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The Vaginal Microbiome - NED Infobite
BANT's scientific NED InfoBites are designed to provide key elements of the latest research using plain language. They provide quick overviews on particular health issues and nutrition topics for a speedy introduction to the science. Visually attractive and easily shareable with clients and social media followers.
2024
Abstract
Disturbances in the vaginal microbiome have effects across many areas of health, from fertility and pregnancy outcomes to development of fungal, viral and bacteria infection. This NED Infobite brings evidence for microbiome changes in relation to pre-term birth, fertility, infant microbial colonisation and cervical cancer.
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The effect of a preconception and antenatal nutritional supplement on children's BMI and weight gain over the first 2 years of life: findings from the NiPPeR randomised controlled trial.
Lyons-Reid, J, Derraik, JGB, Kenealy, T, Albert, BB, Nieves, JMR, Monnard, CR, Titcombe, P, Nield, H, Barton, SJ, El-Heis, S, et al
The Lancet. Global health. 2023;11 Suppl 1:S11-S12
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Rapid weight gain in infancy is associated with future adverse metabolic health. Nutrition intervention before and during pregnancy may promote healthy weight gain in infants, however evidence is lacking. The purpose of this study was to see whether preconception and antenatal supplementation effects the size and growth of children from birth until 2 years of age. 1729 women were recruited for the study and divided into two groups. The intervention group took myo-inositol, probiotics and additional micronutrients, the control group took a standard micronutrient supplement. Measurements of weight and length were obtained from 576 children at multiple intervals during the first 2 years of life. Differences in age and sex standardised BMI at age 2 years (WHO standards) and the change in weight from birth were examined. The results show that the infants where the mother took the intervention supplement before and throughout pregnancy had a lower risk of rapid weight gain and high BMI at age 2 years. Long-term follow-up is required to assess the longevity of these benefits.
Abstract
BACKGROUND Nutritional intervention before and throughout pregnancy might promote healthy infant weight gain; however, clinical evidence is scarce. Therefore, we examined whether preconception and antenatal supplementation would affect the body size and growth of children in the first 2 years of life. METHODS Women were recruited from the community before conception in the UK, Singapore, and New Zealand, and randomly allocated to either the intervention (myo-inositol, probiotics, and additional micronutrients) or control group (standard micronutrient supplement) with stratification by site and ethnicity. Measurements of weight and length were obtained from 576 children at multiple timepoints in the first 2 years of life. Differences in age and sex standardised BMI at age 2 years (WHO standards) and the change in weight from birth were examined. Written informed consent was obtained from the mothers, and ethics approval was granted by local committees. The NiPPeR trial was registered with ClinicalTrials.gov (NCT02509988) on July 16, 2015 (Universal Trial Number U1111-1171-8056). FINDINGS 1729 women were recruited between Aug 3, 2015, and May 31, 2017. Of the women randomised, 586 had births at 24 weeks or more of gestation between April, 2016, and January, 2019. At age 2 years, adjusting for study site, infant sex, parity, maternal smoking, maternal prepregnancy BMI, and gestational age, fewer children of mothers who received the intervention had a BMI of more than the 95th percentile (22 [9%] of 239 vs 44 [18%] of 245, adjusted risk ratio 0·51, 95% CI 0·31-0·82, p=0·006). Longitudinal data revealed that the children of mothers who received the intervention had a 24% reduced risk of experiencing rapid weight gain of more than 0·67 SD in the first year of life (58 [21·9%] of 265 vs 80 [31·1%] of 257, adjusted risk ratio 0·76, 95% CI 0·58-1·00, p=0·047). Risk was likewise decreased for sustained weight gain of more than 1·34 SD in the first 2 years (19 [7·7%] of 246 vs 43 [17·1%] of 251, adjusted risk ratio 0·55, 95% CI 0·34-0·88, p=0·014). INTERPRETATION Rapid weight gain in infancy is associated with future adverse metabolic health. The intervention supplement taken before and throughout pregnancy was associated with lower risk of rapid weight gain and high BMI at age 2 years among children. Long-term follow-up is required to assess the longevity of these benefits. FUNDING National Institute for Health Research; New Zealand Ministry of Business, Innovation and Employment; Société Des Produits Nestlé; UK Medical Research Council; Singapore National Research Foundation; National University of Singapore and the Agency of Science, Technology and Research; and Gravida.
