The Gut-Brain-Axis: Could probiotics help with mood and weight control?

Fiona Clark

Could eating probiotics help your mood or control your weight? Dietitian Sophie Medlin explains what the research shows now.

It’s an emerging area of research but the links between the gut and its microbiome and our brain is starting to be established. It’s known as the gut-brain-axis and it involves the role probiotics (the good bacteria in our gut) could play in helping with mood and weight control. And there may also be benefits for immunity and perhaps even longevity.

But first, what are Probiotics?

According to London-based dietitian, Sophie Medlin, probiotics are the live bacteria and by law must contain bacteria that have a discernible benefit on human health and contain then in sufficient numbers. Kefir, sauerkraut and yoghurt are common sources.

To help them grow in the right number we need a health environment for them to live in and that involves giving them the right foods to eat – the prebiotics.

Prebiotics come from plant-based fibre in our diet or from a fibre supplement. Oats, pulses and green vegetable are good examples.

Going with your gut – a Q & A

Q. In the past we thought that mood impacted on the gut – for example – depression or anxiety could cause you to have a problem with your gut flora, but are we now starting to think it’s the other way around – that a poor gut biome could be a contributory factor in causing depression and anxiety?


A. “Yes, it’s a sort of bi-direction relationship between you gut and mental health. When we’re really stressed we produce cortisol and that’s a fight or flight reaction and your body thinks you’re going to have to run away from something or fight something. This affects your digestion – it can speed up or slow down depending on your genetics and there can be an increase in acid production or a change in gut pH. So it can change the acid balance, the enzymes you secrete and this can have an effect on the gut bacteria profile. What we also know is that those changes in gut bacteria are also seen in people who have long term or chronic anxiety or depression. We also see that there are differences in people who have good mental health and those who suffer with their mental health.

“We think that some of these digestive changes that come as a result of long term stress can have an impact on the bacteria that live in our colon and our gut. And we also know that the bacteria that do well in those conditions release less of the neurotransmitters that are associated with better brain health.

“So interestingly when we have a good gut bacterial profile they’re releasing the neurotransmitters that promote happy hormones like dopamine and serotonin. So it’s a bi-directional relationship – and when you gut health is affected you have a negative impact.”


Sauerkraut – a source of probiotics

Q. It’s early days in terms of research but could probiotics be an adjunct therapy to helping people who have mental health issues?

A. “Absolutely. In Canada here are 2-3 strains of probiotics being used in conjunction in a treatment program. It’s not here yet. And there is an amazing book called ‘The Psychobiotic Revolution’  which is co-written by an academic colleague of mine and it’s a great read and also super enlightening on how our gut and brain interact.”

“The gut produces 90% of the body’s serotonin but what we don’t know is how much of that gut serotonin interacts with our brain – how much of it is affecting our mental serotonin. There’s not necessarily a direct relationship between the amount of serotonin in your gut and in your brain, but certainly it is serotonin and there are receptors in your gut for serotonin and in your brain and we’re on the precipice of discovering how it all interacts.”


Q. Could probiotics help mood swings in menopause?

“Menopause is sadly so under researched. But what I would say from the work I do with women of all ages is that improving your gut health can improve your mood and it’s never time wasted. Anything you can do to improve your gut health – eating well to support your gut biome – will also have an impact.”


Q. Are there any particular strains people should look for?

A. “Using them in combination is important. Taking a lot of the same type all the time won’t as much good as getting a wide variety. Brands like Simprove or Alforex have been shown to have a good effect. They’ve been shown to have the right types of probiotics in the right combination and the numbers.

“Look for ones with the research behind them to support the claims that they’re making. There’s an issue that when they’re put in the capsules they may contain the numbers they say that do, but if they’ve been sitting on a shelf for ages then there will be some loss, lots will die off. So it’s about using brands we know are used in research, that have the data behind them.”


Q. Can we get enough in our diet?

A. “The key thing is to get the best amount of prebiotics to support the probiotics. There’s no point in chucking loads and loads of probiotics in the top end if you’re not going to give them any food to eat or ferment in when they get to your colon. So ultimately, if you don’t make any dietary changes then you’re not going to reap much benefit at all. All of these things live in a symbiotic relationship in our colon and we need to make sure we’re giving them the food they need.

“We all have a microbiome in our colon which contains all of the different species of bacteria. There may be some in lower numbers than others, but if you feed them well you can start to increase the numbers of the good ones while gently starving off the bad ones. They all exist in a sort of competitive environment, so if we increase the good ones they can start to get rid of the less positive ones just because they competing for space. And the positive bacteria release different chemicals in their fermentation process that help get rid of the less positive ones as well – they kill off the ones that you would have heard of that cause food poisoning and the like.

“So the key thing to do is to get more plant food in your diet, which will feed the good bacteria and help get rid of the less positive bacteria.”


Q. Can you cut out foods that feed the less positive bacteria?

A. “We don’t know for sure, but we don’t know that people who eat more processed foods and lots of sugars have a less healthy gut biome but we don’t know if that’s because those foods are displacing the plant foods they need or if they play a role in disrupting the balance. What we do know is that the more plant food we have, the better.”


