Can you take probiotics to lose weight? They have plenty of benefits, from keeping your digestive system healthy to improving your mood, but is weight loss one of them?

Defined by the World Health Organization as “live microorganisms which, when administered in adequate amounts, confer a health benefit on the host,” probiotics live in your colon and other parts of the body, and are most likely to be found in fermented foods or supplements.

Thanks to their myriad of health benefits, many people aim to up their probiotic intake, with yogurt being among the best-known probiotic foods. Other opt for best probiotic supplements. But, can they impact your body composition?

We spoke with Sophie Medlin – Doctify-reviewed consultant dietitian and director of CityDietitians – to discuss the impact that probiotics may have on weight loss, what quantity you should be consuming, and whether you should be getting them from food or supplements. We also combed through all the existing research to find out how gut bacteria affects body weight.

Related: Probiotics vs digestive enzymes

How does gut bacteria affect body weight?

You may have heard the phrase ‘gut feeling’ used to describe an instinctive reaction to something. And, while this may seem like an odd location to encounter emotion at first, a study published in the Journal of Medicinal Food has established strong links between your digestive system and your brain.

This relationship has been labeled the gut-brain axis, with gut bacteria able to send messages to the brain via the vagus nerve, influencing memory, mood and cognition. Research published in Nutrients has also shown gut bacteria’s impacts on several other organ systems, including cardiovascular, neural, immune and metabolic.

So, it’s clear they hold plenty of power, but how does gut bacteria affect body weight?

One study published in Nature found that, in a sample of 123 non-obese people and 169 obese individuals, those with low fecal bacteria diversity (an indicator of the variety of microorganisms in an individual’s gut microbiome) were characterized by higher fat levels, cholesterol and insulin resistance. Obese individuals among those studied with low bacterial richness were also found to gain more weight over time.

Can you take probiotics to lose weight?

“There is data to show that, in mice, having the microbiome of an overweight person can lead to the mouse gaining weight, even if they eat the same diet as they had before,” dietitian Sophie Medlin says.

A study published in Acta Physiologica found that conventionally reared mice had a 40% higher body fat content than germ-free mice (those raised in a sterile environment) resulting in no microorganisms in their gut. On top of this, when distal gut bacteria from normal mice was transplanted into the germ-free animals, they encountered a 60% rise in body fat in two weeks, despite no significant changes being made to their food consumption or energy expenditure.

These findings suggest a link between obesity and the make-up of the gut microbiota, with germ-free mice able to eat more and gain less weight than conventional mice.

“We hypothesize that this might be because of how much additional energy we can harvest when we have an abundance of particular strains of bacteria and, potentially, due to the difference different bacteria can make to our metabolism,” Medlin says.

However, she adds, there are two key factors to remember when interpreting this research. “Firstly, humans are not the same as mice, and the calorie difference this is likely to make in humans compared to mice is very small,” she says. “Secondly, there is still a lot we don’t understand about the microbiome, so it is too reductive to blame weight gain on the bacteria in our bowel.”

As a result, Medlin says you can “definitely not” take probiotics to lose weight. Instead, she recommends making changes to your overall diet to achieve any body composition goals.

“You could certainly eat more plants and less processed food which would feed your beneficial bacteria and, in the medium term, may support you in maintaining a healthy weight. But, no amount of probiotic capsules will promote weight loss if you don’t also adapt your diet.”

Should you get probiotics from food or supplements?

When it comes to consuming probiotics, Medlin says that in general, probiotic capsules will be more likely to have an effect. “But it is really important to also adapt your diet in order to feed the beneficial bacteria,” she says. “You’re much better off feeding the colonic friends you have already by eating more plants and high fiber foods. Adding a probiotic without advice from a specialist dietitian on which type to use to target your symptoms is not going to be of significant benefit.”

However, when looking to positively impact your gut microbiota through the foods you eat, she advises focusing on consuming more prebiotics (dietary fibers that help healthy bacteria grow in your gut) rather than probiotics.

“Prebiotic foods include all plant foods, such as nuts, seeds, wholegrains, fruits, vegetables and herbs and spices. Your beneficial bacteria particularly thrive on plant diversity in your diet so eat as many different plants as you can.”

Medlin adds that both prebiotics and probiotics can be consumed daily. “In terms of foods that contain probiotics, the best evidence is for taking these in dairy form, so foods like kefir and yogurt,” she says. “This is because of the matrix of proteins from dairy foods which form a protective barrier around the bacteria, allowing them to survive our stomach acid.”