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Effect of vaginal probiotics containing Lactobacillus casei rhamnosus (Lcr regenerans) on vaginal dysbiotic microbiota and pregnancy outcome, prospective, randomized study.
Petricevic, L, Rosicky, I, Kiss, H, Janjic, N, Kaufmann, U, Holzer, I, Farr, A
Scientific reports. 2023;13(1):7129
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Preterm delivery (PTD) of babies is thought in some women to be associated with the development of vaginal infections. An imbalance in the bacteria normally present in the vagina and especially a lack of Lactobacillus species can develop into a condition known as bacterial vaginosis (BV), which is a risk factor for PTD. The use of antibiotics is the standard treatment, however concerns on its effectiveness in preventing PTB have been raised. Antibiotics not only eradicate the abnormal bacteria, but also the Lactobacillus. This randomised control trial of 129 women with BV aimed to determine the effect of vaginally applied lactobacilli on the restoration of normal bacteria in the vagina and the rate of PTD. The results showed that women who had a complete lack of Lactobacillus, the vaginal application of Lactobacillus casei rhamnosus, BV was improved and gestational age at delivery and birthweight were both significantly increased, although PTD rate was not. It was concluded that vaginally applied Lactobacillus may be of some benefit to women with BV during pregnancy. This study could be used by healthcare professionals to understand the importance of vaginal microbiota during pregnancy. However, this was a study of very small numbers of women and larger studies are needed before vaginally applied microbiota can be definitively recommended.
Abstract
The intermediate bacterial microbiota is a heterogeneous group that varies in the severity of the dysbiosis, from minor deficiency to total absence of vaginal Lactobacillus spp. We treated women with this vaginal dysbiosis in the first trimester of pregnancy using a vaginally applied lactobacilli preparation to restore the normal microbiota in order to delay the preterm delivery rate. Pregnant women with intermediate microbiota of the vagina and a Nugent score of 4 were enrolled in two groups: intermediate vaginal microbiota and a Nugent score of 4 with lactobacilli (IMLN4) and intermediate vaginal microbiota and a Nugent score of 4 without lactobacilli (IM0N4), with and without vaginal lactobacilli at baseline, respectively. Half of the women in each group received the treatment. Among women without lactobacilli (the IM0N4 group), the Nugent sore decreased by 4 points only in the women who received treatment, and gestational age at delivery and neonatal birthweight were both significantly higher in the treated subgroup than in the untreated subgroup (p = 0.047 and p = 0.016, respectively). This small study found a trend toward a benefit of treatment with vaginal lactobacilli during pregnancy.
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Effects of Probiotics on Glycemic Control and Metabolic Parameters in Gestational Diabetes Mellitus: Systematic Review and Meta-Analysis.
Yefet, E, Bar, L, Izhaki, I, Iskander, R, Massalha, M, Younis, JS, Nachum, Z
Nutrients. 2023;15(7)
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The prevalence of gestational diabetes is increasing worldwide. Gestational diabetes mellitus (GDM) increases obesity and future development of type 2 diabetes in mother and child. Previous research has looked at the beneficial effects of probiotics in reducing metabolic diseases, however, these specific benefits on women with GDM are not fully understood yet. This systematic review and meta-analysis of fourteen randomised controlled trials assessed the beneficial effects of probiotics on glycemic control and metabolic parameters in women with GDM. This study separately assessed probiotic bacterial strains such as Lactobacillus acidophilus, Bifidobacterium bifidum, and Lactobacillus casei to understand their beneficial effects on metabolic parameters. This meta-analysis and systematic review suggest that probiotic supplementation could help improve glycemic control, insulin resistance and lipid levels in women diagnosed with GDM. All probiotic strains showed improvements in metabolic parameters when assessed separately. Further robust studies are required to assess the effect of probiotic supplementation on post- and pre-prandial glycemic control in women with GDM. Healthcare professionals can employ the results of this study to understand the therapeutic benefits of probiotics for improving GDM.