Q. Digestive problems like bloating and flatulence – will plant foods increase these?

A. “Introduce fibre slowly and try to keep it constant. We all have some good eating days and some bad ones and naturally on the bad days the gut biome is going to go to town on it. So, introduce the fibre gradually, drink plenty of water, chew your food well, and the impact should be less. That will improve things over time rather than having a see-saw approach which will see some people say ‘I can’t do this’ and they’ll go back to their normal diet.”


Q. Cognition – can probiotics help memory and brain function?

A. “Unfortunately we don’t know at the moment. There is some tentative research showing the neurotransmitters we want to released may improve cognition and cognitive function but its early days yet. And again all of the foods that we’d want to you eat are probably more likely to have an impact than the lower down the chain things like gut bacteria.”


Q. Is one of  the benefits of probiotics their ability to reduce inflammation?

A. “We do definitely know that any thing that we do in terms of diet and gut biome is important in terms of reducing inflammation and your risk of say developing dementia, but there are so many things that impact on our inflammatory load. There is no cure all. for example going out in the sun, or breathing polluted air are going to increase your inflammatory load, all these things create inflammation. But I think there is this fixation on eliminating inflammation and that doesn’t necessarily lead to all of the beneifts we’d like because if you fixate on one thing you’ll loose out somewhere else. Fixating on improving one thing may create some form of stress and that could in turn increase inflammation as we know stress is a major cause. So we just need to take gentle, nudging steps in the right direction rather than being ultra-fixated on one thing.”


Q. Can probiotics help you keep your weight under control?

A. “So, what we think is that some of the less beneficial bacteria could help you harvest maybe an extra 50 calories a day from your food. So it’s a really low level, but we know that the good bacteria can help you reduce and regulate your appetite. So when we have a disbiosis – a gut biome profile that is pro-inflammatory, it doesn’t release the neurotransmitters that have a positive effect on our brain, then we are more likely to overeat and eat the wrong things.

“We also know, from twin mice studies, where one that is fat and the other that is thin – when we transplant the bacteria from the fat one into the thin one and then feed them the same food, the thin one puts on weight. so we do know from that work that the gut biome does play a role in the weight gain but it’s only part of a much bigger picture- there a drop in the ocean in comparison to how much you eat and how much you move.



Q. Could probiotics play a role helping the gut tell the brain that you’re full?

A. “Potentially but not definitely. They key is to make sure you’re getting the good prebiotics to feed the probiotics.”


Q. Immunity – can probiotics help there?

A. “We have microbiomes all over our body – in our throats, gut, colon, our skin – and we know that people who have a good, healthy biome have a better immune system than those that don’t. It acts as one of our lines of defence – it doesn’t mean that you wont get covid or other infections but it means that your immune system is well armed. It’s one part of your immune system.

“Your immune system is already there and you don’t want it boosted. Your nutritional status can impact on immune system, things like getting enough vitamin C are important, if you don’t have enough it could be compromised. So we want to make sure you get all the things you need to make sure you support your immune system. Support is the important word there. Immune boosting suggests your immune system is on overdrive which can lead to autoimmune or inflammatory diseases. It’s best to avoid anything that says ‘immune boosting’ and just eat a well balanced diet, and try not to get too stressed.”


Q. Longevity and probiotics – can they help?

A. “We know that people who live in the ‘blue zones’ – the regions where people tend to live the longest – have a diverse microbiome but what we don’t know is if that’s the reason they live longer or if it’s one of the other factors. We really can’t single out one factor. Diet, exercise, a sense of purpose in the community – these are all factors.

“I’ve seen it put down to herbs they have on their food, but it’s a combination of all the things that go into their lifestyle that help them live longer.”

“What we know  though is that when we swapped a group of Africans with a group of Americans was that their bacterial diversity changed. The Americans who ate the African diet started to develop a more diverse gut biome but the Africans who started eating a western diet started to show less diversity in their gut biome. And we know that in the west we are fighting a battle with diabetes and heart disease, so giving our body a diverse variety of plant based fibre is important if we want a diverse gut biome. Eating the same foods repeatedly, like chicken, sweet potatoes and broccoli every night – even if it’s supposedly ‘clean eating’ or whatever the latest hashtag is, isn’t going to give you the diversity you need to support your gut health.”


Kefir may be a better source of probiotics than plant-based options

“Stir fry vegetables, a ratatouille with a variety of veggies are easy ways to increase your veggie intake.”

“There are dozens of probiotics on the market. Kefir, kombucha, sough dough. But most of the bacteria in these will be killed off in the stomach which is just like battery acid, so very little will make it through. There is some evidence that some of the probiotics in kefir may make it through as it seems they are wrapped in a protein from the milk that protects it, but the plant based ones aren’t likely to get through.”

“There are very few people who benefit from reducing dairy from their diet. It’s worth trying if you have a skin condition but for most people it will make no difference. Fermented products like yoghurt or kefir are low in lactose so may be ok for people who are lactose intolerance.”


Sophie Medlin is a London-based dietitian and you can find her here:


See the video of our chat here:

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