piper said. "It turns out that the essential amino acids – in that we have to get them from our diet because we can't make them ourselves – these are the potent ones for dictating future protein appetite.
"When we made one of the non-essentials an essential by knocking out the ability of the fly to make it themselves, it actually caused the appetite response of an essential amino acids. What that tells us is that there's not something unique about the essential amino acid in dictating the future appetite, it's just the requirement."
In a separate experiment, they explored the role of the microbiome in that process.
"One of the essential amino acids is called histidine, so when you take that out of the diet the flies become protein hungry," Piper said.
However when they introduced certain bacteria, giving the flies a "complete microbe complement" they stopped becoming protein-hungry, at least in the short term.
"It indicates that the microbes in the gut are somehow compensating for immediate nutritional variations – so it buffers again environmental nutritional variations and it affects decision-making around food intakes," Piper said.
The researchers hypothesised that the gut bacteria was temporarily producing the amino acids, but this was not the case. Rather, a metabolic process took place "tricking" the brain into thinking it was OK without those amino acids.
"Your gut is informing your brain somehow that they are or are not protein hungry in response to a physiological deficiency," Piper said.
This allowed the flies to keep reproducing even without the nutrient typically required to make that happen.
While extrapolating the findings to humans is not straightforward, there are "many, many similar processes between flies and humans," Piper said, and the research helps to establish certain biological processes.
"There are two things to take away in the human context. One is that the microbes in our gut may not just be responding to our diet but they may be partially fulfilling some of our dietary requirements by buffering against our inability to eat a perfectly matched diet to our needs," Piper said.
"They may provide some of our needs to complement what we eat to a small extent. They both provision us with nutrients and – this is the really curious thing for me – they somehow signal us to tell us that we should eat different food in the future so there's a contribution to our decision making process that goes on by the microbes in our guts."
Does it help to explain cravings in humans?
"I don't know," Piper admitted. "We know that food cravings exist in humans and there's no reason to think it doesn't happen.
"If you nutritionally imbalance a diet for a human without them knowing it and then give them free access to foods they do compensate in the future for those previous nutrient deficiencies. We know the same behavioural response exists, we don't know if the microbiota is involved in making those decisions and how."
Understanding how that mechanism works exactly is the next step.
"The interaction between the structure of our microbiota – so the number and types of microbes we have in our gut – and the diet we eat and our healthy state is incredibly complex but hugely interesting and fruitful area for research because you may have instances where individuals who are nutrient-deprived could benefit from a healthy microbiota and that could also affect future decision making about diet," he said.
In the meantime, Piper's advice for keeping our bellies and brains happy:
"Eat a balanced diet."
