In a recent systematic review published in the journal Advances in nutrition, Researchers conducted a systematic review to reveal the relationship between human milk macronutrients and infant growth. Their study included 57 publications on the topic, including 5,976 mother-infant dyads. The study results explain the significant variation in methodological and ethnographic results between different studies. However, digestible carbohydrates resulted in increased infant weight gain, while milk protein content was positively associated with increased infant length. Surprisingly, milk fat was not associated with any growth metric across the studies reviewed here.
Revaluation: Human milk macronutrients and infant growth and body composition during the first 2 years: a systematic review. Image credit: HTeam/Shutterstock
Human breast milk (HM) is considered the best source of nutrition for infants, especially in 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 sources of nutrition for two years or more. HM is a complex nutrient source consisting mainly of water (87%) and macronutrients (13%).
Breast milk contains protein, fat and carbohydrates, the latter being a major source of energy for exclusively breastfed infants. HM’s research has identified the benefits of amino acids and fatty acids (FAs), which together form macronutrients essential for a child’s immune system, ideal metabolic function and development.
Although carbohydrates comprise the second most abundant component of HM (after water), only 4.6–6.0% of these carbohydrates are digestible by the infant’s body. Importantly, studies have revealed that maternal diet has little or no effect on the composition and relative abundance of HM components, suggesting that strong purifying selection prevents alteration of its biological function. Despite substantial research on individual HM components (carbohydrates, proteins, and fats), a considerable amount of successful research exists. Furthermore, little effort has been made to synthesize the overall benefits of these components or to suggest a standardized method for conducting and reporting field results.
About the study
The aim of the present systematic review is to synthesize and validate previous literature on the relationships between HM and child anthropometry, particularly growth, during the first two years of life. Given the large amount of HM literature available, this review comprises the last of three systematic reviews evaluating micronutrients, bioactives and macronutrients.
This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Despite the initial intention to include a meta-analysis, severe heterogeneity between studies made this impossible. This study, therefore, also followed the systematic review without meta-analysis (SWiM) guidelines.
Publications for review were collected from four online scientific databases, namely Medline, EMBASE, Cochrane Library and Web of Science Core Collection. All databases were used from inception to March 2020, with follow-up in March 2022. Inclusion criteria included publications in English and both observational reports and randomized controlled trials (RCTs). Treatment includes breastfeeding as the exclusive source of nutrition or in combination with other food sources. Outcomes of interest included measures of growth, particularly weight, length, body mass index (BMI) and growth velocity.
Included publications were assessed for quality using the Newcastle-Ottawa scale (maximum score = 18), with publications classified as high (score > 13), moderate (7–13) or low (<7) quality. As meta-analyses could not be performed, statistical analyzes included generation of heatmaps to visualize and report summary associations following SWiM guidelines.
The database search identified 9,992 publications for study inclusion screening. Title and abstract screening trimmed the number to 937 articles, and full-text screening resulted in a final dataset of 141 publications, of which 57 related to macronutrients and were therefore used in the present study.
Key research findings have revealed that a mother’s milk does indeed have a profound effect on her baby’s growth. HM protein was found to positively affect infant length, and carbohydrate concentration, particularly the digestible, portion, was positively associated with infant weight gain. Although some studies have identified a positive relationship between child growth and amino acids, the studies presented in these studies differ in study design, and thus these results need to be verified.
Total fat concentration showed an inverse relationship with BMI and a positive relationship with increasing age-for-age Z-score (WAZ) and body fat and weight gain. Again, however, these studies appeared to demonstrate suboptimal sampling methods, particularly the lack of accounting for variation in HM fat concentrations over the day.
“Our finding of no consistent relationship between fat and infant growth outcomes may be related to several factors. Fat content in HM is highly variable within a feed and across feeds during the day. Most of the studies included in the current review used single. “Milk sampling time, usually morning milk was captured and many did not account for changes in fat content between the start and end of the feed.”
Unexpectedly, fructose concentrations positively affected infant growth, weight, fat mass, and fat-free mass. Previous research has identified HM fructose concentration as one of the few HM parameters that varies according to maternal diet. Consequently, high sugar maternal diets may be identified as an important intervention area for future breastfeeding mothers.
Although macronutrients are arguably the most extensively researched components of HM, a vast majority of the literature demonstrates suboptimal sampling and analysis methods, making their findings unreliable for future research. Encouragingly, increased HM analyzer accessibility allows current and future studies to follow consistent analytical techniques, improve sampling optimality, and allow comparison of results between studies.
The present systematic review aimed to investigate the relationships between breast milk and infant growth and found that HM carbohydrate and protein positively influence infant weight and length, respectively. Fat, while presenting a positive association with child growth, requires future verification before the results can be trusted.
“Synthesis of the literature was limited by methodological issues with milk collection techniques and inadequate reporting of results. Moving forward, researchers should use existing validated HM analysis techniques rather than HM analyzers to assess macronutrient content and develop time-reflective sampling protocols.” Variation in HM Macronutrients, Especially the fat content. Furthermore, emphasis should be placed on investigating HM as a biological system that operates within the larger mother-infant biological context rather than examining individual HM components in isolation.”