Abstract
OBJECTIVES To assess the effects of probiotic supplements on glycemic control and metabolic parameters in women with gestational diabetes mellitus (GDM) by performing a systematic review and meta-analysis of randomized controlled trials. The primary outcome was glycemic control, i.e., serum glucose and insulin levels. Secondary outcomes were maternal weight gain, neonatal birth weight, and lipid parameters. Weighted mean difference (WMD) was used. Cochrane's Q test of heterogeneity and I2 were used to assess heterogeneity. RESULTS Of the 843 papers retrieved, 14 (n = 854 women) met the inclusion criteria and were analyzed. When compared with placebo, women receiving probiotic supplements had significantly lower mean fasting serum glucose, fasting serum insulin, homeostatic model assessment for insulin resistance (HOMA-IR), triglycerides, total cholesterol, and VLDL levels. Decreased neonatal birth weight was witnessed in supplements containing Lactobacillus acidophilus. CONCLUSION Probiotic supplements may improve glycemic control and lipid profile and reduce neonatal birth weight in women with GDM.
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Associations between Dynamic Vitamin D Level and Thyroid Function during Pregnancy.
Wang, H, Wang, HJ, Jiao, M, Han, N, Xu, J, Bao, H, Liu, Z, Ji, Y
Nutrients. 2022;14(18)
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Thyroid hormones play a vital role in regulating metabolism. Adequate thyroid hormone levels are also critical during pregnancy for optimal fetal growth and development. The foetus is dependent on maternal thyroid hormones until its own thyroid gland matured in the second half of pregnancy. Furthermore, pregnancy impacts thyroid function leading to an increased demand for thyroid hormones. Thyroid disease has been associated with Vitamin D deficiency. During pregnancy, both thyroid disorders and Vitamin D deficiency can have adverse effects on pregnancy and pregnancy outcomes, hence a potential link between Vitamin D status and thyroid function has been postulated. To fill the gaps in previous research, this retrospective cohort study aimed to explore the associations between Vitamin D status and thyroid function throughout the pregnancy, in each trimester. The analysis of hospital data collected in Beijing demonstrated an association between Vitamin D levels and thyroid function throughout pregnancy. Such interlink appeared to be dynamic and changed depending on the stage of pregnancy. The author's findings affirmed that maintenance of adequate Vitamin D levels supports normal thyroid function which is an important nutritional strategy for a healthy pregnancy.
Abstract
Optimal Vitamin D (VitD) status and thyroid function are essential for pregnant women. This study aimed to explore associations between dynamic VitD status and thyroid function parameters in each trimester and throughout the pregnancy period. Information on all 8828 eligible participants was extracted from the Peking University Retrospective Birth Cohort in Tongzhou. Dynamic VitD status was represented as a combination of deficiency/sufficiency in the first and second trimesters. Thyroid function was assessed in three trimesters. The associations between VitD and thyroid function were assessed by multiple linear regression and generalized estimating equation models in each trimester and throughout the pregnancy period, respectively. The results indicated that both free thyroxine (fT4; β = 0.004; 95%CI: 0.003, 0.006; p < 0.001) and free triiodothyronine (fT3; β = 0.009; 95%CI: 0.004, 0.015; p = 0.001) had positive associations with VitD status in the first trimester. A VitD status that was sufficient in the first trimester and deficient in the second trimester had a lower TSH (β = -0.370; 95%CI: -0.710, -0.031; p = 0.033) compared with the group with sufficient VitD for both first and second trimesters. In conclusion, the associations between VitD and thyroid parameters existed throughout the pregnancy. Maintaining an adequate concentration of VitD is critical to support optimal thyroid function during pregnancy.
