What do probiotics do?

Both Lactobacillus and Bifidobacterium species have shown hypocholesterolemic effects (they reduce cholesterol levels) in lab studies. A meta-analysis published in the European Journal of Clinical Nutrition suggested that yoghurt consumption reduces total cholesterol by 4% and LDL (low density or bad cholesterol) by 5%. Yoghurt is prepared with a culture of Lactobacillus acidophilus and Streptococcus thermophilus.

Several mechanisms have been suggested for the hypocholesterolemic action of probiotics. These include deconjugation of bile salts (part of bile fluid secreted from the liver). Bile salts are basically bile acids attached (conjugated) with glycine or taurine (both glycine and taurine are proteins). They help in the absorption of fats in the intestines. On being deconjugated, bile salts are unable to perform their function and are eliminated with the urine. To make up for the lost bile, the liver uses more fats, thus further reducing your total cholesterol levels.

Additionally, probiotics attach some of the cholesterol to their own surface, keeping them from reaching your blood. 

Read more: Foods that reduce high cholesterol levels

When it comes to the benefits of probiotics in obesity, there are a lot of variables - including the age, sex, and BMI of the person and the dose and duration of intake of the probiotic.

Animal studies show that probiotics affect energy expenditure and the ability of the organism to take energy from food. 

A randomised placebo-controlled study done in Japan found that the consumption of fermented milk that contains 10⁸ CFU/gram of Lactobacillus gasseri starts to reduce adiposity within 12 weeks.

In a clinical trial done in 2014, the administration of probiotics was only found to be effective for women and not men, when combined with calorie restriction and a weight maintenance programme.

More studies are needed to confirm the evidence.

Read more: How to lose weight fast and safely

Antibiotics kill some of the good bacteria in the stomach along with the harmful ones. Thus, prolonged antibiotic use often leads to diarrhoea. Antibiotics also increase intestinal motility, which is the contracting and relaxing motion of your intestines that helps push food forward in your gut. Probiotics have proven benefits for reducing the risk of antibiotic-associated diarrhoea. 

A 2015 review indicated that S. boulardii can help prevent antibiotic-associated diarrhoea in children and adults.

A meta-analysis published in the journal Nutrition in Clinical Practice, a peer-reviewed journal published by the American Society for Parenteral and Enteral Nutrition, suggested that probiotic consumption can reduce the risk of antibiotic-associated diarrhoea in adults but not in the elderly (>65 years of age).

Acute infectious diarrhoea is one of the most common causes of hospitalisation and death in children up to five years old, in developing countries. It is characterised by the presence of liquidy baby poop with or without vomiting and fever. The patient usually has more than eight stools within a day. The most common cause of infectious diarrhoea in children is viral infection, particularly rotavirus infection. Diarrhoea is one of the causes of dehydration in children.

A review article published in the Cochrane Database of Systematic Reviews suggested that probiotics used along with rehydration therapy can reduce the duration of diarrhoea in children.

S. boulardii has been shown to be effective in providing symptomatic relief in infectious diarrhoea in children. Though more studies are still needed to confirm the benefits of this yeast in diarrhoea patients.

IBS patients are often told to take probiotics since an imbalance in gut flora is thought to be one of the possible causes of irritable bowel syndrome - the exact cause of the condition is still unknown.

A meta-analysis of 43 random controlled trials showed that probiotics are effective for the treatment of IBS. In other studies bacteria including Lactobacillus acidophilus, Bifidobacterium breve, and Bifidobacterium longum have been found to be effective in the management of IBS symptoms. 

Clinical studies show that bacteria including Streptococcus thermophilus, Bifidobacterium infantis, B. longum, L. plantarum, L. acidophilus, Lactobacillus bulgaricus and L. casei can be used to reduce flatulence (passing wind) in IBS patients.

However, more studies are still needed to confirm the dosage and duration of treatment required for treating IBS with probiotics. 

Read more: What to eat and what not to eat when you have irritable bowel syndrome

Probiotics are effective against various oral health issues which include periodontal diseases like periodontitis, bad breath and dental caries (cavities). Lab studies suggest that Lactobacillus rhamnosus and Bifidobacterium can keep the caries-causing bacteria Streptococcus from adhering to the teeth surface.

An article published in the American Journal of Dentistry argued that the intake of probiotic cheese is suggested to be effective in reducing dental caries. A randomised double-blind placebo-controlled trial found that the consumption of probiotic yoghurt, specifically that contains Lactobacillus reuteri, can reduce the levels of Streptococcus mutans in the oral cavity by up to 80%.

Clinical studies show that intake of probiotics without any other therapy can reduce gum bleeding in periodontitis and gingivitis. However, in most patients, the condition recurs within a few months or a year. 

Similarly, bacteria including Lactobacillus and S. salivarious have shown a reduction in oral malodour on regular consumption.

Both the oral and topical use of probiotics is said to be good for skin health. Research shows that probiotic bacteria improve skin barrier function by an unknown mechanism, reducing the risk of skin infections. Also, some probiotics release compounds like hyaluronic acid and other proteins that reduce skin dryness and promote skin health.

Intake of Escherichia coli strain Nussle 1917 is found to be beneficial for various inflammatory conditions of the skin. Bacteria like Lactobacillus acidophilus, Bifidobacterium bifidum, L. salivarious and L. casei is suggested to be effective against atopic dermatitis - a chronic condition in which red itchy patches appear all over the skin during a flare-up.

However, most of these studies include different bacteria, dosage, and duration of treatment, so more evidence is needed before probiotics can be used for clinical treatment of skin conditions.