Apr 29th 2017

Do you really choose what you eat – or do your gut microbes decide for you?

by Tim Spector

Professor of Genetic Epidemiology at King's College London.

Image 20170419 6395 1pj6awi Illustration by Gil Costa, with elements from Servier Medical Art

Most of us believe in free will, particularly when it comes to our eating habits. That’s why most people don’t regard obesity as a disease but rather a moral weakness or lack of willpower. But the free will argument has been taking a bit of a beating lately.

For example, we showed in studies using twins and others using families that the reason some people are overweight and others aren’t could partly be down to food preferences. Our food likes and dislikes are not just determined by the horrors of school food (beetroot for me) or family meals. Whether we prefer salads to fries or enjoy garlic or chillies, surprisingly, is more down to our genes than our upbringing. This makes the concept of pure free will, when it comes to eating healthily, increasingly hard to accept.

While our own genes play a role in picking what foods to eat and then metabolising them in a unique way, we are now discovering that other processes or microbes could also be involved.

Bacteria-controlled fruit flies

A study from Lisbon and Monash, published in PLOS Biology, further expanded our insight into nutritional choice and free will by manipulating the microbes inside fruit flies to see how it affected their eating habits. The experiment involved studying the trillions of gut microbes that all animals contain (the “gut microbiome”).

We recently realised that these microbes are crucial for our digestion of foods, such as complex carbohydrates, and are key to regulating a normal immune system and making many essential hormones and vitamins that the body cannot produce.

Microbes also produce brain chemicals, such as serotonin, and there are an increasing range of studies in humans showing associations between a dysfunction of gut microbes and brain and mood-related disorders like depression, anxiety and autism. Some animal studies have shown these traits can be “transmitted” to sterile animals via microbial transplants, suggesting the microbes themselves produce chemicals that may be causal.

What also has been suspected is that individual microbes could influence the behaviour of their host to improve their own chances of evolutionary survival. There are several examples in nature of this, including the many species of fungus that can infect the brains of ants. These fungi make the ants climb certain trees helping the microbe to survive at the cost of the poor zombie ants whose heads explode, spreading the fungal spores in prime leafy locations.

As you can guess, it is very hard to test the “selfish microbe” theory in humans, so the Portuguese researchers used fruit flies – a much simpler animal which is used to establish the rules of nature, especially for many genetic studies. As with all animals, fruit flies contain microbes in their primitive intestines which coexist and help them digest food. During stressful periods, and mating (which may be stressful or fun, I guess) fruit flies vary in whether they prefer protein or carbohydrates.

By manipulating the microbes inside the fruit flies, using special flies brought up in germ-free conditions, the researchers found they could alter the flies’ choices of food, especially for protein intake. This directly involved two microbes (in this case, acetobacter and the yogurt bacteria lactobacillus) acting together.

When one type of essential amino acid protein was depleted in the flies’ diet, these microbes sent signals to the fly to eat more yeast (the main source of protein) and at the same time signals to stop them reproducing for a while. This means the two microbes, which benefit from eating some of the amino acids from yeast protein, can proliferate at the expense of other microbes and win their evolutionary arms race.

How this translates into humans is still speculative, but we all have thousands of microbial species and sub-strains all highly specialised and all competing for food and their byproducts inside us. Like us, they are driven to want to pass on their genes to their descendants.

We know that restricted diets can dramatically alter the balance of our microbes. For example, ten days of only eating high fat and sugary junk food severely reduced the number of species surviving in my son after his McDonald’s ten-day eating experiment (and he still hasn’t fully recovered).

Food zombies

If one species of gut microbe only reproduces well when it has access to a particular type of fat and otherwise would die out, for example, it could mutate one of its genes to produce a chemical to make its host eat more of that fat. And as some microbes reproduce every 30 minutes, the required mutation could happen quickly.

Indeed, many of us have experienced changes to our taste and appetite when we take antibiotics. These could be due to changes in our microbes rather than the direct effect of the drug.

Although we have no direct evidence for this microbial signalling in humans, and we don’t yet know the chemicals involved, it could be a factor in explaining why habits are so hard to break. For example, why it’s so difficult for hardened meat eaters to become vegetarians. Perhaps it’s because their microbes won’t allow it.

The good news is that, unlike our genes, we can modify our gut microbes. By having a varied high fibre and high polyphenol diet, we can maintain a diverse and healthy gut microbe community and prevent one group taking over the community and running it like a dictatorship.

And as we learn more about ourselves, we also have yet another excuse for eating that extra piece of cake: “It’s not just my genes, my upbringing or slick marketing – my microbes made me do it.”

This article was originally published on The Conversation. Read the original article.

