It’s an unpleasant thought: even when you’re healthy your bowels, skin, mouth, genitals are home to trillions of bacteria, fungi, viruses and parasites. Occasionally, one strain of bacteria grows out of control, causing an infection. However, many of these bacteria are essential to a person’s health and wellbeing by, for example, helping to digest plant fibre, producing several vitamins, shaping immune responses, controlling levels of disease-causing bacteria and breaking down (metabolising) some potentially harmful chemicals. As a result, many people take pro- and prebiotics in the hope of preventing or alleviating diseases or boosting their general wellbeing. But when do probiotics really help?
Probiotics deliver bacteria directly. Prebiotics supply nutrients that certain healthy bacteria use to grow. Synbiotics combine pro- and prebiotics that work together. And there’s little doubt that these popular supplements can help certain conditions.
For example, up to 30 per cent of people taking antibiotics develop upset bowels – so-called antibiotic-associated diarrhoea (AAD). When researchers combined the results of 17 trials in a meta-analysis, people taking probiotics were 51 per cent less likely to develop AAD than controls (8.0 per cent and 17.7 per cent respectively). Lactobacillus rhamnosus GG (71 per cent reduction) and Saccharomyces boulardii (59 per cent reduction) seemed to be the most effective. No significant differences in adverse events emerged with probiotics.
Probiotics, used alongside rehydration, can also alleviate acute infectious diarrhoea. A Cochrane review looked at 63 studies, 56 of which enrolled infants and young children. Probiotics reduced the average duration of diarrhoea by almost 25 hours, the likelihood that the diarrhoea would last for at least four days by 59 per cent, and the frequency of stools on the second day by 20 per cent – about one fewer diarrhoeal stool. Researchers now need to determine which probiotic is best for which patient.
Some people find that probiotics alleviate irritable bowel syndrome (IBS). NICE recommends that people who try probiotics for IBS should use the product for at least four weeks, while keeping a note of any benefits, and stress that people should take probiotics at the dose recommended by the manufacturer.
One study compared the probiotic Bifidobacterium infantis 35624 and placebo in 362 women with IBS in the UK. Four-weeks treatment with B. infantis 35624 reduced abdominal pain, bloating, bowel dysfunction, incomplete evacuation, straining and the passage of gas. The overall improvement in symptoms was more than 20 per cent greater than placebo.
The NHS website adds that some probiotics, such as Lactobacillus acidophilus, may alleviate the symptoms of lactose intolerance, such as stomach cramps, flatulence and diarrhoea. People who are lactose intolerant could try probiotic preparations (not yoghurts) containing L. acidophilus. Again, it’s worth suggesting that people keep a note of the benefits.
Despite these apparent benefits, nhs.uk makes it clear that there is a lack of evidence that probiotics help other conditions, such as boosting the immune system, treating bacterial vaginosis or vaginal thrush, or alleviating the symptoms of eczema, Crohn’s disease or ulcerative colitis. Sometimes, studies hint at a benefit but further research is needed.
For example, a decline in the number of lactobacilli in the vagina can allow the growth of candida, a fungus. A Cochrane review looked at 10 studies that included 1,656 non-pregnant women with vaginal thrush. Used alongside antifungals, vaginal or oral probiotics increased the likelihood of a clinical cure (no symptoms) within five to 10 days by 14 per cent, the likelihood of a microbiological (laboratory) cure by six per cent and the risk of relapse at one month by 66 per cent. But there was no difference in cure rates one and three months after treatment. More, larger and better-designed studies are needed to see if probiotics really can help vaginal thrush.
Meanwhile, the role of pre- and probiotics is set to grow, in sometimes surprising ways. For instance, for a number of years healthcare professionals learnt that healthy urine was sterile. However we now know urine from healthy women actually seems to contain about 120 bacterial species, which are similar to those in the vagina, but different from those of the gut. It’s possible that changes in the urine microbiota could contribute to urinary tract infections.
