Breastfeeding is optimal for infants, but many receive formula. This study aimed to compare the metabolic responses to different infant formulas (varying in protein type and quantity) with human milk (HM) in adults, as a model to understand infant responses. Infant formulas use either intact or partially hydrolyzed protein, with varying whey-to-casein ratios and protein concentrations. While partially hydrolyzed whey formulas are used for infants at risk of atopy, their metabolic effects compared to standard formula are unclear. Diets with low glycemic and insulinemic responses are beneficial for preventing type 2 diabetes and cardiovascular disease. Milk, including infant formula and breast milk, produces a disproportionately high insulin response relative to its low glycemic index, influenced by protein and lipid content. The insulin response differs based on the protein source (casein vs. whey) and its form (intact vs. hydrolyzed). Some studies suggest that formula-fed infants have higher postprandial insulin than breastfed infants, but these studies have methodological limitations. This study hypothesized that insulin responses to intact and partially hydrolyzed formulas would be similar, and aimed to investigate the metabolic effects of different forms and quantities of protein in infant formula compared with HM.

The literature review highlights the benefits of breastfeeding and the prevalence of formula feeding. It discusses the composition of human milk, including its protein content and whey-to-casein ratio. The review also examines studies on the relationship between glycemic and insulinemic responses and the effects of different types of protein on glucose metabolism. Previous research showed varying results on the comparison of insulin responses to intact versus hydrolyzed proteins, with some studies showing no significant difference while others showed a greater response to hydrolyzed protein. Methodological limitations in infant studies prompted the use of an adult model in this study.

This was a randomized, double-blinded, crossover study. Thirty-five healthy adults (aged 20-50 years, BMI 19-25 kg/m²) were recruited. Participants consumed 600 ml of one of four products: intact protein formula (INTACT), partially hydrolyzed whey formula (PHw), high-protein partially hydrolyzed whey formula (HPPHw), or human milk (HM, n=8). The study used a crossover design, with participants consuming each of the assigned products on separate test days. Blood samples were taken before and at intervals after ingestion to measure glucose, insulin, C-peptide, GLP-1, glucagon, free fatty acids, triglycerides, and amino acids. A two-week washout period for men and a four-week washout period for women separated test days. Statistical analysis employed a mixed model, using a likelihood ratio test and permutation tests to account for the non-validity of asymptotic statistics for mixed models. Bioequivalence was assessed using the European Medicines Agency (EMA) guideline.

Twenty-nine participants completed the protocol for the infant formula analysis and 8 for HM comparison. Postprandial insulinemia and glycemia were similar for INTACT and PHw, and close to those of the HM group. However, HPPHw significantly increased postprandial insulin (iAUC: p<0.001 for INTACT, PHw, and HM comparisons) and glucose (iAUC: p=0.003 for INTACT; p=0.001 for PHw; p=0.41 for HM) responses compared to all other groups. HPPHw also significantly increased C-peptide, triglycerides, and essential amino acid levels compared to the other groups. No significant differences in GLP-1 and free fatty acids were observed among the formula groups. Bioequivalence between INTACT and PHw was demonstrated for both insulin and glucose, based on EMA criteria, meaning that their responses are similar. The study confirms that there is no apparent difference in insulin response between intact and partially hydrolyzed whey protein for up to 20g/day of protein in an adult setting.

This study provides evidence for similar metabolic responses to intact and partially hydrolyzed whey protein formulas at lower protein levels, comparable to human milk. The results support previous research showing no significant difference between hydrolyzed and intact protein formulas. The higher protein load in HPPHw resulted in significantly higher insulin and glucose responses, which could support the early protein hypothesis suggesting that excess protein in infancy may increase growth mediators, leading to weight gain and adiposity. The increased responses to HPPHw observed for C-peptide, triglycerides, and essential amino acids support this interpretation. Differences in glucagon responses across treatments were observed in this study, but the clinical significance requires further investigation. Differences in other metabolites, such as free fatty acids and active GLP-1 were not statistically significant among the groups. These findings provide evidence to guide the formulation of infant formulas.

The study demonstrated similar metabolic profiles for INTACT and PHw formulas, comparable to human milk. The high-protein HPPHw formula showed significantly higher insulin and glucose responses, potentially supporting the early protein hypothesis. While the study used an adult model, the relative differences between the formulations might be similar in infants. Future research should investigate the metabolic effects of different formulas in infants and explore factors like gastric emptying rate to improve understanding.

The study used an adult model, which may not perfectly replicate infant metabolic responses. The smaller sample size in the human milk group limits the statistical power of comparisons with this group. The volume of formula consumed was higher than a typical infant feeding, though this was chosen to match a small adult meal. The study didn't directly measure gastric emptying rate, which could further explain metabolic differences.