Gut microbiota and obesity

This summary is adapted from “Gut Microbiota and Obesity: A Role for Probiotics” by Abenavoli et al. 2019

The worldwide prevalence of obesity has almost tripled since 1975, with it acting as a contributing factor in the deaths of at least 2.8 million people annually1. There has long been interest in the gut microbiome, which constitutes the microorganisms which live in the human intestinal tract, but research in this area has really taken off in recent times. The number of annual publications on this topic has quadrupled since 2005. The human gut microbiome mainly consists of bacteria, with 500-1000 distinct bacterial species potentially influencing various aspects of human health, including obesity. This paper is a literature review, which aimed to provide an overview of the evidence for association between gut microbiota and obesity. To achieve this, the authors searched for papers related to metabolism, gut microbiota, dysbiosis, and obesity, and summarised them in this review.

The colonisation of the gut and the development of the gut microbiome begins at birth, as the gut is essentially free from microorganisms at delivery. This immediate colonisation is influenced by multiple factors, such as the microorganisms present in the mother and in the birth environment2. Following this, are three main periods of change in the gut microbiome during life: birth to weaning, weaning to a normal diet, and adult to elderly. Infant feeding has important implications for the first period, birth to weaning. The bacterial composition in the infant gut is distinctly different in breastfed subjects compared to those fed with formula3. Benefits have been associated with the microbiota of breastfed infants, such as improved digestion and absorption function of particular vitamins, along with stimulating the immune system to reduce the risk of allergies3.

Related to obesity, the number of a specific group of bacteria in the gut, Bifidobacteria, was higher during infancy in children who remained at a normal weight compared to children who became overweight4. This was validated in another study which showed reduced levels of Bifidobacteria in children at three months old who were obese at 10 years old. The number and composition of bacteria in the gut in obese and normal children has not been found conclusively to differ, therefore the composition of the infant microbiome influenced by feeding has a lasting effect and appears to play a role in obesity development. This is further emphasised when the gut microbiome begins to move towards the adult composition after solid food is introduced, as there is relatively little change in composition after childhood.

There has also been evidence of interaction between the gut microbiome and obesity in pregnancy. There are significant microbial differences in pregnant women depending on their body mass index, along with variations in microbial composition depending on the amount of weight gain in the mother during pregnancy. These compositions have been associated with health measures such as levels of HDL cholesterol and folic acid levels5.

In adults, there have been multiple studies showing an altered ratio of the groups of bacteroidetes and firmicutes bacteria in the human gut microbiome in obesity. However, many studies have contradictory results so the association appears to be complex, and is yet to be fully elucidated. Twin studies have been carried out to eliminate the environmental and genetic influences, but they have yet to make the picture clearer.

Another part of the association between the gut microbiome and obesity that is still being investigated is the mechanisms through which the bacteria in the gut exert their effects, both beneficial and detrimental. Gut bacteria closely interact with metabolism, by converting complex nutrients to more easily digestible forms, transforming dietary fibres into simple sugars for absorption, and influencing the gut immune system. How these functions are altered to influence the development of obesity has not yet been explained. Related to the treatment of obesity, there have been some positive effects seen with the use of probiotics in children who are obese, but results have been conflicting and have not been replicated in adults. 

The overall relationship between the gut microbiota and obesity is still to be fully understood. However, there is some evidence that breast milk promotes a gut microbiome which can have a protective effect on obesity. In the coming years we can expect to see more high-quality studies further classifying the relationship due to progress in the methods used to analyse the microbiome.

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  2. Francino MP. Birth mode-related differences in gut microbiota colonization and immune system development. Annals of Nutrition and Metabolism. 2018;73(3):12-6.
  3. Goldsmith F, O’Sullivan A, Smilowitz JT, Freeman SL. Lactation and intestinal microbiota: how early diet shapes the infant gut. Journal of mammary gland biology and neoplasia. 2015 Dec;20(3):149-58.
  4. Kalliomäki M, Carmen Collado M, Salminen S, Isolauri E. Early differences in fecal microbiota composition in children may predict overweight. The American journal of clinical nutrition. 2008 Mar 1;87(3):534-8.
  5. Collado MC, Isolauri E, Laitinen K, Salminen S. Distinct composition of gut microbiota during pregnancy in overweight and normal-weight women. The American journal of clinical nutrition. 2008 Oct 1;88(4):894-9.
Gut microbiota and obesity

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