The range of climates, micro-climates, technological level, cultivar availability, weather patterns, land use patterns, farming/gardening practices - to name a few - of urban areas, considered globally, make it impossible to determine useful comparative statistics.

In general, horticultural techniques yield more food value per land area and diverse agricultural production operations (many different crops and mixed-use areas) yield the highest food value.

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre

Hello technopolitical.  I take the point of the next comment below : it is almost impossible to make valuable statistics on comparative horticultural productivity because of the dramatic differences in location, climatic variation, soil quality etc.

Data on adult diets and the number of kilocalories or joules are undeniably important. However, one should also take into account that the daily consumption of fresh food (vegetables and fruits) plays an extremely important role in public health, particularly for children, because of a significant enhancement in vitamins and mineral elements. Having a small family garden or a school garden will perhaps not solve ALL the food problems of the rural or urban poor, but it certainly will avoid a number of classical diseases and hunger.

Prof. Dr. Willem Van Cotthem Beeweg 36 - B 9080 Zaffelare (Belgium)