How gut bacteria control your weight

The gut microbiota is the hottest of hot research fields today. The microbes that inhabit our intestine are more than passive freeloaders living off the food we eat. With a thriving mix of beneficial bacteria, our gut microbiome can keep us healthy.

Immunity, mental health, diabetes and even body weight make up the fascinating areas covered in the ever-growing research field of the gut microbiota. That last point about body weight is the one getting a lot of attention from obesity researchers. New evidence finds that gut bacteria can alter the way we store fat, balance levels of glucose in the blood, and even how we respond to hormones that make us feel hungry or full.

The gut microbiota and microbiome explained

The gut microbiota is a complex system made up of our resident bacteria, viruses and other microbes that have colonised the gastrointestinal tract. You may sometimes hear the term gut microbiome used. All the genes inside the microbial cells are what create the microbiome. The two terms microbiota and microbiome are often used to mean the same thing.

The microbiota creates its own mini ecosystem in the same way that plants, animals and insects live together in their own delicate ecosystem in a rainforest. We have a skin microbiota, mouth microbiota and in females, a vaginal microbiota. Then there is the one that receives the most attention: the gut microbiota. The entire microbiota in our gastrointestinal tract weighs between 0.5 to 2 kilograms.

One-third of our gut microbiota is common to most people, while two-thirds are specific to each one of us. This makes our gut microbiome as unique as our fingerprints.

The gut microbiome is not a static thing. It changes throughout life after first colonising the gut shortly after birth and continuing to gather new members from the environment throughout life. Variation is highest during childhood, and it gradually decreases with age. Illness, antibiotic use, fever, stress, injury and dietary changes all affect the blend of microbes that make up the microbiome.

Gut bacteria and weight

The spotlight on what influences our weight usually focuses on a poor diet, sedentary lifestyle and the genetic cards dealt to you at birth. But as evidence continues to snowball, should we also be looking further inside us at our gut microbes?

An early clue that gut microbes might play a role in obesity came from studies comparing intestinal bacteria in obese and lean twins. Lean twins have a more varied ecosystem of gut microbes compared to an obese twin sibling.

The twin studies make fascinating observations, but don’t prove that it is the gut bacteria that are the cause of the weight difference. The twin studies illustrate how complex a condition obesity is. Genetics and early life colonisation by bacteria only explain part of it. Diet can affect the mix of gut bacteria, with a highly processed diet low in food variety and fibre translating into less diversity of gut microbes.

From the human twin studies, the next move was to manipulate the gut bacteria of mice to see if body weight changes would follow. Identical mice inoculated with microbes from human obese and thin twins served as the model. Mice inoculated with bacteria from an obese human twin gained more weight and had a less diverse community of microbes compared to mice inoculated with bacteria from the thin twin.

So how could microbes affect our weight? One plausible mechanism is by altering our appetite. A French research team found that microbes make hormones involved in appetite regulation 20 minutes after being given nutrients. Giving these appetite hormones to mice saw them eat less food. It is no coincidence that the 20-minute mark after a meal is around the time it takes for a person to feel full. Eating slowly makes for sage advice so that gut microbes can ramp up production of appetite hormones, meaning you’ll stop eating earlier.

The research field is moving rapidly with clinical trials now up and running to ‘transplant’ freeze-dried bacteria from the faeces of lean and healthy donors into volunteers as a weight loss treatment. It’s probably not the most pleasant treatment that you can imagine, but there’s good evidence that faeces and the microbes they harbour are good for the microbiome environment inside our gut.

How what we eat affects our bacteria

You are what you eat, and so are the bacteria that live in your gut. The different strains of bacteria change depending on the types of food eaten. We have some way to go to know what the ‘perfect’ diet may be for gut health, but it will likely be very individual.

The dietary pattern linked most to an adverse change in bacterial species is the highly refined typical Western diet high in sugar and low in fibre. The good news is that a shift to a healthier diet can change the bacteria mix in a few days.

The key theme of healthy long-lived communities around the world is a diet high in plant-based foods. Across the spectrum, the Mediterranean diet gets top marks as a healthy balanced diet. It is distinguished by a beneficial fatty acid profile that is rich in both monounsaturated and polyunsaturated fatty acids, high levels of polyphenols and other antioxidants, high amounts of fibre, and lots of plant-based foods.

The Mediterranean dietary pattern is one that stands out for the variety of foods eaten and ticks most of the boxes for a microbe-friendly diet. Legumes, fruits, wholegrains, olive oil, yoghurt, dairy, nuts, seeds, vegetables and fish are a feast for your gut microbes.

