In this era of acrimonious debate about what goes..........when one is not deficient is not helpful and can be harmful.

In this era of acrimonious debate about what goes into intelligence, it may be a surprise to hear that in China alone, 480 million IQ points have been lost for want of a simple chemical, costing less than a few cents per person each year.

Around the world, more than 20 million people are less intelligent than they might have been because they did not receive this vital nutritional supplement. About 1.6 million people, or a quarter of the planet's population, are at risk from this deficiency. The lack of this chemical has caused the IQ bell curve of some countries to shift thirteen points to the left of where it might have been otherwise.

The substance in question is iodine, the 53rd element of the periodic table. No longer scarce in the diet of the world's wealthy nations, this micronutrient is the key to what may be the least recognized epidemic on the globe.

Micronutrients are substances required in minuscule amounts to maintain health. A person needs one ten-thousandth of a gram of iodine a day, two grams over a lifetime.

A worldwide effort is under way to eliminate iodine-deficiency disorders in this decade by fortifying the world's salt supply. Although there are other ways to deliver iodine, salt is the cheapest and easiest route.

Iodine deficiency is the world's leading cause of mental defects. It produces not only severe mental retardation, deaf-mutism and partial paralysis, but also more subtle problems such as clumsiness, torpor and reduced learning capacity.

Iodine is an essential part of the thyroid hormone, a substance that contributes to brain development during foetal life and is the main throttle of metabolism thereafter. Without enough iodine there cannot be enough thyroid hormone.

The thyroid hormone is made in a gland that wraps around the front of the neck. A shortage of this hormone causes the thyroid to grow, a condition known as goiter. Sometimes goiters are grotesquely large. Other conditions, including hormone excess, can cause the problem, but iodine deficiency is the most common.

In lower vertebrates, the thyroid hormone is responsible for such events as the metamorphosis of tadpoles to frogs and the migration of the juvenile flounder's eyes to one side of its head. In human beings, the hormone's development activities are mostly confined to the brain.

A profound lack of the thyroid hormone before birth causes cretinism (a permanent disability characterized by retardation), deaf-mutism, muscle rigidity and, often, immature skeletal growth.

Research has shown that giving pregnant women iodine supplements before the second trimester of pregnancy prevents cretinism. Supplementation of iodine later, or during infancy, can decrease, but not eliminate, neurological damage.

Studies have shown that supplementing the diets of those who are mildly iodine-deficient can measurably improve brain function. In the most dramatic study, researchers in Malawi (where in some villages 86 per cent of children were mildly iodine-deficient) supplemented the diet of grade-schoolers with iodine.

The treated group showed significant improvements on measures of verbal fluency, visual memory and eye-hand coordination. Although IQ was not measured, the investigators estimated the average gain was 21 points.

Iodine, however, should not be viewed as some all-purpose brain food. Taking extra iodine when one is not deficient is not helpful and can be harmful.

Summary: Millions of people are less intelligent and a quarter of the world's population risk neurological defects due to an iodine deficiency. It is also the main cause of mental retardation. This is because iodine is essential for brain development. In fact a person needs only as little as two grams of iodine over a lifetime. This is easily and cheaply supplemented by iodized salt. Research has shown that giving iodine supplements to pregnant women in their first trimester eliminates neurological defects in their infants. Also, research carried out among the Malawi village children who were mildly iodine-deficient showed great improvements in their IQ after they had been given iodine supplement. Although iodine is necessary for healthy brain. development, an overdose of it can be harmful.