Zinc is an essential trace element for humans. Its physiological demands are very high in adolescents that are in risk of deficiency, especially if their diets are based on cereals and legumes with high content of zinc absorption inhibitors, as in deprived populations [1
]. Zinc deficiency affects growth, sexual maturity, and the immune system [2
]. Therefore, it is necessary to reduce risk and one possibility for doing so is to use fortification of highly consumed foodstuffs or food ingredients [3
In Mexico, there is a program for fortification with zinc and other micronutrients in all commercial wheat and corn flours as well as in milk only for targeted groups. We found previously that zinc-fortified milk was effective in increasing both, intake and plasma zinc levels of adolescent girls from the northwest area of the country [5
]. Under basal conditions, 35% of the studied adolescents did not meet their zinc requirements and were facing a borderline deficiency.
There are few studies in humans about the control of zinc homeostasis after high or low supplementary levels of zinc in deprived or in borderline deficient populations [6
]. The objective of the Mexican supplementation programs is to protect individuals with high deficiencies and do not affect those with adequate intake. Therefore, it is important to understand the relationship between zinc dietary intake by fortification and its uptake and excretion, which drives the homeostatic regulation. After intake variations, the control of homeostasis can be affected by changes in the expression of zinc transporters [8
]. Some zinc transporters are tissue specific and maintain intracellular zinc concentrations in a narrow physiological range. The ZnT family of transporters decreases cytoplasmic zinc concentrations by secretion, sequestration, or efflux, whereas the ZIP family increases cytoplasmic zinc by influx or release of stored zinc [9
]. Therefore, it is likely that the expression of the ZnT genes would be up-regulated, whereas the ZIP genes could be down-regulated in response to the increase of dietary zinc intake. The aim of this study was to evaluate the changes in zinc dietary intake, absorption and plasma concentrations, as well as to investigate the expression levels of ZnT1 and ZIP1 in peripheral white blood cells from 24 adolescent girls after drinking 500 mL/day of milk fortified with zinc and other micronutrients, for 27 days.
The characteristics of the 24 adolescent girls in this intervention were almost the same of those reported in our former study [5
], except by the mean age (14.1 instead of 15.1 years old). However, there were no differences in the mean plasma zinc levels of adolescent girls in the current study, after the 27 days intervention, while in our former study zinc levels increased after drinking fortified milk in the same period. According to Andree et al.
] and Aydemir et al.
], plasma zinc levels are not the best markers for zinc status because these values would not reflect changes in zinc intakes since they are under tight homeostatic control. However, a transient increase in plasma zinc occurs during supplementation and it may be more evident if the status is initially low [18
Mocchegiani et al.
] supplemented, using 10 mg/d zinc during 48 days, old European subjects selected on the basis of low plasma zinc levels and IL-6-174 polymorphism (G/C or C+, and G/G or C− genotypes). G/G genotypes are associated with an impaired zinc status. Since zinc parameters as NK cell cytotoxicity, nitric oxide releasing and zinc erythrocyte increased in all the supplemented groups (plasma zinc ≤ 10.5 µM), including C− with unstable plasma zinc. So that the sole assessment of plasma zinc level is not a reliable indicator of zinc status at least for old people. Mariani et al.
] suggest that the potential interaction among circulating zinc increments, changes in immunological parameters and the interactive influence of +647 MT1a and −174 IL-6 polymorphic alleles, could be important determinants for evaluating the efficacy of zinc treatment and for identifying groups of subjects that can take advantage of therapeutic intervention.
To explain our findings in plasma zinc changes between both interventions using the same zinc-fortified milk for 27 days, it is important to consider the basal proportion of adolescents not achieving their requirement (EAR = 7.3 mg/day) for zinc that was 35.2% at the former study and at the present study such proportion was 16.6%. In addition, it can be related to the diet, where phytate presence and protein intake may influence zinc absorption [21
], although estimated zinc absorption appear to be similar for both studies (≈3 at baseline and ≈5 at the end of the study, respectively). In the present study done in the winter, participants consumed the same amount of phytate, 14% more energy and 20% more protein than in the former study that was made during the summer and autumn. It has been shown that higher energy and protein intakes in north-western Mexican women are mainly taken between winter and spring compared with summer and autumn [22
], and this pattern is related to the extreme desert weather.
It is obvious that the Mexican north-western adolescent population in this study had a wide variability in zinc intake and absorption enhancers. According to a Mexican national survey [23
], the highest zinc intake in adolescent girls was in the studied region with respect to the rest of the country.
On the other hand, plasma zinc concentration was negatively related to BMI in group A of the present study. As it is known, zinc metabolism is altered in obesity [24
]. In México, obesity and overweight are a public health problem, with 35.8% of the adolescent girls suffering them [25
], with similar prevalence for girls in our study area [26
]. Overweight people present low serum zinc levels [27
], and zinc content of erythrocytes is inversely related to BMI in obese women [28
]. Therefore, the high prevalence of overweight and obesity in the studied demographic area, together to insufficient zinc intake and high content of dietary phytates, could be negative factors for the zinc nutritional status of adolescent girls.
Therefore, it could be expected to find a wider variation in zinc status for the general Mexican population, with the same zinc fortification of food ingredients from the national program. Thus, it is very important to understand the relationship between zinc dietary intake by zinc-fortified milk with zinc uptake and excretion, which drive the regulation. The obtained information could be useful to provide feedback for the national fortification program if necessary.
We measured the mRNA expression of two zinc transporters of peripheral blood cells from adolescents of groups A and B, as biomarker of zinc status [6
]. Plasma zinc in group A increased by 31 µg/dL, while in group B, it decreased by 25 µg/dL. As ZIP1 (importer) and ZnT1 (exporter) are among the most abundantly expressed zinc transporters [24
] and their expressions were down-regulated and up-regulated, respectively, after short periods of zinc supplementation [6
], we selected these markers for evaluation.
In the adolescents of group A of our study, whose plasma zinc increased after they drank zinc-fortified milk for 27 days, ZIP1 was down-regulated while ZnT1 mRNA was not significantly affected. The same result of decreased ZIP1 and no change in ZnT1 expression was shown in a study with young women after zinc supplementation of 22 mg/day, for 27 days [6
]. However, the ZnT1 expression gave maximum accumulations after 2 and 8 days supplementation of 15 mg zinc/d to young healthy men [17
]. Our evaluation was done after 27 days of drinking the zinc-fortified milk, when mRNA expression probably was back to its normal level. Additionally, ZnT1 does not fit well with the modus operandi
of traditional transporters, but it affects zinc homeostasis through regulating L-type calcium channels [30
The girls in group B whose mean plasma zinc decreased, showed a wide variability in mRNA expression amounts for ZIP1 and ZnT1. Therefore, mean differences between baseline and after supplementation period with zinc-fortified milk, were not significantly different neither for ZIP1 nor for ZnT1. According to Liuzzi et al.
], the ZIP1 transporter shows a high transcript level when zinc intake is low, but when zinc repletion is reached, it rapidly down-regulates its expression to basal conditions. Zinc intake of participants from group B ranged from 9.04 to 17.07 mg/day and none of the participants had zinc intake under the requirement; their expression of mRNA ZIP1 could be in a wide range of responses. Although we found no relationship between the ZIP gene expression with plasma or zinc intake levels, Noh et al.
] suggest that the expression of zinc transporters may be altered in people with obesity, and then affect zinc homeostasis. So, it is possible that the zinc nutritional status is compromised in a high proportion of the Mexican population, including adolescent girls.