Malnutrition is one of the world’s biggest challenges, afflicting 1 of every 6 of us. Although we are moving in the right direction—despite adding more than 70 million people to the global population each year, the number of those suffering from malnutrition has been falling—more than 3 million people will die this year from poor nutrition. Some 800 million are chronically undernourished.

The most well-known form of malnutrition is a lack of calories. But there is another, more prevalent form. It isn’t obvious or easily photographed, and so it attracts scant attention. Yet it could be solved with remarkable ease. It is the unsexy-sounding “micronutrient deficiency”—a lack of iodine, vitamin A, and iron.

Children lacking iodine do not develop properly, either physically or intellectually. All they need is salt fortified with iodine. An absence of vitamin A increases the risks of blindness. The nutrient could easily be made more readily available in staple food items, such as genetically modified golden rice.

Iron deficiency affects as many as 3.5 billion people—more than half the world’s population. An iron deficit stunts growth and impedes mental abilities—stealing up to 15 IQ points from the average child. It reduces a person’s ability to perform manual labor by as much as 17 percent. Today, it’s battering the health and energy of half a billion women and stunting the growth of 40 percent of the developing world’s children. Yet we already know how to solve this problem: The fortification of flour, rice, and salt is cheap and simple. In other cases, iron cooking pots, which slowly emit iron, could be distributed in poor countries.

Dealing with micronutrient deficiency would quickly and cheaply improve the lot of billions of people. According to the Copenhagen Consensus,...