Yo-yo dieting is bad news for your gut microbes

Weight rebound is a far too common result of most weight loss attempts. For some, the weight comes back on with interest. Gut microbes could be playing a part here too.

Using a yo-yo dieting model in lab mice, Israeli scientists found that yo-yo dieting mice had less diversity in their gut microbes compared to mice eating normally. They also regained weight rapidly. The changes to the gut microbiome brought about by obesity stayed put for far longer than the time spent dieting. This could explain why it is so hard to lose weight when you try your next diet – your gut microbes are still reeling from the effects of the last diet

So take a guess what happened when normal-weight mice received microbes from yo-yo dieting mice? Yes, they gained weight. These inoculated mice also digested fibre less efficiently. Fewer beneficial by-products were made from a class of plant chemicals called polyphenols. Polyphenols can act as a prebiotic, increasing the number of healthy bacteria in the gut. Fruits, vegetables, dark chocolate and tea are good sources of polyphenols and make great food for our gut microbiome.

What does all this research mean for me?

Laboratory studies in mice are all well-and-good to explore a theory, but what does it mean for us? Can a change in diet change the population of our gut microbes? And will that mean a change in weight?

A recent study involving African-Americans who swapped their meat-heavy, highly processed diet for a diet typical of African foods rich in beans and vegetables saw a positive change gut microbes within 2 weeks. And the reverse swap that saw rural Africans switch to a typical American diet gave them a microbe profile that was more in line with a higher risk of colon cancer.

Two weeks is a short time, but long enough to make changes to the microbe population that can alter our risk of disease. Will this guarantee weight loss? Not always, but the food swaps recommended are in line with guidelines already in place for better health and lowering chronic disease risk. Bringing in the angle of the gut microbiome gives these guidelines even more credence.

What about probiotics?

A less studied area is the potential effect of probiotics on body weight. Gut bacteria have been linked to how fat is stored, how glucose levels are regulated and even how hormones involved in making us feel hungry or full are regulated. The wrong mix of microbes could worsen problems for someone struggling to control how much food they eat.

So the big question is: can taking a probiotic supplement help with weight loss? And the answer appears to be ‘yes’; at least according to a systematic review and meta-analysis of clinical trials. Twenty-five randomised controlled trials involving close to 2,000 people informed the review. The trials were made up of a mixture of healthy adults, people with type 2 diabetes, people with high cholesterol or hypertension and people with non-alcoholic fatty liver disease. Most of the people were overweight or obese at the start of the studies.

All trials included both a group receiving either one or a combination of several probiotic supplements and another group receiving a placebo. The difference in weight loss between the two groups was in favour of the probiotic supplement with an additional 0.54 kg lost. This may not seem like a lot, but it represented additional weight lost compared to any other diet or physical changes the people were making.

Digging a bit deeper, it seemed that trials that used multiple strains of probiotics gave a greater weight loss benefit. And the longer the study ran for, the more likely that weight loss happened. Because so many different strains of probiotics were used it is not possible to narrow in on which protocol may work best for weight loss.

A probiotic supplement or food is not some magical panacea for obesity. But the findings from this latest review give some validity to the theory that our gut microbes have some sway in controlling what we eat and how we regulate our weight. There are too many over-hyped ineffective weight-loss supplements that fill shelves, but at least for probiotics the case for their use has some validity.

Top foods to improve your gut health

Look after your gut microbes and they will look after you. And to do that, you want to feed them the right food. Gut microbes ferment prebiotic foods which can then stimulate the growth of beneficial bacteria. Here are my top tips to keep your gut microbes well fed and thriving.

• Eat as many different plant-based wholefoods as possible. Some top prebiotic foods to include are:
  • Onions
  • Leeks
  • Garlic
  • Jerusalem artichokes
  • Chicory root
  • Asparagus
  • Barley and oats
• Foods high in resistant starch such as legumes, green bananas and cooked and then cooled potatoes are a great fermentable fuel source for bacteria
• Fermented probiotic foods like sauerkraut, kimchi, kefir and yoghurt are good sources of beneficial bacteria.

The gut microbiome into the future

Understanding the health implications of the gut microbiome is very much at an early stage. A fascinating peek into what could lie ahead is having dietary advice tailored to a person’s own unique microbiome signature.

Only two years ago, Israeli researchers attracted worldwide attention by successfully predicting a person’s blood glucose response to food by personalising their diet according to their gut microbiota profile as one of several factors.

The practical implications of the gut microbiome research field boil down to some simple basics: eat a variety of minimally processed plant-based foods high in fibre and move away from the ultra-processed foods that are over-represented in the Western diet. In the future, a personal consultation for diet advice for health, weight loss and disease risk could also involve a swab, petri dish and microbiologist.