Infants and elderlies are susceptible to zinc deficiency

The importance of zinc for human health has been recognized since the early 1960s, but today there is little concern about zinc deficiency in developed countries. In this study, we measured the zinc concentration in hair from 28,424 Japanese subjects (18,812 females and 9,612 males) and found that 1,754 subjects (6.17%) had zinc concentrations lower than 2 standard deviations (86.3 ppm) below the control reference range, which qualifies as zinc deficiency. In particular, a considerable proportion of elderlies and children (20% or more) were found to have marginal to severe zinc deficiency. A zinc concentration of 9.7 ppm was the lowest observed in a 51-year-old woman; this concentration was approximately 1/13 of the mean reference level. The prevalence of zinc deficiency in adults increased with aging to a maximum of 19.7% by the 8th decade of life, and decreased to 3.4% above 90-year-old. The proportion of zinc deficiency in infants 0–4 years was 36.5% in males and 47.3% in females; these percentages were higher than the maximum prevalence in elderly subjects. These findings suggest that infants and elderlies are prone to zinc deficiency and that intervention of zinc deficiency is necessary for normal human development, health and longevity.

also increased by decade from 1.3% at the 2 nd decade to 1.5, 1.9, 3.6, 8.5, 15.4% and reached a maximum of 19.7% by the 8 th decade; it then decreased to 3.4% for ages above 90 years (Fig. 3). A significant (p < 0.001) inverse   correlation between the log of the zinc concentration and age (r = − 0.12 and − 0.14 for male and female groups, respectively) was observed (Fig. 4A,B).
The prevalence of zinc deficiency in the male and female groups of children aged 0-9 years was 29.9% and 33.8%, respectively; these rates were both higher than the maximum of 19.7% observed in the adult groups. In particular, in infants aged 0-4 years, the prevalence of zinc deficiency was 36.5 and 47.3% in males and females, respectively, and higher rates, over 50%, were observed in the age group including 2-and 3-year-old (Fig. 5). In addition, a highly significant correlation between the zinc concentration and age was observed in the child group (r = 0.298, p < 0.0001), with a plateau at ages over 10 years (Fig. 6). These findings indicate that infants are more prone to zinc deficiency than elderly individuals, which suggests that an early intervention to correct zinc deficiency is necessary for normal child development and health.