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Contribution of Lactobacillus iners to Vaginal Health and Diseases: A Systematic Review.
Zheng, N, Guo, R, Wang, J, Zhou, W, Ling, Z
Frontiers in cellular and infection microbiology. 2021;11:792787
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The vaginal microbiome is an important contributor to vaginal health. Here the dominance of Lactobacilli species, alongside their antimicrobial compounds play a critical role in contributing and protecting the vaginal milieu. Conversely the disruption or absence of Lactobacilli dominance is frequently associated with vaginal disease and infections. One of the Lactobacilli species prevalent in the vaginal microbiome is Lactobacillus iners. It was long overlooked in research due to it being so difficult to culture, and it was first identified in 1999 thanks to DNA sequencing. Researchers since discovered that the relationship between L. iners and vaginal health is far more complicated and ambiguous compared to the other Lactobacilli species. This systematic review explores the current knowledge of the characteristics of L. iners and its role in vaginal health and disease. The article discusses L. iners identification, genetic make up and differences to other Lactobacilli species and how they relate to vaginal health. The article also summarizes L. iners nutrient requirements and its role in diseases like dysbiosis, bacterial vaginosis, sexually transmitted infections and biofilm formation. Furthermore the authors look at the relation between L. iners and premature birthing, fertility and menstrual cycles. A final section in discusses the antimicrobial and immune sytem activating qualities of L. iners. In light of all these findings the authors describe L . iners as a very unique Lactobacilli due to its unusual characteristics. Whether L. iners is beneficial or pathogenic for the host remains controversial, as it can adapt to high and low pH environment and is seen in health and equally dysbiotic states of infection. Hence some describe it as a transitional species that colonizes the vagina after disturbances. It may be a risk factor for infections by contributing to the onset and maintenance of dysbiotic disturbances. Further studies are needed to clarify the role of L. iners and its role on vaginal health and whether it could serve as a biomarker for vaginal inflammation. This article is a useful summary about the characteristics and role of L. iners in vaginal health in disease.
Abstract
Lactobacillus iners, first described in 1999, is a prevalent bacterial species of the vaginal microbiome. As L. iners does not easily grow on de Man-Rogosa-Sharpe agar, but can grow anaerobically on blood agar, it has been initially overlooked by traditional culture methods. It was not until the wide application of molecular biology techniques that the function of L. iners in the vaginal microbiome was carefully explored. L. iners has the smallest genome among known Lactobacilli and it has many probiotic characteristics, but is partly different from other major vaginal Lactobacillus species, such as L. crispatus, in contributing to the maintenance of a healthy vaginal microbiome. It is not only commonly present in the healthy vagina but quite often recovered in high numbers in bacterial vaginosis (BV). Increasing evidence suggests that L. iners is a transitional species that colonizes after the vaginal environment is disturbed and offers overall less protection against vaginal dysbiosis and, subsequently, leads to BV, sexually transmitted infections, and adverse pregnancy outcomes. Accordingly, under certain conditions, L. iners is a genuine vaginal symbiont, but it also seems to be an opportunistic pathogen. Further studies are necessary to identify the exact role of this intriguing species in vaginal health and diseases.
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Implications of the vaginal microbiome and potential restorative strategies on maternal health: a narrative review.
