The community of microorganisms colonizing the gut changes during the first postnatal years of life. This ecosystem, henceforth described as the microbiome, modulates infant physiology and health, but uncertainty remains about the significance of variation in microbiome composition and function. Some may be tolerable, yet some microbiomes may be less healthy than others. Most efforts to identify parameters of microbiome health focus on adults, and derived concepts may not directly translate to early life that is characterized by dynamic and sequential changes. Data suggest that an orderly progression from an immature neonatal microbiome to a mature adult state is preferable to delayed or over-rapid development. This can be parameterized as a "microbiome development trajectory". Diet modifies early life microbiome development and is the principal modifiable factor to this end. Infants fed with infant formulas show different microbiome development trajectories from breastfed infants. Early data suggest that formulas containing a specific blend of human milk oligosaccharides partially mitigate this difference. Introduction of a complementary diet complexifies the identification of diet-microbiome development interactions. A better understanding will only be achievable through detailed, longitudinal characterization of large cohorts.

Humans maintain symbiotic relationships with complex microbial communities in their intestinal tracts that are paramount to their host's health and development. Given their importance, it is essential for the host to reliably acquire key members of the gut microbiota and assemble communities that provide benefits during important windows of host development. Epidemiological studies over the last 2 decades have convincingly shown that clinical and nutritional factors that disrupt early-life microbiome assembly predispose humans to infections and chronic noncommunicable diseases. These connections emphasize the importance of understanding host-microbiome assembly on a mechanistic level, the time windows that are most important for host-microbe crosstalk, and the clinical and lifestyle factors that shape and disrupt symbiotic interactions to develop therapeutic and nutritional strategies to prevent noncommunicable diseases. In this article, I will provide an evolutionary and ecological perspective on when and how humans acquire their gut microbiome, the factors that shape the assembly process, and how the process can be disrupted. I will discuss the most important time windows for both microbiome assembly and the microbiome's impact on development of the immune system. Finally, I will discuss how evolutionary and ecological principles inform strategies to support and restore the gut microbiome early in life.

This article explores the challenges and opportunities of applying Evidence-Based Medicine (EBM) to the field of gut microbiota research. EBM has revolutionized healthcare by integrating the best available evidence with clinical expertise and patient values. However, EBM has also faced criticisms such as overemphasizing results of randomized controlled trials and a lack of patient involvement. The article discusses these criticisms in the broader context of EBM and how they are particularly relevant to studies on gut health. This article also discusses the emergence of next-generation EBM methods, examining their potential strengths and limitations. For example, integrating next-generation EBM methods into gut microbiota studies offers the potential for improved understanding and patient-centered interventions. Still, it also raises questions about data quality, privacy, and patient involvement. This article concludes that as EBM evolves, careful attention must be paid to ensure that new methods are robust, transparent, and patient-centric, thereby contributing to better outcomes in gut microbiota research.

There has been a significant increase in the number of people shifting towards plant-based dietary patterns over the past decade due to interest in protecting the health of the planet as well as improving human health. Studies have shown that vegetarian diets are associated with a lower prevalence of obesity in adults and children; therefore, moving towards a vegetarian diet in childhood may help prevent obesity later in life. The VeChi study in Germany found that on average vegetarian and vegan children grew equally well as omnivorous children. It is important to ensure that children following plant-based diets have adequate amounts of key nutrients, such as energy, fats, iron, calcium, iodine, vitamin B12, and omega-3 fats. In the VeChi studies, vegan children had the lowest intakes of calcium and iodine out of the three diet groups. The vegan children also had the lowest vitamin B12 intakes without supplements, but when supplements were taken into account, they had the highest vitamin B12 intakes. Iron intake in vegetarian children is consistently reported as higher than in omnivorous children. However, iron stores (indicated by low ferritin levels) tend to be lower in vegetarian compared to omnivorous children, due to decreased bioavailability of non-haem iron found in plant foods. When introducing solids, iron-rich foods should be offered early and paired with iron enhancers such as vitamin C and beta-carotene to improve iron absorption.

Our diets are the greatest determinant of health, and what we eat is sustained and shaped by the food we produce. Food systems have increased production to feed the growing world population, which has also led to a dietary transition, with increases in energy and protein intakes, and only modest improvements in micronutrient density. Thus, undernutrition has decreased globally, while non-communicable diseases are dramatically increasing. Today, food systems are being threatened by global warming. Conversely, food systems are a major contributor to climate change and environmental degradation, generating one-quarter of all greenhouse gasses, using half of the world's habitable land, and are the largest source of water pollution. The greatest contributor to this environmental degradation is the production of animal-based foods, particularly meat. Food systems must ensure access to safe, nutritious, and sustainable foods (e.g., improving yields, reducing waste, and greenhouse gas emissions), decreasing animal-based food production, and increasing plant-based foods, which can positively impact our diets. On the "demand side", shifting our current diets from high animal-based foods to plant-based diets will decrease global mortality and disability. Our diet is at the intersection of our health and our planet's health and, thus, a major instrument to improve both.

Although malnutrition in the form of child wasting, stunting, and micronutrient deficiencies remain prevalent on many of the poorest and war-torn places on earth, there has been major progress in other regions and the direction of travel remains generally good. However, as countries pass through the economic transition there has been a seemingly inevitable rise in overweight and obesity with its attendant personal health costs (reduced life span due to obesity-related chronic conditions) and a rise in the societal costs of care. Strategies, by healthcare professionals and others, to combat the two sides of the malnutrition coin must be built on a solid foundational knowledge of the causes of each condition. The individual, nutritional, and environmental drivers are summarized here. It is sometimes helpful to focus on a single unifying concept as a way of rationalizing the causes and required solutions; namely the nutrient density of foods. Malnutrition is caused, inter alia, by foods lacking in sufficient energy, protein, and micronutrients. The same is true for obesity which, in large part, is driven by foods overly dense in energy but lacking other critical nutrients. Food quality therefore emerges as a key concept that healthcare professionals can adopt as they educate parents and children at the microlevel and schools, health systems, and government bodies at the macrolevel.

