Severe iodine deficiency in utero can cause the profound mental retardation of cretinism, but milder deficiencies also take an intellectual toll. In the republic of Georgia, for instance, a widespread iodine deficiency is estimated to have robbed the country of 500,000 IQ points in the 50,000 babies born in 1996 alone. The depletion of human intelligence on such a scale is a profligate, even criminal, waste. Vitamin A deficiency, which affects about 100 million young children worldwide, was long known to cause blindness. But it has become increasingly clear that even mild vitamin A deficiency also impairs the immune system, reducing children's resistance to diarrhoea, which kills 2.2 million children a year, and measles, which kills nearly 1 million annually.
Many children suffer from multiple types of malnutrition, so numbers tend to overlap.
But it is estimated that 226 million children are stunted, shorter than they should be for their age. A study in Guatemala found that severely stunted men had, on average, 1.8 fewer years of schooling than those who were non-stunted, while severely stunted women had, on average, one year less. Every additional year of schooling translated into 6 per cent more in wages. Nearly 67 million children are estimated to be wasted, which means they are below the weight they should be for their height. And about 183 million children weigh less than they should for their age.