When we look at a biological variable in a population we can establish a set of "normal" values. It is perfectly possible, however, that the "normal" population does not represent a healthy population. One example of this is weight. In many Western countries it is now "normal" to be overweight or obese and thus at greatly increased risk for diseases such as heart disease, stroke and cancer. In this case measuring a population to establish a "normal" set of values for weight would set the "normal" upper limit of weight in the obese or grossly obese range.

There is a body of evidence that the normal values for laboratory tests of iron levels include many individuals who are iron deficient. Iron is necessary for the formation of haemoglobin and low iron can cause low haemoglobin levels. However, symptoms of iron deficiency and fatigue will occur before anaemia becomes apparent. This is because iron is necessary for several steps in the conversion of glucose to energy. In particular the mitochondria.

One laboratory blood test of iron stores is Ferritin. At our local laboratory the Reference Range for Ferritin is set at 4.6 (4.6mcg/l) to 204 for women and 21.8 to 274 for men.¹ The lower limit for women has recently been adjusted downwards. A recent UK study of women with fatigue showed that there was a positive response when iron supplementation was given to women who had Ferritin levels below 50.² There is much debate but it is probable that Ferritin levels should be at least 30^3,4 for optimal function and we advise our athletes to maintain Ferritin levels above 50 for maximum aerobic performance.

Iron deficiency is of particular concern in children. Ferritin levels below 30 have been shown to correlate with attention deficit.⁵ Also there have been several studies that have shown that iron deficient children have poor cognitive development, particularly of verbal learning and memory,^6,7 that is reversible with iron supplementation. It is rational that iron deficiency causes cognitive impairment as the brain, despite its relatively small size accounts for 20% of the body's energy consumption. A recent study found children with ADHD to have lower levels of iron, which correlated with cognitive abnormalities.⁸ Restlessness and irritability in children who are deficient in iron has been related to increased concentrations of catecholamines, which return to normal after treatment with iron.⁹ Adolescent females with low iron levels have been shown to have cognitive impairment and lowered IQ's, that is treatable with iron supplementation.¹⁰

I recently looked at 40 consecutive Ferritin measurements in our expatriate paediatric patients. The mean was 22 and the range 5.3 - 61.5. It is not at all uncommon to see Ferritin levels in single figures in expatriate children. This should be seen in the context that 10% of children in some English Schools Foundation schools have special educational needs.¹¹ By contrast the mean Ferritin level of 20 of our Chinese paediatric patients was 37.9 (range 17.8 to 94), which may reflect a difference in the Western diet of the expatriate families and that used by the local Chinese population here.

Iron is just one of several important trace elements including zinc and selenium. It is unlikely that deficiency in one trace element (Iron) will occur without deficiency of other trace elements, particularly in children. Trace element deficiency is a worldwide problem with 30% of the world's population estimated to be zinc deficient.¹² Zinc, Iron and Selenium are necessary for proper immune function and Zinc supplementation has been shown to decrease hyperactivity in children.

How has this come about? The most important factor is dietary change. Our ancestral diet had a far higher nutrient density and far lower energy density. Farming was only introduced 10,000 years, or 400 generations ago, far too soon for significant genetic adaptation to take place. Since then the proportion of simple carbohydrate and fat in the diet have increased dramatically. Simple carbohydrates such as rice, wheat, corn, potatoes and sugar are products of agriculture, and wild animals (including humans) are never fat. A UK study showed that between 1961 and 1985 consumption of fruit fell by a quarter and green vegetables by a half.¹³ In a large American survey 17% of the population ate no vegetables (including potatoes) on the day of the survey.¹⁴ It is a myth that a modern Western diet is adequate. 90% of UK females of childbearing age do not achieve the recommended daily intake of elemental iron (14.8 mg) from their diet.¹⁵ In addition to a decrease in micronutrient levels modern diets contain high levels of phytate and phosphate, which prevent absorption of iron.

Iron deficiency is more common in adult women than adult men, as shown by the "normal" values which is probably mostly due to menstruation. Among the higher apes only humans show a difference in iron status between adult males and females.¹⁶ Family planning and bottle-feeding may account for this difference. Along with a diet high in micronutrients our far female ancestors had more pregnancies and breast fed for far longer and so had far fewer periods (and thus less iron loss) in a lifetime.

There are probably several nutrients and measures of nutrition where normal values do not reflect healthy values. As local diets have changed from traditional to Western there has been an increase in childhood obesity. It is possible that iron and other micronutrient deficiency will become more common if a low micronutrient, high energy density Western diet is adopted by the Hong Kong population.

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