Discussion
The importance of zinc in human nutrition and health has been recognized since the early 1960s 1,2 . Because zinc is required for the synthesis and repair of DNA, RNA and proteins, this micronutrient appears to affect the basic biochemical and physiological processes involved in cell growth, cell division, cell differentiation, development and aging [1][2][3][4] . Clinical signs of zinc deficiency include acrodermatitis, suppressed immunity, diarrhea, poor healing, stunted growth, hypogonadism, fetal growth failure, teratogenesis and abortion. Zinc deficiency is also known to be associated with various diseases such as malabsorption syndrome, chronic liver disease, chronic renal disease, sickle cell disease, diabetes, malignancy, neurodevelopment disorders and other chronic illnesses [1][2][3][4][18][19][20][21][22] . Zinc deficiency also causes significant impairment in adaptive and innate immune responses and promotes systemic immune dysfunction in older populations 2,4,7,23-26 .
Hair zinc concentration is commonly used in marginal zinc deficiency studies of children, and its usefulness has been documented in many industrialized countries including Canada and the USA 27-31 . Symptomatic zinc deficiency in infants was first reported in the early 1980s, with most cases occurring in breast-fed preterm infants [32][33][34][35] because the zinc concentration of human milk is much lower than that of cow's milk, and the demand for zinc increases rapidly in thriving preterm infants 36 .
In the present epidemiological study of 28,424 Japanese subjects, we demonstrated that the prevalence of zinc deficiency in infants is higher than that in adult and elderly subjects. In children aged 0-9 years, nearly one-third (male: 29.9%; female: 33.8%) of the subjects exhibited marginal to severe zinc deficiency. In infants aged 0-4 years, the incidence of zinc deficiency was particularly high: 36.5 and 47.3% in males and females, respectively (Fig. 5). Furthermore, a highly significant correlation (r = 0.298, p < 0.0001) between zinc concentration and age was observed in these children (Fig. 6). These results suggest that children, particularly infants, are susceptible to zinc deficiency.
The mechanisms that lead to zinc deficiency in infants may include unbalanced meals, lower absorption ability in the intestinal tract 37,38 , and low zinc concentration in maternal breast milk 39 . In addition, maternal dieting and cigarette smoking have been reported to be associated with lower zinc and higher cadmium and lead concentrations in neonates 40 . Toxic metals that have accumulated in the maternal bone matrix appear to be co-transferred with calcium and magnesium to fetuses and neonates through accelerated bone-resorption during pregnancy and lactation [40][41][42] .
Severe zinc deficiency in the rare inherited human disease acrodermatitis enteropathica has been reported to result from defective intestinal absorption of zinc due to mutations in the Zip4 transporter located in the intestinal tract 43,44 . Furthermore, recent genetic studies have indicated that mutations in the ZnT2 transporter gene in mothers produce zinc-deficient milk and cause breast-fed infants to develop a severe zinc deficiency 45-47 that can  be reversed by zinc replacement therapy 47 . These genetic factors, as well as various environmental factors, also contribute to some zinc deficiencies in infants.
In adults, highly significant (p < 0.001) inverse correlations between zinc concentration and age (r = − 0.12 for males and − 0.14 for females) were observed (Fig. 4A,B). In addition, a significant age-dependent increase in the prevalence of zinc deficiency was observed in males and females: from 2.0% to 15.1% by the 7 th decade of life and from 1.3% to 19.7% by the 8 th decade of life, respectively (Fig. 3). A study conducted in five European countries reported zinc deficiency in 31% of people over 60 years of age, with some country-specific differences in prevalence 48 . Another study of hospitalized elderly patients reported 28% zinc deficiency 49 . These findings indicate that elderly individuals are prone to zinc deficiency, even in developed countries.
It is interesting that male subjects over 85-years-old and females over 90 years old exhibited a low prevalence of zinc deficiency: 3.4% or less (Fig. 3). This finding that zinc deficiency is rare in the older age groups in both genders suggests that the mineral is essential for healthy aging and longevity and that zinc supplementation is probably effective and necessary for aging well.
Recently, dietary-restriction-induced zinc deficiency has been reported to up-regulate the intestinal zinc-importer (Zip4) and induce an increase in Zip4 protein present on the plasma membrane of enterocytes 50 . This adaptive response to zinc deficiency is known to increase the risk of high-uptake of toxic metals such as cadmium and lead. Thus, individuals with zinc deficiency are likely at an increased risk of absorbing large amounts of toxic metals and retaining them in their bodies. In fact, we have demonstrated a highly significant inverse relationship between hair zinc concentration and lead or cadmium concentration 20 . Thus, not only zinc deficiency itself but also the consequent increased risk of toxic metal burden seem to induce various physical and mental disorders [18][19][20][21]51,52 .
In conclusion, the present study of 28,424 Japanese subjects demonstrates that many cases of zinc deficiency are detected in infants and elderly individuals, indicating that these populations are prone to zinc deficiency. It remains to be established whether early intervention to correct zinc deficiency leads to normal development and health and to healthy aging and longevity.

Methods
Sampling and zinc analysis. After obtaining informed consent, scalp hair samples from 28,424 (male: 9,612; female: 18,812) subjects aged 0-100 years were collected from June 2005 to September 2007 (Table 1). Hair sampling was conducted by cutting hair as close as possible to the scalp of the occipital area.
A 75 mg hair sample was weighed into a 50 ml plastic tube and washed with acetone and then with a 0.01% Triton solution, in accordance with the procedures recommended by the Hair Analysis Standardization Board. The washed hair sample was mixed with 10 ml 6.25% tetra methyl ammonium hydroxide (TMAH, Tama Chemical, Kawasaki, Japan) and 50 μ L 0.1% gold solution (SPEX Certi Prep, Metuchen, NJ, USA), and then dissolved at 75 C with shaking for 2 hours. After cooling the solution to room temperature, an internal standard solution was added. After adjusting its volume gravimetrically, the obtained solution was used for zinc analysis. The zinc concentrations were determined with inductively coupled plasma mass spectrometry (ICP-MS; 7500ce, Agilent Technologies, Santa Clara, CA, USA) by the internal standard method and expressed as ng/g hair (ppb) or μ g/g (ppm) 18,19 .
The ethical committee of the La Belle Vie research laboratory reviewed and approved this study. The methods were carried out in "accordance" with the approved guidelines. Informed consent was obtained from all subjects. All of the data collected were held securely in such a form as to ensure anonymity. Statistical analysis. Because scalp hair mineral concentration follows a log-normal distribution, the log of the zinc concentration and the geometric rather than arithmetic mean were used to represent the hair zinc concentration. The relationship between age and zinc concentration was examined using Pearson's correlation coefficient.