Improved maternal nutritional status is hypothesized to promote good pregnancy and infant health outcomes but trial evidence supporting the commencement of nutritional supplementation before conception is sparse. The NiPPeR (Nutritional Intervention Preconception and During Pregnancy to Maintain Healthy Glucose Metabolism and Offspring Health) multinational double-blind randomized controlled trial conducted in the United Kingdom, Singapore, and New Zealand tested a nutritional formulation containing myo-inositol, probiotics, and multiple micronutrients (intervention), compared with a standard micronutrient supplement (control), taken at preconception and throughout pregnancy. The primary outcome of gestational glycemia at 28 weeks' gestation showed no difference. However, differences in several prespecified secondary outcomes were notable. The intervention reduced the incidence of preterm delivery particularly those associated with preterm prelabor rupture of membranes, operative delivery for delayed second stage, and major postpartum hemorrhage. It may also shorten time to conception in overweight women, to that similar to nonoverweight/obese women. Importantly, the intervention associated with a reduction in the incidence of rapid infant weight gain and high body mass index at 2 years among offspring. Such evidence indicates the potential for preconception maternal nutritional interventions to have appreciable impact in shaping the long-term health of an individual and building resilience against noncommunicable chronic diseases in the future.

Malnutrition is present in most countries of the world. This ranges from general undernutrition due to insufficient food, or poor-quality diets low in some essential nutrients, to overnutrition and obesity with energy-rich but nutrient-poor diets. The fundamental aim of dietary recommendations is to prevent deficiency diseases, and the assumptions which underpin these recommendations need to be understood when considering what advice to give to the general public or individual patients. This is particularly relevant in early life as the nutritional state and dietary intake of the mother are of major importance for both her and her baby's health. There is a particular concern with pregnancy in teenage women, as they are still likely to be growing and have different nutrient requirements compared to older women. There is now evidence of beneficial effects for both the mother and baby of supplementation of the mother's diet in those with a low nutritional status. For infants, early gut microbiome development is supported by human milk components (including oligosaccharides) and the reported health benefits are of growing interest and offer potential areas for future developments. Yet, the increasing overweight and obesity in children are a serious concern, in both developed and developing economies. Considerations of the achievements, challenges, and future directions of early life nutrition need to be addressed in a global environment in which every country in the world is experiencing some form of malnutrition. The term malnutrition encompasses a number of different scenarios ranging from undernutrition, which encompasses an inadequate nutrient intake in a diet with a low level of diversity, up to overnutrition where there is an excess of energy intake in a diet which is predominantly composed of nutrient-poor foods. The major feature of malnutrition is that there is micronutrient inadequacy, and even deficiency, which is particularly concerning in early life. The present chapter will consider the major achievements and future challenges in relation to achieving optimal nutrition in early life as well as in older children. Clearly, when considering nutrition in children, it is important to also consider the nutritional state of women before, during, and after pregnancy, as this can have a major impact on the fetus and young child. Before considering these issues in detail, this chapter will begin by addressing the basis on which nutritional recommendations are founded and the challenges that have to be met in getting novel recommendations approved by the appropriate authorities.

Eating behavior and food preferences are shaped in early life and contribute to lifelong food choices. Much of the current dietary advice for infants and toddlers focuses on the nutritional quality of foods, with less emphasis on food sensory qualities. However, exposure to age-appropriate sensory properties, such as tastes and textures, are key in shaping early-life eating behaviors and food preferences. During weaning, new-borns rely on reflexes such as sucking and rooting to get sufficient nutrient intake. Around 6 months of age infants transit from dependent feeding with liquid foods such as breast or bottle feeding, to independent feeding with solid foods. During this rapid learning period, the infant must learn to sit upright and balance their head and quickly develop in terms of oral anatomy, emerging of teeth as well as the muscle coordination needed to orally process food. Different product textures require unique oral processing skills that have to be acquired through experience with food oral breakdown and swallowing. These early food experiences shape the eating behaviors that become habitual and are carried forward into later childhood. Early life feeding strategies vary widely across populations but become all the more challenging in specific child populations such as children who received early life tube-feeding and children with developmental challenges are further complicated by anatomical issues and acquired negative associations with food. Due to the significance of early life food sensory exposure in shaping dietary behavior, there is a need for science-based recommendations to help guide this sensory learning to inform dietary behaviors in both healthy and clinical child populations.

Human milk is the recommended sole source of nutrition for infants during the first 6 months of age, thanks to its composition rich in nutritious and bioactive components. Progress in analytics has allowed for a detailed description of its components and their variability within and among mothers. This is especially valid for the human milk oligosaccharides (HMOs) that represent one of the major human milk compound groups. The stages of lactation and maternal genotypes are the main contributors to the variability of HMOs, although other maternal and environmental factors also contribute to the variation, which may be important for adaptation in evolutionary terms. Today, mainly individual HMOs or structural groups of HMOs were associated with infant outcome measures, ranging from anthropometry to immunity and brain development (social and cognitive skills). Mechanistic insights can partly explain some findings, yet there is a lack of consistency between the different observational studies of breastfed infants. Gaining a better understanding of the reasons behind these disparate findings is the key element going forward. Furthermore, studying human milk components, like HMOs, and their expected benefits using a systems biology approach can reveal further important insights. Here, we discuss recent findings with the perspective to learn more about the link to health outcomes.