Browse articles by author

More Essays

Jul 10th 2019


The eight-mile ‘river of flowers’ that grows alongside a motorway nea
Jul 5th 2019
"........since World War II, 97% of unimproved grassland habitats have vanished from the UK. This has contributed to the loss of pollinating insects – and the distribution of one third of species has shrunk since 1980."
Jun 25th 2019
"For many of us, eating a meal containing meat is a normal part of daily life. But if we dig deeper, some sobering issues emerge. Every year, 66 billion terrestrial animals are slaughtered for food. Predictions are that meat consumption will rise, with increasing demand for meat from China and other Asian countries as their standards of living increase. The impact of grazing animals on the environment is devastating. They produce 18% of the world’s greenhouse gases, and livestock farming is a major contributor to species extinctions."
Jun 22nd 2019
"Throughout history, people who have gained positions of power tend to be precisely the kind of people who should not be entrusted with it. A desire for power often correlates with negative personality traits: selfishness, greed and a lack of empathy. And the people who have the strongest desire for power tend to be the most ruthless and lacking in compassion."
Jun 21st 2019
"In this era of Trump, it should perhaps come as no surprise to find supposed experts lacking in historical perspective. Yet it is still disappointing to find this deficit in the New York Times, which prides itself on clinging to a pursuit of the truth. So it is a bit sad to read the plaintive cry of Allison Schrager’s op-ed of May 17, lamenting that the domination of art markets by the super-rich will somehow force smaller galleries to go out of business, and imperil the careers of young artists."
Jun 17th 2019
Extract: "ust as an earlier generation resisted the limiting post-War era "white middle class" definition of being American by giving birth to an awakening of cultural pluralism and ethnic pride, it falls to our generation to fight for an expanded view of the idea of being American that rejects the narrow view projected by Trump and white nationalists. The idea of America isn't theirs. It's bigger than they are and unless our national cohesion is to unravel, this challenge must be met by projecting an inclusive vision of America that celebrates our inclusive national identity in an increasingly globalized world."
May 28th 2019
Whatever other attributes Homo sapiens may have – and much is made of our opposable thumbs, upright walking and big brains – our capacity to impact the environment far and wide is perhaps unprecedented in all of life’s history. If nothing else, we humans can make an almighty mess.
Apr 29th 2019
A century ago, unspeakable horrors took place on every continent that were known only to the victims and the perpetrators. Not so today. As a result of advances in communications – from the telegraph and radio to satellite television and the internet – the pain and loss of global tragedies are brought home to us in real time.   Because of this expanding consciousness, the post-World War II era has witnessed the rise of visionary leaders and the birth of countless organizations dedicated to alleviating suffering and elevating the causes of peace, human rights, and tolerance among peoples. Individually and collectively, they have championed the rights of peoples in far-flung corners of the world, some of which had been previously unknown to those who became their advocates. These same leaders and groups have also fought for civil rights and for economic, social, political, and environmental justice in their own countries. 
Apr 23rd 2019


“Cursed be that mortal inter-indebtedness which will not do away with ledgers. I would be free as air; and I’m down in the whole world’s books. I am so rich… and yet I owe for the flesh in the tongue I brag with” (Moby Dick, chapter cviii). 