Moumne, O, Hampe, ME, Montoya-Williams, D, Carson, TL, Neu, J, Francois, M, Rhoton-Vlasak, A, Lemas, DJ
Journal of perinatal medicine. 2021;49(4):402-411
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The vaginal microbiome is a complex community of bacteria that maintains and contributes to vaginal health and works together to protect the host from disease. Lactobacilli are the predominant species in a healthy vaginal microbiome. This review sought to summarize current evidence on how changes in the vaginal microbiome composition before and throughout pregnancy influence and contribute to several health conditions. The authors investigated the role of the vaginal microbiome in fertility and in-vitro fertilization (IVF), where evidence is still conflicting, followed by conditions that can present during pregnancy such as preterm labour, gestational diabetes, and preeclampsia. In preterm labour, lower levels of Lactobacillus and its D-lactic acid secretions have been observed as a common denominator; in gestational diabetes, it remains unclear whether the changes in the vaginal microbiome associated with the condition are causative or rather a result of the condition. Little research has been done regarding preeclampsia, but initial studies indicate there may be a link between the condition and the vaginal microbiome. The authors also reviewed the evidence on methods of restoring vaginal health, including oral and vaginally applied probiotics as well as vaginal microbiome transplants (VMT) - the transfer of vaginal secretions from a healthy donor. The paper discusses the evidence on the use of oral and locally applied probiotics for the aforementioned conditions, and the promising potential of VMTs alongside the risks and regulatory challenges that come with it. This review provides a helpful summary of the current understanding of the role of the vaginal microbiome in fertility and throught pregnancy.
Abstract
The vaginal microbiome undergoes dramatic shifts before and throughout pregnancy. Although the genetic and environmental factors that regulate the vaginal microbiome have yet to be fully elucidated, high-throughput sequencing has provided an unprecedented opportunity to interrogate the vaginal microbiome as a potential source of next-generation therapeutics. Accumulating data demonstrates that vaginal health during pregnancy includes commensal bacteria such as Lactobacillus that serve to reduce pH and prevent pathogenic invasion. Vaginal microbes have been studied as contributors to several conditions occurring before and during pregnancy, and an emerging topic in women's health is finding ways to alter and restore the vaginal microbiome. Among these restorations, perhaps the most significant effect could be preterm labor (PTL) prevention. Since bacterial vaginosis (BV) is known to increase risk of PTL, and vaginal and oral probiotics are effective as supplemental treatments for BV prevention, a potential therapeutic benefit exists for pregnant women at risk of PTL. A new method of restoration, vaginal microbiome transplants (VMTs) involves transfer of one women's cervicovaginal secretions to another. New studies investigating recurrent BV will determine if VMTs can safely establish a healthy Lactobacillus-dominant vaginal microbiome. In most cases, caution must be taken in attributing a disease state and vaginal dysbiosis with a causal relationship, since the underlying reason for dysbiosis is usually unknown. This review focuses on the impact of vaginal microflora on maternal outcomes before and during pregnancy, including PTL, gestational diabetes, preeclampsia, and infertility. It then reviews the clinical evidence focused on vaginal restoration strategies, including VMTs.
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Effects of date fruit (Phoenix dactylifera L.) on labor and delivery outcomes: a systematic review and meta-analysis.
Bagherzadeh Karimi, A, Elmi, A, Mirghafourvand, M, Baghervand Navid, R
BMC pregnancy and childbirth. 2020;20(1):210
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Since the 1970s the rate of Caesarean sections (C-section) has risen, particularly in some developing countries. This increases the occurrence of C-section-associated delivery and neonatal complications. Various factors may have contributed to this trend. Labour is an energy-intensive process and requires extra calories to maintain the process. Some research showed that consuming carbohydrates during labour can reduce the rate of instrumental-assisted deliveries, whilst lack of eating during labour can lead to ineffective contractions and increase C-section rates. The fruit of the Date Palm (Phoenix dactylifera) contains valuable vitamins, proteins, fatty acids, salt and minerals and has a high percentage of carbohydrates and sugars, which are readily absorbed. In Traditional Persian Medicine the date is regarded as a medicinal food for labour. The fatty acids in dates contribute to the formation of prostaglandins, which influence cervix ripening, labour induction, uterine contractions and accelerated delivery. Thus, dates may help to reduce the need for induction. Dates also increase antioxidant capacity, which can lead to a transient increase in pain tolerance. This systematic review examined eight clinical studies investigating the effects of date fruits on labour outcomes. It looked at the duration of labour stages, Bishop Score (a calculation that predicts the likelihood of normal vaginal labour initiation) and frequency of C-section when compared to routine care. The results showed that the consumption of date fruits significantly reduced the duration of the active phase of labour and improved the likelihood of vaginal labour going by the Bishop Score, and therefore may reduce the frequency of C-sections when compared to control groups. Due to the small study size and the low-medium quality of the studies included, the authors advocate for further research. As dates are an easily accessible and low-risk food for many, it is worth considering their inclusion into the diet in the last weeks of pregnancy to support the processes for vaginal delivery.
