Objective The aim of this article is to review why the first 1000 days of life are a vulnerable period of human development and the long‐term effects of a nutrition experiment carried out in Guatemala (1969–1977). Methods In 1969–77, a supplement called Atole, containing high quality protein, energy and micronutrients, was provided to women during pregnancy and lactation and to children <7 years of age in two villages while in two control villages a low‐energy drink called Fresco was provided. The villages were assigned at random to the treatment groups. Results Several reasons explain the vulnerability of the first 1000 days: rapid growth and development, high nutritional requirements, greater susceptibility to infections, high sensitivity to programming effects and full dependence on others for care, nutrition, and social interaction. Compared with Fresco, Atole improved total nutrient intakes (protein, energy, and micronutrients) and reduced stunting, but only in children < 3 years of age. A study in 2002–2004 showed that schooling, reading, and intelligence were improved in Atole villages, but only in those who received Atole before the age of 3 years. Wages of men were increased by 46% in those provided Atole through the age of 2 years. Findings for cardiovascular disease risk factors were inconclusive, perhaps because of the young age of the sample. A new study focusing on chronic diseases is ongoing (ages 38–54 years). Conclusions The Guatemalan studies indicate that substantial improvement in adult human capital and economic productivity resulted from the nutrition intervention. This provides a powerful argument for promoting improvements in nutrition in pregnant women and young children in low income countries.