In a recent systematic review published in the journal Advances in Nutrition, researchers conducted a systematic review to reveal the associations between human milk macronutrients and infant growth. Their study included 57 publications on the topic, comprising 5,976 mother-infant dyads. Study findings elucidate significant heterogeneity between methodologies and anthropometric outcomes between different studies. However, digestible carbohydrates resulted in increased infant weight, while milk protein components were positively associated with increased infant length. Surprisingly, milk fat was not associated with any growth metrics across all research reviewed herein.
Review: Human milk macronutrients and child growth and body composition in the first 2 years: a systematic review. Image Credit: HTeam / Shutterstock
Mother's milk
Human breast milk (HM) is considered the best nutrient source for infants, especially during their first two years of life. The World Health Organization (WHO) recommends exclusive breastfeeding for the first six months, followed by a combination of HM and other nutrient sources for two years or longer. HM is a complex nutrient source comprising predominantly water (87%) and macronutrients (13%).
Breast milk contains proteins, fats, and carbohydrates, the latter of which is a major source of energy for exclusively breastfed babies. Research into HM has identified the benefits of amino acids and fatty acids (FAs), which together form essential macronutrients for infant immunity, ideal metabolic functioning, and development.
Despite carbohydrates comprising HM's second most abundant component (following water), only 4.6-6.0% of these carbohydrates are digestible by infants' bodies. Interestingly, research has revealed that maternal diet has little to no impact on the composition and relative abundance of HM components, suggesting that strong purifying selection prevents alterations to its biological functionality. Despite ample research into individual HM components (carbohydrates, proteins, and fats), substantial confound exists between successive studies. Furthermore, little effort has been made to consolidate these components' overall benefits in synergy or suggest a standardized methodology for conducting and reporting findings in the field.
About the study
The present systematic review aims to synthesize and validate previous literature on the associations between HM and infant anthropometry, specifically growth, in the first two years of life. Given the vast amount of HM literature available, this review comprises the last of a cohort of three systematic reviews evaluating micronutrients, bioactives, and macronutrients.
This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Despite initially intending to incorporate meta-analyses, severe heterogeneity between studies made this impossible. This study, therefore, also followed the Systematic Review without Meta-Analysis (SWiM) guidelines.
Publications for the review were collated from four online scientific databases, namely Medline, EMBASE, the Cochrane Library, and the Web of Science Core Collection. All databases were perused from initiation until March 2020, with a follow-up In March 2022. Inclusion criteria comprised publications published in English and included both observational reports and randomized controlled trials (RCTs). Treatments included breastfeeding as an exclusive nutrient source or in combination with other food sources. Outcomes in interest included growth measures, specifically weight, length, body mass index (BMI), and growth velocity.
Included publications were quality assessed using the Newcastle-Ottawa scale (max score = 18), with publications classified as high (score >13), moderate (7-13), or low (<7) quality. Since meta-analyses could not be carried out, statistical analyses comprised the generation of heatmaps to visualize and report summary associations following SWiM guidelines.
Study findings
The database search identified 9,992 publications for study inclusion screening. Title and abstract screening pruned the number to 937 articles, and full-text screening resulted in a final dataset of 141 publications, 57 of which pertained to macronutrients and were hence used in the present study.
The key study findings revealed that a mother's milk does indeed have a profound effect on the growth of her infant. HM protein was found to influence infant length positively, and carbohydrate concentrations, particularly the digestible portion, were positively associated with increases in infant weight. While some studies identified positive associations between infant growth and amino acids, these studies presented confounds in study designs, and thus these findings need verification.
Total fat concentration displayed inverse associations with BMI and gain in weight-for-age Z-Scores (WAZ) and positive associations with body fat and weight increases. Again, however, these studies were observed to display suboptimal sampling methodologies, especially a lack of accounting for variations in HM fat concentrations across a day.
"Our finding of no consistent relationship between fat and infant growth outcomes could be related to a variety of factors. Fat content in HM is highly variable within a feed and across feeds during the day. The majority of studies included in the current review used single milk sampling times, usually capturing early morning milk and many did not account for transitions in fat content between the beginning and end of a feed."
Unexpectedly, fructose concentrations positively impacted infant growth, weight, fat mass, and fat-free mass. Previous research has identified HM fructose concentrations as one of the few HM parameters that vary according to mothers' diets. As a result, high-sugar maternal diets could be prescribed to breastfeeding mothers in the future as an important intervention area.
Conclusions
Despite macronutrients being arguably the most extensively researched components of HM, a vast majority of the literature displays suboptimal sampling and analysis methodologies, making their findings unreliable subject to future research. Encouragingly, increased HM analyzer accessibility allows current and future studies to follow consistent analytic techniques, improving sampling optimality and allowing for comparing findings between studies.
The present systematic review aimed to investigate the associations between breast milk and infant growth and found that HM carbohydrates and proteins positively influenced infant weight and length, respectively. Fat, while presenting positive associations with infant growth, needs future verification before findings can be trusted.
"Synthesis of the literature was limited by methodological issues with milk collection strategies and insufficient reporting of findings. Moving forward, researchers should consider using existing validated HM analytic techniques rather than HM analyzers to assess macronutrient content and developing sampling protocols that are reflective of the temporal variation in HM macronutrients, specifically fat content. Further, increased emphasis should be placed on investigating HM as a biological system that operates within the larger maternal-infant biological context rather than examining individual HM components in isolation."
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