One of the main causes of world hunger is the diversion of vast quantities of protein, fat, and carbohydrates from the agricultural system to feed livestock. The livestock are then used to produce animal-source foods: beef, pork, poultry, farmed fish, dairy products. But the amount of macronutrients that we receive from animal products is significantly less than the amount of macronutrients fed to those animals. The problem is particularly intense in the developed world, where livestock are fed corn (maize) and soy.
According to one estimate, caloric losses to animal feed constitute about 1,700 kcal per person per day globally, whereas the calories obtained from animal foods provide only 500 kcal/person-day. [Tristram Stuart, Waste, “Losses in global food supply from field to fork”, p. 304, citing: Smil (2001), Bender (1994), and Lundqvist (2008).] The net loss is 1,200 kcal/person-day, which is about 3.0E+15 kcal per year. The one billion hungry in the world need 1.0E+15 kcal/year [2740 kcal/person-day times 365 days times one billion persons]. So the net loss is enough calories to feed 3 billion persons.
However, as I’ve discussed before in this blog, the world agricultural system produces far too many carbs, and not enough protein and fat. So the cropland used to grow cattle feed would need to be used to grow more protein and fat, and less carbs (less maize), in order to make more efficient use of the land in addressing world hunger.
Reducing the amount of land used to feed livestock means reducing our intake of animal-source foods. A one-third reduction saves enough kcal for one billion persons. But the hungry of the world generally have some food, especially carbs. They mostly need protein and fat. By my estimate (explained in my forthcoming book), the one billion hungry of the world need only 3.45E+14 kcal/year in additional calories — if those calories were appropriately distributed between protein, fat, and carbohydrates. That figure is 11.5% of the net loss of 3.0E+15 kcal per year. So a reduction of about 12% in the animal-source foods used by the world would save enough kcal, essentially meaning that it would save enough land used to grow those kcal, in order to end world hunger.
The world does not need to become vegetarian or vegan in order to end world hunger. One effective approach would be to cut back on our consumption of animal-source foods. However, as the developing nations become developed, their populations increase their intake of animal-source foods. The average world diet is about 15% animal foods, but that percent is much higher for wealthier populations and for wealthier nations.
See the 2012 FAO Hunger Report for a breakdown of the types of food eaten by different populations (figures 9 and 10).