Undernutrition is a critical global health issue affecting nearly one in five children worldwide, often leading to impaired growth, developmental delays, and increased susceptibility to infections. However, it's not just a matter of inadequate food intake or nutrient deficiencies. Recent research has unearthed a fascinating player in this complex issue: the gut microbiome. This microscopic community inhabiting our intestines plays a crucial role in how we digest food, train our immune systems, and ultimately, how we grow and develop.
Traditionally, undernutrition has been classified into three types: wasting (acute weight loss resulting in low weight-for-height), stunting (chronic undernutrition leading to low height-for-age), and being underweight. According to estimates, 149 million children under five are stunted, and 45 million are wasted, predominantly in low- and middle-income countries (LMICs). Alarmingly, undernutrition is implicated in nearly half of all deaths in children under five, as it weakens the immune system and worsens the risk of fatal infections.
While food scarcity and poor nutrition are significant factors, they don't paint the full picture. Pathogen exposure and impaired gut function, known as environmental enteropathies, also play a role. Studies have shown that merely increasing food intake often fails to fully correct stunting. For example, nutritional supplements only correct about 10% of the linear growth deficit in children with stunting. This suggests that other overlooked biological pathways could be at play.
The microbiome, consisting of trillions of microorganisms, primarily bacteria, residing in our gut, emerges as a crucial biological mediator in this context. Research has revealed that the gut microbiome heavily influences weight gain, immune system training, and overall health. Factors such as genetics, age, sex, and environment—ranging from geographic location to lifestyle—shape this intricate microbial ecosystem.
Early life, particularly the first three years, is a critical period for both microbiome and child development. The establishment of the gut microbiome begins at birth, mainly through vertical transmission from the mother. The initial colonizers are facultative anaerobes from the mother’s skin, gut, and oral cavity, such as Escherichia, Enterobacter, Enterococcus, Staphylococcus, and Streptococcus. These set the stage for subsequent microbes, primarily obligate anaerobes like Bifidobacterium, Bacteroides, and Clostridium.
Breastfeeding plays a pivotal role in early microbiome development, significantly increasing the levels of beneficial Bifidobacteria in infants. As solid foods are introduced and breastfeeding declines, the gut microbiome undergoes significant changes, marked by increased diversity and metabolic shifts. A longitudinal study of 903 infants across four high-income countries—Germany, Finland, Sweden, and the US—highlighted three distinct phases in microbiome development. The developmental phase (3-14 months) sees a predominance of Bifidobacterium, followed by increasing diversity of major phyla. The transitional phase (15-30 months) is characterized by a rise in Bacteroidetes and a decrease in Proteobacteria, while the stable phase (31-46 months) shows little change in composition and diversity.
While these trends are consistent in industrialized countries, they vary significantly across different lifestyles. For instance, infants in non-industrialized regions or those with traditional lifestyles show higher levels of Prevotella and Faecalibacterium and more uncharacterized species, compared to their industrialized counterparts dominated by Bacteroides. These differences underscore the significant impact of environment and lifestyle on microbiome development.
Disturbances in the coevolution of the host and microbiome can have profound implications on child development, affecting immune functions, intestinal development, energy harvest, neurodevelopment, growth, and weight gain. For example, children with slower weight gain in a Swedish cohort displayed lower microbial diversity and reduced abundance of beneficial bacteria like Faecalibacterium and Ruminococcus.
One of the critical periods for modifying the gut microbiome is during the transition from exclusive milk diets to complementary foods. This phase is also crucial for the child's nutritional status. Poor feeding practices and higher pathogen exposure during this time can disrupt the microbiome, influencing the child's development. Studies in Bangladesh have shown significant differences in the gut microbiota of acutely undernourished infants compared to healthy ones, with an increase in Proteobacteria (including pathogens like Klebsiella, Escherichia, Shigella, and Streptococcus) and a decrease in beneficial Bacteroidetes and overall microbial diversity.
Another study highlighted the lower abundance of Bifidobacterium infantis in severely acute malnourished (SAM) infants in Bangladesh compared to healthy peers. This bacteria is known for its ability to metabolize a wide range of human milk oligosaccharides (HMOs). In African and Asian countries, children with SAM showed a depleted presence of several Bacteroides species, indicating profound differences in microbiome composition between healthy and malnourished children.
Given the vital role of the microbiome, leveraging this knowledge could revolutionize interventions for undernutrition. Rather than merely increasing food supply, future strategies could focus on enhancing the microbiome to improve nutrition absorption and boost immune function. Probiotics and prebiotics, which are beneficial bacteria and their nutrients, respectively, can be tailored to correct microbiome imbalances in undernourished children.
However, the study field is not without challenges. Data collection in LMICs can be particularly tough due to limited resources, and variability in microbiome samples between populations adds another layer of complexity. Additionally, the observational nature of many studies hampers our ability to make causal inferences. Future research must aim to overcome these limitations with larger, more diverse, and controlled studies.
Understanding the intricate relationship between the microbiome and child development opens new avenues for combating undernutrition. As researchers delve deeper, the promise of microbiome-centered therapies grows. One potential aspect of future research could focus on enhancing the child microbiome through maternal health interventions during pregnancy, emphasizing a holistic approach that starts before birth. Tailoring interventions to various environments and lifestyles will also be crucial, ensuring that solutions are effective across diverse populations.
Ultimately, solving undernutrition might not solely hinge on food availability but also on a deeper understanding of these microbial allies in our gut. By harnessing the power of the microbiome, we have the potential not only to nourish but to fundamentally transform childhood development and health outcomes worldwide. As stated in the research, “The microbiome might not just be a new organ but an exciting frontier in the fight against one of our oldest enemies—childhood undernutrition."
