As a window of opportunity for prevention,nutrition in the first 1000 days of lifeoffers the chance to achieve ideal growth,metabolic advantages, and optimal health inlater life. Nutritional components of infants’diets and their impact on anthropometricaldevelopment and metabolic mechanismsare therefore studied with huge interest.Protein, as a substantial macronutrient inhuman milk and in infant formula is a keydriver in this process of early metabolicand anthropometrical programming.The“Early Protein Hypothesis” is based on theobservation that the increased protein intakeby infant formula in contrast to humanmilk increases early weight gain, which is aknown risk factor for later obesity.With the European Childhood ObesityProject “CHOP” we examined the impactof protein supply by formula on differentgrowth parameters and metabolic changesthroughout early to late childhood. In fiveEuropean countries, about 1600 infantswere recruited to be either randomizedto formula with higher (HP, 2.05 g/dl) orlower (LP, 1.25 g/dl) protein content or tobe followed as a breastfed reference group.The difference in protein intake achieved bythe first year’s intervention was about 1 g/kgbody weight which equates the differencesobserved between conventional formulafeeding and breastfeeding in observationalstudies. Energy intake did not differ betweengroups Both formula groups showed a more rapidweight gain in the first year compared tobreastfed infants. But the increased proteinintake in the HP group was associated witha 0.23 SD higher BMI z-score (P<0.01) at2 years compared to the LP group. The LPgroup closely resembled the growth patternobserved in breastfed infants. Since heightwas not different between the groups,BMI differences may be explained by anincreased accumulation of body fat mass. Inthe long-term follow-up we observed a 0.51kg/m² higher BMI (P<0.01), a higher FMI(0.16 kg/m², P=0.02), and a 2.4 fold (P=0.02)increased obesity risk comparing the HP tothe LP group at the age of 6 years.The power of programming growthpatterns up to early school age by modifyingthe infants’ diet in the first year of life offershuge possibilities for the improvement oflater health. Breastfeeding with its beneficialeffects should be promoted. Modification ofprotein quality and quantity in infant formulamay help to fill the gap, if breastfeeding isnot possible. Lowering the protein supplyof infants and toddlers may be an effectivestrategy for obesity prevention.