"Commonly claimed benefits of probiotics include the decrease of potentially pathogenic gastrointestinal microorganisms, the reduction of gastrointestinal discomfort, the strengthening of the immune system, the improvement of the skin's function, the improvement of bowel regularity, the strengthening of the resistance to cedar pollen allergens, the decrease in body pathogens, the reduction of flatulence and bloating, the protection of DNA, the protection of proteins and lipids from oxidative stress damage, and the maintaining of individual intestinal microbiota in subjects receiving antibiotic treatment."
Probiotics are under considerable research, as the concept holds promise for human health and well-being, and corresponding commercial opportunities. Protection of consumers requires health claims to be confirmed with sufficient scientific evidence. Overall scientific demonstration of probiotic effects requires defining a healthy microbiota and interactions between microbiota and host, and the difficulty to characterize probiotic effectiveness in health and disease. Recent developments of high-throughput sequencing technology and the consequent progresses of metagenomics represent a new approach for the future of probiotics research.
Studies are examining whether probiotics affect mechanisms of intestinal inflammation, diarrhea,or urogenital infections. Through 2012, however, in all cases proposed as health claims to the European Food Safety Authority, the scientific evidence remains insufficient to prove a cause and effect relationship between consumption of probiotic products and any health benefit.
Research into the potential health effects of supplemental probiotics has included the molecular biology and genomics of Lactobaccillus in immune function, cancer, and antibiotic-associated diarrhea, travellers' diarrhea, pediatric diarrhea, inflammatory bowel disease, and irritable bowel syndrome. Testing of a probiotic applies to a specific strain under study. The scientific community cautions against extrapolating an effect from a tested strain to an untested strain.
Although research does suggests that the relationship between gut flora and humans is a mutualistic relationship, very little evidence supports claims that probiotic dietary supplements have any health benefits. Improved health through gut flora modulation appears to be directly related to long-term dietary changes.
In a 2009 blog post, one expert reasoned that preliminary clinical results exist for some applications, such as treating diarrhea, but wider health benefits claimed by probiotic proponents lack plausibility since the body's "ecosystem" is sufficiently complex that adding a few bacteria is unlikely to have the claimed effect. Accordingly, he reasoned, "the alleged health benefits of probiotics are often an example of spin". Since then, there has been an increase in the body of scientific evidence supporting the use of specific probiotics to improve health. Although the body's complex microbial community is incompletely understood at present, there is strong scientific consensus on the benefits of using of probiotics to address certain medical states or conditions.
Claims that some lactobacilli may contribute to weight gain in some humans remain controversial.
Probiotics are ineffective in preventing allergies in children, with the possible exception of eczema.
Some probiotics are suggested as a possible treatment for various forms of gastroenteritis, and a Cochrane Collaboration meta-analysis on the use of probiotics to treat acute infectious diarrhea based on a comprehensive review of medical literature through 2010 (35 relevant studies, >4500 participants) reported that use of any of the various tested probiotic formulations appeared to reduce the duration of diarrhea by a mean of 25 hours, also noting, however, that "the differences between the studies may be related to other unmeasured and unexplored environmental and host factors" and that further research was needed to confirm reported benefits.
Some of the best evidence in support of probiotic health benefits is in the treatment of antibiotic-associated diarrhea (AAD). Antibiotics are a common treatment for children, and 20% of antibiotic-treated children develop diarrhea. AAD results from an imbalance in the colonic microbiota caused by antibiotic therapy. Microbiota alteration changes in carbohydrate metabolism, with decreased short-chain fatty acid absorption and osmotic diarrhea as a result. The preventive role of some probiotics has been correctly assessed in randomized, controlled clinical trials. A review assessing the work of 16 different studies representing the evaluation of more than 3,400 patients concluded that the evidence gathered suggested a protective effect of some probiotics in this condition. In adults, some probiotics showed a beneficial role in reducing the occurrence of AAD. Another consequence of antibiotic therapy leading to diarrhea is the overgrowth of potentially pathogenic organisms such as Clostridium difficile.
Probiotic treatment might reduce the incidence and severity of AAD as indicated in several meta-analyses, For example, treatment with probiotic formulations including L. rhamnosus may reduce the risk of AAD, improve stool consistency during antibiotic therapy, and enhance the immune response after vaccination. However, further documentation of these findings through trials is required to confirm specific effects and obtain regulatory approval, which currently does not exist.
The potential efficacy of probiotic AAD prevention is dependent on the probiotic strain(s) used and on the dosage. A Cochrane Collaboration systematic review, in which 16 randomized clinical trials were analyzed, concluded that treatments with less than 5000 million CFUs/day did not show a significant decrease of AAD. However, patients treated with ≥5000 million CFUs/day (including L. rhamnosus and Saccharomyces boulardii) had 60% lower relative risk (95%CI: 44–71%) of experiencing AAD than untreated patients.
Ingestion of certain active strains may help lactose-intolerant individuals tolerate more lactose than they would otherwise have tolerated.