More remarkably, increasing evidence links the microbiota’s composition with psychiatric and neurological conditions, including: autism, schizophrenia, attention deficit hyperactivity disorder, Parkinson’s disease, Alzheimer’s disease and multiple sclerosis.
The finding isn’t as surprising as it might appear. For instance, gastrointestinal bacteria release signalling molecules that may affect the brain. Researchers have even transplanted depression. People with depression show marked changes in their gastrointestinal microbiota. So researchers transplanted faecal microbiotas from people with depression and healthy people into rats who had their gastrointestinal bacteria depleted with antibiotics. Rats that received a transplant from people with depression showed an abnormal microbiota and more anhedonic (non-pleasure seeking) and anxiety-related behaviours compared to the animals that received a transplant from controls. This and other studies raise the prospect of using probiotics to treat depression.
Another study enrolled 40 healthy people who took a probiotic containing eight species of bacteria or a placebo, once daily for four weeks. Probiotic users showed less emotional and mental reactivity – such as rumination and aggressive thoughts – to sad mood, a trait that indicates the person might be vulnerable to depression. The authors say that the “results provide the first evidence that the intake of probiotics may help reduce negative thoughts associated with sad mood”. Studies in people with depression are now needed to confirm these promising results.
In one study, of 22 healthy volunteers, taking Bifidobacterium longum 1714 for four weeks seemed to reduce stress-related increases in the hormone cortisol and anxiety. B. longum 1714 also seemed to reduce daily stress reported by the volunteers and subtly improved some aspects of memory.
An astounding number of bacteria call your body home. Genetic fingerprinting identified more than 2,000 species in the human bowel, although the exact composition varies from person to person. The total number of bacteria in your body (your microbiota) may reach 100 trillion – roughly 300 times the number of stars in the Milky Way. You host 10 times more bacterial cells in your intestines than all the human cells in the rest of your body. Collected together, your microbiota would be about the same size as your brain.
Biologists divide the microbiota into five habitats – skin, nose, mouth, lower gastrointestinal tract and vagina – each of which is subdivided. For example, the human mouth’s microbiome consists of more than 700 bacterial species. But bacteria in your cheek lining differ from those in your plaque. Indeed, your microbiome is probably as individual as your fingerprint.
According to the NHS, “for most people probiotics appear to be safe”. Provided the person has a healthy immune system, probiotics “shouldn’t cause any unpleasant side effects.” However, the microbiome may interact with some medicines, which could increase the risk of side effects or undermine effectiveness.
The bacteria in a healthy gut break down potentially harmful chemicals, including some pollutants and carcinogens. But bacteria can’t distinguish between a possible toxin and a medicine. So the gut microbiota seems to contribute to the metabolism of, among other drugs, paracetamol, prednisolone, ranitidine, chloramphenicol, lovastatin, simvastatin and digoxin.
Antibiotics can disrupt the bacteria in the gut, which is why they cause AAD. So it’s not surprising that antibiotics can alter the metabolism of several drugs – including lovastatin, sulfasalazine, nitrazepam and digoxin – by changing the gastrointestinal microbiota. Blood levels of digoxin can up to double in some people taking antibiotics such as erythromycin and tetracycline, which could lead to side effects. A recent review concluded that antibiotics’ effect on the metabolism of drugs and other chemicals “may be more extensive and potent than previously recognised”. In the meantime, the review suggests carefully monitoring the effects of oral drugs administered with antibiotics.
So it’s worth checking with the pharmacist or the prescriber if someone who is taking a medicine wants to try a pre- or probiotic, although the study of this area is in it’s infancy. And people should never stop taking a prescription medicine or reduce the dose without speaking to the prescriber first, even if they feel better after taking a pre- or probiotic.
The thought that trillions of bacteria and other micro-organisms call us home isn’t pleasant. Nevertheless, disruptions to our healthy bacteria can increase the risk of several conditions and biologists are only just beginning to map out the microbiome’s role in health and disease. But for some people, pre- and probiotics can give the microscopic squatters that are essential for our health and wellbeing a boost.