Thirty five percent of the Giardia-free and 29% the Giardia-infected children showed daily zinc intakes below the recommended values . In 1999, the National Survey of Nutrition estimated that about 60% of the general population of Mexico and 57% the northern region had daily zinc intakes below the recommendation . The present study suggests that schoolchildren in northwestern Mexico have fewer incidences of zinc-deprived diets than the general national population. Low levels of parental education, poor income and household conditions, high overcrowding, poor sanitary conditions, and low quality of drinking water are associated with a high prevalence of intestinal parasitic infections [4, 40]. Our study population was attending public schools located in underprivileged areas, and no differences were found in the socioeconomic variables between the Giardia-free and Giardia-infected groups.
A Mexican study revealed a higher prevalence of parasitic infections in 65 out of 372 (22.5%) children with lower nutritional status, compared to 32 out of 285 (11%) in better-nourished schoolchildren . In 2005, another study in Turkey  showed higher Z scores for H/A and W/A in a Giardia-free group than in the corresponding group of Giardia-infected schoolchildren. Intestinal parasites are associated with childhood malnutrition, where malnutrition may increase the susceptibility to parasites, or parasites may deteriorate the nutritional status of the host [2, 43]. In our study, the children free of Giardia and/or other pathogenic parasites showed significantly higher Z scores for W/A and H/A than the Giardia-infected children.
In the present study, no cases of wasting were observed, in agreement with the National Survey of Nutrition in 1998 . Probably, in regions with both chronic nutrient deficiency and infections, children adapt their stature to their lower weight. In this way, children may appear to have a normal W/H, but in reality, they are children of low stature. This condition is referred as "homeorrhesis" .
One published Mexican study  has shown that 20% of 219 schoolchildren from rural southern Mexico had serum zinc levels less than 1.6 μmol/L. This suggested that our schoolchildren are less nutritionally zinc-deprived than those from the southern region, who probably consume less dietary zinc. In 1999, 66% of the southern general population did not meet the daily zinc recommendation, and 34% of the children under five yr old showed zinc deficiency .
In the present study, giardiasis was the difference in the observed results as evidenced by the parasitic treatment. This finding is supported by both the non-significant increase in the mean serum zinc levels from baseline to six months after in the Giardia-free group and the significant larger increased serum zinc value observed in the Giardia-infected group than in the Giardia-free group. Although the association between malabsorption and giardiasis is well documented, little is known about the giardiasis-zinc interaction. Recently, two studies from Turkey [15, 16] showed that Giardia-infected groups of children (45 and 34 respectively), 2 yr to 14 yr old, had lower mean serum zinc levels than their matched Giardia-free group (10.3 μmol/L vs. 220.2 μmol/L, and 16.7 μmol/L vs. 20.8 μmol/L, respectively). Another study from Turkey  found a significant increase in the mean serum zinc levels after treatment (8.7 μmol/L vs. 14.8 μmol/L) in 20 Giardia-infected children of ages 3 mo to 14 yr. In contrast, a Spanish study  found no change in the mean serum zinc levels before (14.1 μmol/L) and three months after treatment (14.1 μmol/L) in 25 Giardia-infected children of ages 6 yr to 9 yr. It must be remarked that the Turkish and Spanish children were from low and medium socioeconomic levels respectively, and the baseline means for serum zinc in the Turkish children were lower than in the Spanish children. The children in our study were from low socioeconomic level, and showed a mean serum zinc level at baseline similar to that of the Spanish children. In addition, the mean serum zinc levels were always above 10.7 μmol/L. On the other hand, it is possible that a no detected increase in zinc intake due to a raised awareness of the issue in mothers during the study course could have been related to the increased serum zinc levels in these children. However, recalls revealed no zinc supplementation or increased food intake in this study. In addition, the main foods recognized as major zinc contributors to the children's diets were milk, corn tortilla, ground beef, eggs, sausage, flour tortilla, beans, cheese and chicken, and they were invariably present in the recalls collected at baseline and follow-up.
How zinc metabolism is compromised by G. intestinalis is not well known, but it is hypothesized that the increased intestinal absorption of zinc associated with anti-Giardia treatment may be explained by the restoration of intestinal mucosa that had been impaired by giardiasis . However, our cross-sectional data analysis showed no significant differences between the Giardia-free and the Giardia-infected groups in this study. Although some limitations in the cross-sectional analysis may explain these findings: a) The distinction between the Giardia-free and Giardia-infected groups may hide the real association between giardiasis and low serum zinc, because all the children could have been infected with Giardia sometime prior to this study. b) If Giardia-free children showed no infection because of recent treatment, their serum zinc levels might yet not have been restored to a point significantly different from that in the Giardia-infected children. c) The duration of the Giardia infection may not have been long enough to significantly decrease the serum zinc levels in the Giardia-infected children. d) The sample size may not have been large enough to find a significant difference between the groups. Despite this, the validity of this study was supported by the findings of the longitudinal data analysis in the Giardia-infected and Giardia-free groups.