Human milk (HM) is the gold standard of early infant nutrition. The World Health Organization (WHO) recommend that newborns be exclusively breastfed for the first 6 months of life. It has beed reported that an exclusive breast feeding at 6 months of age range from 10 to 46%. Thus, the intake of infant formulas (IF) is quite widespread, including infants that have less than 6 months that are not receiving breast milk. Most of the commercial IF are cow's milk-based formulas that have a higher concentration of protein than breast milk and have a different protein composition. Commercial IF have a low proportion of alpha lactoalbumin, and A1 β-casein; during its digestion, a β-casomorphin 7 peptide is produced. In addition, it has a high concentration of β-lactoglobulin. Both, β-casomorphin 7 and β-lactoglobulin have been associated with a higher risk of diabetes obesity and allergies. Human β-casein does not produce β-casomorphin 7 during its digestion, it has a high concentration of alpha-lactoalbumin and does not have β-lactoglobulin.

The purpose of this study is to evaluate growth, gastrointestinal tolerance, and β-casomorphin 7 in urine of infants that are exclusively breastfed compared to infants fed three IF with different proteint content.

Developing an IF with A2 β-casein, high proportion of alpha-lactoalbumin and less protein that commercial IF, similar to breast milk (1g/100 mL), could be and important strategy to decrease the risk of diseases. This infant formula was compared with breast milk and two other IF with different protein composition. The three formulas were packed in the same presentation and identified by a number. Mothers voluntarily elected to exclusively breast-feed or formula-feed her newborn. the present study

Infants' mothers were recruited from six health centers from the Health Ministry in Querétaro, México. Directors of each health center authorized the protocol study. The field workers were read and explained the informed consent letter, the objectives, procedures, risks and benefits of the study in detail.

Inclusion Criteria:

• Full-term newborn (≥37 weeks of gestation)
• Birth weight ≥2500 g and ≤4000 g
• Apgar score > 8
• Infants from birth to 40 days of age at the time of enrollment
• Mothers voluntarily elected to exclusively breast-feed or formula-feed her newborn.

Exclusion Criteria:

• Infants with congenital heart defect, congenital illness or malformations, severe gastrointestinal disease, kidney, liver, central nervous system, or metabolic disease, or born from mothers with gestational diabetes.

The participant did not know which formula was assigned. Infant formulas were packed in the same presentation and identified by a number.