Abstract
BACKGROUND The rate of cesarean section is increasing in all over the world with different drafts in various countries. This growth increases unpleasant outcomes of delivery. Recent studies explained the benefits of date palm fruit on labor process improvement. Date fruit can be considered as a factor for increasing vaginal delivery and also reducing the frequency of caesarean section in order to prevent its great complications. This systematic review has been designed to review clinical studies that investigate the effects of date palm fruit on labor outcomes (duration of labor stages, bishop score, and frequency of cesarean section) compared with routine cares. METHODS This study was performed in 2019. Required data has been collected from electronic databases and manual searches. All randomized clinical trials evaluating the effects of date palm fruit on labor and delivery that were published from January 2000 to August 2019 in English and Persian languages, were incorporated in this systematic review. The methodological quality of the included studies was evaluated according to the risk of bias assessment of Cochrane handbook of systematic reviews, and were then reported using Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) statement. RESULTS Eight studies were included in the qualitative and quantitative synthesis. Meta-Analysis showed that date fruit consumption can significantly reduce active phase of labor (three trials with 380 participants; (MD = - 109.3, 95%CI (- 196.32, - 22.29; I2 = 89%), P = 0.01), and also it can significantly improve the bishop score (two trials with 320 participants; MD = 2.45, 95%CI (1.87, 3.04; I2 = 0%), P < 0.00001). Date fruit consumption had no effects on the duration of first, second, and third stages of labor, and the frequency of cesarean section. CONCLUSION Date can reduce the duration of active phase and improve the bishop score; however, due to from the low to mediate quality of the studies; it seems that the other studies are needed to prove these results better than this.
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A Systematic Review of Organic Versus Conventional Food Consumption: Is There a Measurable Benefit on Human Health?
Vigar, V, Myers, S, Oliver, C, Arellano, J, Robinson, S, Leifert, C
Nutrients. 2019;12(1)
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The demand for organic products has risen rapidly over the last decades. The reasons why consumers may favour organic over conventional products are varied. They may be for personal health and wellbeing, environmental considerations, animal welfare or perceived higher nutritional profile - which is true for some, but not all components. While the long-term safety of pesticide consumption through conventional food production has been questioned, organic foods clearly show lower levels of toxic metabolites, like heavy metals and synthetic fertilizer and pesticide residues. This systematic review aimed to assess the current evidence of organic diet consumption and human health compared to conventionally produced foods. Included were 35 papers on clinical trials and observational studies. The clinical trials studied pesticide and phytochemical excretion, antioxidant capacity, body composition, lipids and inflammatory markers. The observational studies were focused on fertility, foetal and childhood development, pregnancy, lactation and levels of pesticides in children and adults, as well as nutritional biomarkers and cancer risk in adults. An increased intake of organic produce in long-term studies appeared to reduce the incidence of infertility, birth defects, allergies, middle ear infection, pre-eclampsia, metabolic syndrome, high BMI, and non-Hodgkin lymphoma. Organic intake was also linked to reduced urinary levels of organophosphorus pesticides and herbicides. Yet, the author highlighted that organic consumers are more likely to be health conscious, physically active, eat a more plant-based diet, have higher education levels and income, and therefore are not representative of the general population. They also argue that the possible benefits from an organic diet may be partially due to the quality and composition of the diet rather than a direct effect of organic food consumption. Whereby a growing number of findings demonstrate the health benefits of organic food consumption, according to the authors, the current evidence does not yield a solid and definitive answer.