Preliminary human and animal studies have demonstrated the efficacy of some strains of lactic acid bacteria (LAB) for reducing serum cholesterol levels, presumably by breaking down bile in the gut, thus inhibiting its reabsorption (where it enters the blood as cholesterol).
A meta-analysis that included five double-blind trials examining the short-term (2–8 weeks) effects of a yogurt with probiotic strains on serum cholesterol levels found a minor change of 8.5 mg/dL (0.22 mmol/L) (4% decrease) in total cholesterol concentration, and a decrease of 7.7 mg/dL (0.2 mmol/L) (5% decrease) in serum LDL concentration.
A slightly longer study evaluating the effect of a yogurt with probiotic strains on 29 subjects over six months found no statistically significant differences in total serum cholesterol or LDL values. However, the study did note a significant increase in serum HDL from, 50 to 62 mg/dL (1.28 to 1.6 mmol/L) following treatment. This corresponds to a possible improvement of LDL/HDL ratio.
Studies specifically on hyperlipidemic subjects are still needed.
The consumption of probiotics may effect a modest benefit in helping to control high blood pressure.
Immune function and infections
Some strains of LAB may affect pathogens by means of competative inhibition (i.e., by competing for growth) and some evidence suggests they may improve immune function by increasing the number of IgA-producing plasma cells and increasing or improving phagocytosis, as well as increasing the proportion of T lymphocytes and natural killer cells. Clinical trials have demonstrated that probiotics may decrease the incidence of respiratory-tract infections and dental caries in children. LAB products might aid in the treatment of acute diarrhea, and possibly affect rotavirus infections in children and travelers' diarrhea in adults, but no products are approved for such indications.
Some strains of LAB may affect Helicobacter pylori infections (which may cause peptic ulcers) in adults when used in combination with standard medical treatments, but no standard in medical practice or regulatory approval exists for such treatment.
Some strains of LAB may modulate inflammatory and hypersensitivity responses, an observation thought to be at least in part due to the regulation of cytokine function. Clinical studies suggest they can prevent reoccurrences of inflammatory bowel disease in adults, as well as improve milk allergies. How probiotics may influence the immune system remains unclear, but a potential mechanism under research concerns the response of T lymphocyts to proinflammatory stimuli.
Irritable bowel syndrome and colitis
Probiotics may help people with irritable bowel syndrome, although uncertainty remains around which type of probiotic works best, and around the size of the effect.
No good evidence indicates taking probiotics helps maintain remission from ulcerative colitis.
Several clinical studies provide evidence for the potential of probiotics to lower the risk of necrotizing enterocolitis (NEC) and mortality in premature infants. One meta-analysis indicated that probiotics reduce all-cause mortality and risk of having NEC by more than 50% compared with controls.
Probiotic treatment has been studied as a means of addressing maladies associated with vitamin deficiency, e.g., of Vit K, folic acid, and Vit B 12.
Probiotics are commonly given to breast-feeding mothers and their young children to prevent eczema, but some doubt exists over the strength of evidence supporting this practice.
Probiotic treatment of bacterial vaginosis is the application or ingestion of bacterial species found in the healthy vagina to cure the infection of bacteria causing bacterial vaginosis. This treatment is based on the observation that 70% of healthy females have a group of bacteria in the genus Lactobacillus that dominate the population of organisms in the vagina. Currently, the success of such treatment has been mixed since the use of probiotics to restore healthy populations of Lactobacillus has not been standardized. Often, standard antibiotic treatment is used at the same time that probiotics are being tested. In addition, some groups of women respond to treatment based upon ethnicity, age, number of sexual partners, pregnancy, and the pathogens causing bacterial vaginosis. In 2013, researchers found that administration of hydrogen peroxide producing strains, such as L. acidophilus and L. rhamnosus, were able to normalize vaginal pH and rebalance vaginal flora, preventing and alleviating bacterial vaginosis.
In some situations, such as where the person consuming probiotics is critically ill, probiotics could be harmful. In a therapeutic clinical trial conducted by the Dutch Pancreatitis Study Group, the consumption of a mixture of six probiotic bacteria increased the death rate of patients with predicted severe acute pancreatitis.
In a clinical trial aimed at showing the effectiveness of probiotics in reducing childhood allergies, researchers gave 178 children either a probiotic or a placebo for the first six months of their lives. Those given the probiotic were more likely to develop a sensitivity to allergens.
Some hospitals have reported treating Lactobacillus septicemia, which is a potentially fatal disease caused by the consumption of probiotics by people with lowered immune systems or who are already very ill.
Probiotics taken orally can be destroyed by the acidic conditions of the stomach. A number of microencapsulation techniques are being developed to address this problem.
One 2009 paper cited a 2007 study in chickens as a support for causally linked probiotic products such as yogurts with obesity trends. However, this is contested as the link to obesity, and other health-related issues with yogurt may link to its dairy and calorie attributes.
Some experts are skeptical on the efficacy of many strains and believe not all subjects will benefit from the use of probiotics.