Gut Microbiota Diversity in the Young and Old

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A factor in disease conditions and aging?

In recent decades, there has been an explosion of information regarding the role that the human microbiota plays in both health and disease. Projects such as the National Institutes of Health Human Microbiome Project,[1] and similar collaborative efforts in other countries, have focused on the collection of human microbiome data from large populations for the purpose of increasing our knowledge about the microbes inhabiting the body.[2],[3] Combined with population data, these projects offer information about the impact of host and environmental factors on microbiota in average, healthy populations, as well as microbiome-associated variables that may point to disease risks.

Our microbiome includes the microbes that find their home not only in the gastrointestinal (GI) tract, but also the on the skin, respiratory tract, urogenital tract, and even the brain. These microbes predominantly consist of bacteria, although nonbacterial organisms such as viruses and fungi are also represented. The term “microbiome” refers not only to the microbiota and their habitat but also the collective genomes of the microbes, known as the “metagenome.”[4] Although some references estimate that the 100 trillion organisms comprising the human microbiome represent 10 times the number of cells in the human body,[5] a recent paper suggests that the ratio of microbes to human cells is closer to 1.3:1.[6] Regardless, both of these estimates reflect a substantial microbial population, whose function impacts not only the interfacing epithelial tissues, but also the function of our body within.

By far, the largest population of these microbes is found in the GI tract, which is home to nearly 10¹³ to 10¹⁴ microorganisms that represent between 500 to 1,000 unique bacterial species.[7],[8] The most significant influences on the development of the infant gut have been well established to be mode of birth and early feeding.[9] Compared to infants born vaginally, infants born by cesarean section have decreased bacterial diversity, or “richness” in the gut.[10],[11] Infants who are formula-fed also have decreased bacterial richness in the gut compared to those who are breastfed.[12] The neonatal gut continues to populate and diversify through childhood until it reaches a high level of diversity similar to the adult gut by about two to three years of age.[13] Studies have found an increased risk of obesity in children born by cesarean section,[14] while in adulthood, a lack of microbial diversity has been shown to be associated with an increased risk of obesity, metabolic syndrome, high cholesterol levels and inflammation.[15]

Gut microbial diversity in the elderly

A reduction in in gut microbiota diversity is not only seen with obesity,[16] but also in the process of aging,[17] particularly with early frailty and the use of multiple medications.[18],[19] Reduced diversity can contribute to the increased representation of pathogens, as well as a decline in the healthy immune system response to infections. Probiotics have been shown to be effective for restoring the microbiota decline seen with aging,[20] and in clinical studies with elderly populations they have been observed to improve digestive symptoms and immune system function.[21],[22] Probiotics also have evidence indicating they may prevent C. difficile infection which the elderly, particularly those in hospitals and residential facilities, are more at risk of.[23] Bifidobacterium lactis is one specific strain being shown in the elderly to increase immune surveillance, which is not only important for infections, but also the prevention of cancer.[24]

Gut microbial diversity in children

In children, a healthy and diverse gut microbiome is important for reducing infection and may help reduce the occurrence of common pediatric conditions. Reduced gut microbiota diversity in children has been shown to be associated with allergic disease, asthma and inflammatory bowel disease (IBD) in children.[25],[26],[27] Good results with the probiotic combination of Lactobacillus acidophilus DDS® -1 and B. lactis UABLA-12™ have been seen in clinical trials in settings of atopic dermatitis and respiratory tract health. In children ranging from one to three years in age, the combination was shown to significantly improve eczema symptoms and reduce the need for topical hydrocortisone cream.[28] It also reduced the time to recovery from acute respiratory tract infections as well as symptom severity and the need for medications.[29],[30]

The gut microbiota will clearly be a focus of research for microbiologists and physicians for years to come, however, studies such as these increasingly shed light on the importance of the healthy bacteria in the gut. Select strains of probiotic bacteria such as B. lactis and L. acidophilus have been demonstrated to deliver therapeutic benefit and are important to consider when supporting the health of the young and the elderly.

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