Apr 20th 2019
Economists speak in numbers only, clinging to statistical data and quantitative models. We do so in the hope of looking objective. But this is counter-productive – “data” cannot tell us everything. Other social sciences such as sociology and anthropology use a broader range of methods, and consequently have a broader perspective on society. If we take our societal role of adviser on economic matters seriously, we will need to open up and adopt the insights that these other disciplines bring us about how the economy works.Politics and economics are inextricably intertwined, as Adam Smith, David Ricardo, Karl Marx knew all too well. Somehow this has been forgotten. This does not mean economists need to get political or choose sides. But it does mean that we ignore politics at our own peril – by blindsiding ourselves or dismissing it as “external stuff”, we hamper our understanding of the very system we study.
Apr 16th 2019
Although it is not likely that many visitors who pass by the Giacometti sculptures on their way to Las Meninas will ponder it, the contrast between these works underscores the single greatest transformation in the history of western art, from a regime in which artists tailored their works to the aims of individual patrons, to one in which artists choose their techniques and motifs according to their own concerns, and only then present the products to an anonymous competitive market
Apr 4th 2019
On March eleventh, the world lost someone who was very special, who made a mark and touched people with his voice, as a singer, a humorist and writer..........I had the great good fortune to know him and spend time with him, playing music, talking with him – he was a man of immense culture, fluent in Hebrew, German, English, and Romanian. He loved New York City and Vienna and we would often swap apartments so that he could stay in New York while I lived at his place in Vienna.
Apr 1st 2019
The ongoing controversy over admissions to American universities has overlooked the one of the most telling aspects of the scandal—that it took place with the connivance and active participation of administrative bureaucracies able to act with impunity in the pursuit of their interests. Neither the professoriate, often the target of opprobrium from the left and the right, nor the student body, also the target of criticism from both sides of the political spectrum, bore any of the responsibility.  Current debates over “what ails” U.S. colleges and universities consistently ignore the single most important dynamic of all institutions—their structure of power. I suggest that the way in which power is allocated within American universities is strikingly similar to that of Soviet-type regimes. Presidents, chancellors, provosts, deans, and their bureaucratic apparatuses preside over vast real-estate and financial holdings, engage in the economic equivalent of central planning, have inordinate influence over personnel, and are structured hierarchically, thereby forming an enormously powerful “new class” like that described by the renowned Yugoslav dissident, Milovan Djilas, in the mid-1950s. 
Mar 22nd 2019
When you think of religion, you probably think of a god who rewards the good and punishes the wicked. But the idea of morally concerned gods is by no means universal. Social scientists have long known that small-scale traditional societies – the kind missionaries used to dismiss as “pagan” – envisaged a spirit world that cared little about the morality of human behaviour. Their concern was less about whether humans behaved nicely towards one another and more about whether they carried out their obligations to the spirits and displayed suitable deference to them. Nevertheless, the world religions we know today, and their myriad variants, either demand belief in all-seeing punitive deities or at least postulate some kind of broader mechanism – such as karma – for rewarding the virtuous and punishing the wicked. In recent years, researchers have debated how and why these moralising religions came into being.
Mar 19th 2019
European food and ingredients have become staple food choices for the British. The use of ingredients such as garlic, peppers, avocados, Parmesan cheese and all those other European ingredients that are now taken for granted are relatively new and were still rare in the 1990s. When I was growing up in rural Devon in the 1970s, olive oil was only really readily available in chemists as a cure for earache – now it is found in most food cupboards. And wine drinking has permeated through all social classes.
Mar 12th 2019
The Guggenheim’s strange and wonderful exhibition of Hilma af Klint’s groundbreaking, yet largely unknown body of abstract art is an important event – one that challenges us to not only rethink the early history of twentieth century abstract art, but to recognize her vision of art and reality as unique, authentic, and deliciously puzzling. 
Feb 25th 2019
Looking at the world today, it's clear that the consequences of this imperial legacy are still with us. If anything has changed it is that we are now beyond just viewing the former "natives" as far-away oddities. They are now living within our borders, having come to find the opportunities they were denied at home. So when I hear the reactions in the West to the influx of South Asians going to the UK, or North Africans going to France, or Central Americans migrating to the US, I can only say "Guys, these are the fruits of your conquest – your chickens coming home to roost."
Feb 25th 2019
Extracts: "The new novel Sérotonine by Michel Houellebecq, the bad boy of French literature, is a saga of depression and death told with such irony and wit that readers seem to love it despite the unsettling themes. Maybe it’s just me but I found myself laughing out loud.......True to form, the French don’t agree on Houellebecq – or anything else, for that matter. The impact of his new novel has divided the readers into opposite love-hate camps with hardly any middle ground. Houellebecq cannot leave you indifferent, notes a literary friend of mine"........Picture: Michel Houellebecq, by the reviewer Michael Johnson. 
Feb 19th 2019
The term “smiling depression” – appearing happy to others while internally suffering depressive symptoms – has become increasingly popular. Articles on the topic have crept up in the popular literature, and the number of Google searches for the condition has increased dramatically this year. Some may question, however, whether this is actually a real, pathological condition. While smiling depression is not a technical term that psychologists use, it is certainly possible to be depressed and manage to successfully mask the symptoms. The closest technical term for this condition is “atypical depression”. In fact, a significant proportion of people who experience a low mood and a loss of pleasure in activities manage to hide their condition in this way. And these people might be particularly vulnerable to suicide.
Feb 19th 2019
Outstanding, experienced journalist Michael Johnson, whose articles, often accompanied by his striking portraits, has now brought his love of music and of pen, ink, gouache and watercolor to create a study of remarkable insight, strong opinions and beauty in this gorgeous book. Written in both French and English the brief descriptions of musicians he has met, studied, interviewed are accompanied by distinctive portraits that, as his title suggests, some may be caricatures. I argue that the author/artist has created insightful studies of the human face engaged in the pursuit of music. The only caricature is his own self-deprecating, slyly wry self-portrait that opens the book—and it is worth the book’s purchase on its own.