Abstract
The current review aims to systematically assess the evidence related to human health outcomes when an organic diet is consumed in comparison to its conventional counterpart. Relevant databases were searched for articles published to January 2019. Clinical trials and observational research studies were included where they provided comparative results on direct or indirect health outcomes. Thirty-five papers met the criteria for inclusion in the review. Few clinical trials assessed direct improvements in health outcomes associated with organic food consumption; most assessed either differences in pesticide exposure or other indirect measures. Significant positive outcomes were seen in longitudinal studies where increased organic intake was associated with reduced incidence of infertility, birth defects, allergic sensitisation, otitis media, pre-eclampsia, metabolic syndrome, high BMI, and non-Hodgkin lymphoma. The current evidence base does not allow a definitive statement on the health benefits of organic dietary intake. However, a growing number of important findings are being reported from observational research linking demonstrable health benefits with organic food consumption. Future clinical research should focus on using long-term whole-diet substitution with certified organic interventions as this approach is more likely to determine whether or not true measurable health benefits exist.
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Pregnancy outcomes in women taking probiotics or prebiotics: a systematic review and meta-analysis.
Jarde, A, Lewis-Mikhael, AM, Moayyedi, P, Stearns, JC, Collins, SM, Beyene, J, McDonald, SD
BMC pregnancy and childbirth. 2018;18(1):14
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It has been suggested that probiotics might help prevent premature birth, but two previous systematic reviews found possible increases in risk. The objective of this meta-analysis was to perform an up-to-date review of the risk of premature birth and other pregnancy outcomes in pregnant women taking probiotics or prebiotics. The authors pooled data from 27 studies, one using prebiotics and the rest probiotics. Taking probiotics or prebiotics during pregnancy did not change the risk of premature birth, or other pregnancy outcomes. The authors concluded that more studies are required to assess the safety and effects of taking probiotics and prebiotics during pregnancy.
Abstract
BACKGROUND Probiotics are living microorganisms that, when administered in adequate amounts, confer a health benefit. It has been speculated that probiotics might help prevent preterm birth, but in two previous systematic reviews possible major increases in this risk have been suggested. Our objective was to perform a systematic review and meta-analysis of the risk of preterm birth and other adverse pregnancy outcomes in pregnant women taking probiotics, prebiotics or synbiotics. METHODS We searched six electronic databases (MEDLINE, EMBASE, CINAHL, Cochrane Central Register of Controlled Trials, Web of Science's Core collection and BIOSIS Preview) up to September 2016 and contacted authors for additional data. We included randomized controlled trials in which women with a singleton pregnancy received a probiotic, prebiotic or synbiotic intervention. Two independent reviewers extracted data using a piloted form and assessed the risk of bias using the Cochrane risk of bias tool. We used random-effects meta-analyses to pool the results. RESULTS We identified 2574 publications, screened 1449 non-duplicate titles and abstracts and read 160 full text articles. The 49 publications that met our inclusion criteria represented 27 studies. No study used synbiotics, one used prebiotics and the rest used probiotics. Being randomized to take probiotics during pregnancy neither increased nor decreased the risk of preterm birth < 34 weeks (RR 1.03, 95% CI 0.29-3.64, I2 0%, 1017 women in 5 studies), preterm birth < 37 weeks (RR 1.08, 95% CI 0.71-1.63, I2 0%, 2484 women in 11 studies), or most of our secondary outcomes, including gestational diabetes mellitus. CONCLUSIONS We found no evidence that taking probiotics or prebiotics during pregnancy either increases or decreases the risk of preterm birth or other infant and maternal adverse pregnancy outcomes. TRIAL REGISTRATION We prospectively published the protocol for this study in the PROSPERO database ( CRD42016048129 ).