Obesity makes sufferers anxious or depressed by upsetting gut bacteria

Obesity fuels anxiety and depression by disrupting gut bacteria that affect the brain, study finds

  • Obesity affects more than one-third of Americans and depression is the leading cause of disability in the US 
  • Links between mental health and obesity are well-established, but scientists are still learning the underlying biology 
  • Harvard University researchers found that high fat diets disrupt gut bacteria
  • But changing the gut microbiome treated the symptoms, even without diet changes
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Obesity makes people more anxious and depressed by changing the bacteria in the gut – our ‘second brain’ – new research suggests.   

Scientists found that a high fat diet alters the type of bacteria that live in the gut and these play a role in these mental disorders.

Obese people and those with type 2 diabetes suffer more negative feelings that others, but scientists have not previously been able to work out why this happens.

Researchers at Harvard Medical School  discovered that mice fed a high-fat diet showed significantly anxious, depressive and obsessive behaviors than those on regular diets – but treating their gut bacteria with antibiotics improved their moods.

Obesity may raise anxiety levels by changing gut bacteria, a new Harvard study suggests 

Even without altering their diets, once the mice were given antibiotics to alter the gut bacteria the symptoms completely ceased or were reduced.

This study is just the latest to show that mental health and obesity affect one another.

These discoveries underscore the ‘gut-brain’ connection and the concept that the gut itself acts like a second brain. 

Obesity, which now affects more than a third of American adults, comes with its own long-term health risks, including type 2 diabetes, heart disease and a greater risk of an early death. 

Mental health, meanwhile, is the leading cause of disability in the US. 

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Recently, scientists have established strong links between measures of obesity – such as waist size – and levels of anxiety and depression, but the mechanisms at play are still being investigated.  

The Harvard researchers’ work, published in Molecular Psychiatry, suggests that the psychological changes associated with obesity begin with disruptions to the second brain. 

An estimated 100 trillion bacteria live harmlessly in the human body. But they exist in a delicate balance and, if that balance is upset, it can set off a chain of reactions.  

Lead study author Dr Ronald Kahn explained: ‘As endocrinologists, we often hear people say that they feel differently when they’ve eaten different foods.

‘What this study says is that many things in your diet might affect the way your brain functions, but one of those things is the way diet changes the gut bacteria or microbes.

‘Your diet isn’t always necessarily just making your blood sugar higher or lower, it’s also changing a lot of signals coming from gut microbes and these signals make it all the way to the brain.’

To further cement the findings, the gut bacteria was extracted from the mice on a high-fat diet and transferred to germ-free rodents.

The mice which received the bacteria began to suffer from the same symptoms even though they were on a standard diet.

However, those who received microbes from mice on a high-fat diet plus antibiotics did not, even though they did not receive the antibiotics themselves.


Dr Rob Knight, co-founder of the American Gut Project, said his team is working on many ‘science fiction’ ideas to revolutionize how we understand our personal gut health. 

He tabled three ideas which he is exploring to work towards:

‘This proves that these behaviors are driven to some significant extent by the gut microbiome,’ said Dr Kahn.

Dr Kahn and his team found the areas of the brain that control metabolism, mood and behavior were becoming insulin-resistance because of the high fat diet.

He said: ‘We demonstrated that, just like other tissues of the body, these areas of the brain become insulin resistant in mice on high-fat diets.

‘And this response to the high fat is partly, and in some cases almost completely, reversed by putting the animals by antibiotics.

‘Again, the response is transferable when you transfer the gut microbiome from mice on a high-fat diet to germ-free mice.

‘So, the insulin resistance in the brain is mediated at least in part by factors coming from the microbiome.’ 

Dr Kahn added: ‘All of these behaviors are reversed or improved when antibiotics that will change the gut microbiome were given with the high fat diet.’

He and his colleagues are now working to identify specific groups of bacteria that contribute to insulin resistance in the hope of achieving healthier brain function.

He said: ‘Antibiotics are blunt tools that change many bacteria in very dramatic ways.

‘Going forward, we want to get a more sophisticated understanding about which bacteria contribute to insulin resistance in the brain and in other tissues.

‘If we could modify those bacteria, either by putting in more beneficial bacteria or reducing the number of harmful bacteria, that might be a way to see improved behavior.’ 

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