Probiotic Supplements Sometimes Do More Harm Than Good

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Probiotics have been a hot topic in the world of health and wellness for quite some time. Many scientists are interested in targeting the gut microbiome (the microorganisms living in the gut) in order to boost many aspects of human health, from immune system function to brain health. Probiotic supplements introduce beneficial bacteria into the gut, and studies suggest that they could be used to treat certain diseases as well as to improve overall health and wellbeing. 

Many people take probiotic supplements to improve general health and reduce their risk of disease, yet some researchers caution that this could sometimes do more harm than good, and that there may be better approaches to improving gut health.

Probiotics vs prebiotics, and why the latter may be more important

The human intestine is home to at least 500 different bacterial species – that’s a lot of biodiversity to exist within a single organ system. Different bacteria are good at breaking down different food components. In doing so, they produce molecular signals that can affect the nearby gut and resident immune cells, which can have wide ranging effects on even distant organs. These bacteria also protect us against infectious diseases by outcompeting pathogenic bacteria that invade the gut. In other words, what we eat nourishes the gut microbiome, which in turn has a powerful effect on overall health.

Probiotics are live microorganisms that are found in certain foods and supplements. They contain bacterial species that we believe to have beneficial effects within the gut. These beneficial bacteria can become depleted and outcompeted by ‘bad’ gut bacteria due to factors like poor diet and advancing age. Probiotics can replenish good bacterial species and address imbalances in the microbiome. However, the bacteria contained in probiotic supplements aren’t actually native to the human gut. This means that they do not survive that long within the intestine, so you need to keep taking them in order to maintain their beneficial effects.

Prebiotics, on the other hand, are non-digestible fibres that feed the good bacteria in your gut. While probiotics add good bacteria to your gut, prebiotics help support the growth of the good bacteria already present. We know in medicine that preventing a health problem is almost always easier than reversing it once it has occurred, and that remains true for the gut microbiome. Currently, the only way of permanently restoring a healthy microbiome once it has been lost is by faecal transplantation from someone with a healthy microbiome. 

Can you have too much of a good thing?

The description of ‘good’ and ‘bad’ bacteria can sometimes give the impression that these groups are only in competition with each other. In reality, any strain of bacteria is capable of outcompeting another strain, regardless of whether they are beneficial for the gut or not. Some scientists are concerned that too many good bacteria can crowd out other good bacteria, thereby doing more harm than good.

There is certainly plenty of evidence that probiotics can be beneficial in the treatment of some diseases. However, there may also be some significant drawbacks. For example, one clinical trial gave human volunteers broad-spectrum antibiotics followed by either faecal transplant, probiotic supplements or a placebo. Broad spectrum antibiotics damage the gut microbiome, so one might have expected the probiotics to be particularly beneficial in these people. However, the results showed that people taking the placebo treatments recovered their original microbiome composition much faster than those taking probiotics. This suggests that the non-native probiotics may have competed with the native bacteria and prevented them from recolonising the gut as quickly. This doesn’t necessarily mean that the probiotics didn’t benefit participant’s health in the short term – just that there appears to be a trade-off.

Probiotics can be inconsistent

Even in conditions for which probiotics appear to have clear benefits, these benefits can be inconsistent. Insulin resistance is one such condition. Previous studies suggest that probiotics can help lower blood sugar by improving sensitivity to the hormone insulin. However, a study from last year suggests that this isn’t always the case. They randomised people with metabolic syndrome (a cocktail of diabetes risk factors including high blood sugar and blood pressure) to receive a placebo or a set of probiotics thought to benefit metabolic health. While some people experienced benefits such as reduced blood pressure after taking probiotics, some people actually fared significantly worse than those taking the placebo, experiencing a rise in blood sugar. 

How could the same treatment be good for some people and bad for others when they all have the same disease? The most likely answer is diet. Probiotics only have access to the nutrients we are feeding them, and those nutrients will affect how they behave within our gut. In other words, we can’t afford to overlook the importance of prebiotics, even when taking bacteria that are well established to be beneficial.

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Probiotic supplements remain a promising intervention for treating disease and for improving general health. Unfortunately, there’s still a lot we don’t know about how these bacteria interact with our native gut bacteria, diet, and current health status. Rather than relying solely on probiotics, incorporating prebiotic-rich foods and supplements into your diet may be a better option.