Consumer-level food loss and protein overconsumption in Japan largely contribute to food nitrogen footprint

The current agro-food system requires chemical nitrogen (N) fertilizer input to maintain food production. However, the system is leaky in terms of N use efficiency, and leaking N to the environment perturbs natural N cycle in the Earth system. The present study focused on consumer aspects of the N loads that affect the agro-food system, i.e., consumer-level food loss (wasting edible food) and protein overconsumption (eating more than nutritionally recommended intake). We looked into the case of Japan, home to the world's oldest population, from 1961 to 2015, considering gender and age differences when possible. Consumer-level food loss as N was evaluated as the difference between net supply and consumption of protein N. Protein overconsumption as N was evaluated as the difference between consumption and World Health Organization-recommended intake of protein N. Consumer-level food loss was negligible in Japan until the 1970s, and then began to increase slowly, and accounting for an average of 13.2% of the net supply from 2011 to 2015. In contrast, Japanese people have overconsumed protein relative to the WHO-recommended intake since 1961, the beginning of the study period. Protein overconsumption peaked at about double of the recommended intake in the 1970s, accounting for an average of 32.3% of the recommended intake from 2011 to 2015. The tendency of protein overconsumption was a common feature across gender and age classes in Japan after 1995 when detailed data were available. Consumer-level food loss and protein overconsumption respectively averaged 0.6 and 1.3 kg N capita^-1 yr^-1 from 2011 to 2015, contributing to a total N footprint of 7.4 kg N capita^-1 yr^-1 calculated from the N-calculator method. The average N footprints of consumer-level food loss and protein overconsumption from 2011 to 2015 accounted for 39% of the total food N footprint in the same period, showing a large room for reduction. Several reduction scenarios were evaluated; a feasible scenario, the combination of a 50% reduction in food loss and a 25% reduction in protein overconsumption combined with the introduction of a diet halving the relative consumption ratio of livestock meat, was estimated to reduce the N footprint by 2.8 kg N capita^-1 yr^-1, or 15% of the Japanese current food N footprint.