Nitrogen was the most commonly yield-limiting nutrient in all pre-industrial agricultures. Only the Haber-Bosch synthesis of ammonia broke this barrier. The rising dependence on nitrogenous fertilizers, which represents the largest human interference in the biospheric N cycle, has two different roles. In affluent nations it helps to produce excess of food in general, and of animal foods in particular, and it boosts agricultural exports. But for at least a third of humanity in the world's most populous countries the use of N fertilizers makes the difference between malnutrition and adequate diet. Our understanding of human N (protein) needs has undergone many revisions and although some uncertainties still remain it is clear that average protein intakes are excessive in rich countries and inadequate for hundreds of millions of people in Asia, Africa, and Latin America. More dietary protein will be needed to eliminate these disparities but the future global use of N fertilizers can be moderated not just by better agronomic practices but also by higher feeding efficiencies and by gradual changes of prevailing diets. As a result, it could be possible to supply adequate nutrition to the world's growing population without any massive increases of N inputs.