Dietary Zinc Deficiency Affects Blood Linoleic Acid: Dihomo gamma linolenic Acid (LA; DGLA) Ratio; a Sensitive Physiological Marker of Zinc Status in Vivo (Gallus gallus)

Zinc is a vital micronutrient used for over 300 enzymatic reactions and multiple biochemical and structural processes in the body. To date, sensitive and specific biological markers of zinc status are still needed. The aim of this study was to evaluate Gallus gallus as an in vivo model in the context of assessing the sensitivity of a previously unexplored potential zinc biomarker, the erythrocyte linoleic acid: dihomo gamma linolenic acid (LA:DGLA) ratio. Diets identical in composition were formulated and two groups of birds were randomly separated upon hatching into two diets, Zn( plus ) (zinc adequate control, 42.3 μg per g zinc), and Zn( minus) (zinc deficient, 2.5 μg per g zinc). Dietary zinc intake, body weight, serum zinc, and the erythrocyte fatty acid profile were measured weekly. At the conclusion of the study, tissues were collected for gene expression analysis. Body weight, feed consumption, zinc intake, and serum zinc were higher in the Zn( plus ) control versus Zn(  minus ) group. Hepatic TNFalpha (TNFa), IL1beta (IL1b), and IL6 gene expression were higher in the Zn( plus ) control group, and hepatic delta (d)6 desaturase was significantly higher in the Zn( plus ) group. The LA to DGLA ratio was significantly elevated in the Zn( minus  ) group compared to the Zn( plus ) group (22.6  plus or equal to  0.5 and 18.5 plus or equal to 0.5, per cent  w per w, respectively). This study suggests erythrocyte LA to DGLA is able to differentiate zinc status between zinc adequate and zinc deficient birds, and may be a sensitive biomarker to assess dietary zinc manipulation. (Authors abstract)

Zinc is one of the most abundant trace minerals in cells, and is essential for growth and development of nearly all organisms. With 1.5 to 2.5 g of zinc present in the average adult , zinc is second only to iron in total body trace mineral content. It is found primarily in tissues such as the brain, kidneys, pancreas and liver with smaller concentrations in hair, skin and fingernails. However, despite an increasing understanding of zinc homeostasis, the paucity of sensitive zinc biomarkers, as well as a representative animal model in which to test them, has made assessment of zinc deficiency difficult to both quantify and categorize. Purported biomarkers such as hair, urinary, and fecal zinc  have shown mixed efficacy as sensitive biomarkers of zinc status during dietary intervention; these discrepancies may be independent of differences in experimental protocol. The need to develop additional robust indicators of zinc status and expound upon the already known clinical markers, for which limited data of reliability exists, is evident. The broiler chicken (Gallus gallus) matures relatively quickly and is sensitive to dietary manipulation of various micronutrients such as zinc. Using this animal model, the authors identified and implemented a previously unexplored biomarker of zinc status pertaining to erythrocyte d6 desaturation. Desaturase enzymes have both a requirement for zinc and a relatively low binding constant, thus their activity is quite sensitive to early stage zinc deficiency. What ensues is a disturbed ratio of their substrates and products, in this case linoleic acid and dihomo gamma linolenic acid, respectively. The d6 catalyzed step required for conversion of LA to DGLA is usually the highest flux pathway, so an elevation in the LA to DGLA ratio may be a sensitive marker for zinc deficiency. The authors hypothesize that the LA to DGLA biomarker assessed in this study will respond to changing levels of dietary zinc. Such a biomarker could be used to rapidly screen zinc status in vivo. Therefore, the primary objective of this study was to evaluate the response of the LA to DGLA ratio, as well as other indices of zinc nutriture, to dietary zinc deficiency in the Gallus gallus model. This study reports on the initial findings of the development and implementation of the LA to DGLA biomarker. In the present study, body weight, zinc intake, and serum zinc appear to be accurate markers of zinc deficiency, as they responded to zinc depletion in a dose dependent fashion. Feather and nail zinc concentrations seem to beefficacious biomarkers of zinc status, as well. These results are in agreement with previous studies in humans which have found serum, hair and nail zinc concentration  reflective of dietary zinc intake. The relative increase in gene expression of the cytokines TNFa, IL1b, and IL6 in the Zn( plus ) group is in agreement with previous in vitro and in vivo studies where expression of various cytokines, especially TNFa and IL 6, was downregulated in zinc deficient birds due to the dependency of the immune system on zinc. The increase in hepatic d6 desaturase expression in the Zn( plus ) group was also expected, since zinc is an essential cofactor for the d6 desaturase enzyme. As such, zinc deficiency would impede functioning and gene expression of hepatic d6 desaturase.

The results of the LA to DGLA ratio indicate that it is sensitive to changes in supplemental zinc intake. The results suggest the erythrocyte LA to DGLA ratio may be a sensitive tool to investigate the effects of in vivo dietary zinc manipulation under controlled dietary and environmental conditions. Further, an additional value of this biomarker is that it can be used to assess outcomes of changing levels of dietary zinc rapidly, as demonstrated in the current study where significant differences between groups occurred within seven days. This biomarker could possibly be used to detect early stage zinc deficiency before the onset of symptoms and the progression to a more serious disease state. Among purposed zinc biomarkers, LA to DGLA ratio appears to be sensitive to dietary zinc depletion between bird groups over time. (Editors comments)