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Is serum zinc a better way to assess for suboptimal zinc levels compared to RBC zinc?

Assessment of individual nutrients is not as cut and dried as most would think and both methods have benefits and drawbacks.

Measuring serum levels of zinc is most commonly used to detect overt zinc deficiency or toxicity. However, serum levels, which represent a fraction of the body’s stores of zinc, can be acutely affected by stress, infection, starvation, plasma binding proteins, and diurnal rhythms. Also, during insufficiency, plasma zinc may be maintained at the expense of other tissues.

Red blood cells levels are more stable and also reflect zinc-containing enzymes. However, RBC zinc may be slow to reflect a deficiency of zinc compared to serum levels.

Wood, R J. “Assessment of marginal zinc status in humans.” The Journal of nutrition vol. 130,5S Suppl (2000): 1350S-4S. 

Braun, L A, and F Rosenfeldt. “Pharmaco-nutrient interactions - a systematic review of zinc and antihypertensive therapy.” International journal of clinical practice vol. 67,8 (2013): 717-25.

Noland, Diana, Jeanne A. Drisko, and Leigh Wagner, eds. Integrative and Functional Medical Nutrition Therapy: Principles and Practices. Springer Nature, 2020.

Adequate zinc is essential to maintaining zinc-dependent metabolic processes including antioxidant enzyme function (catalase, glutathione peroxidase, copper-zinc superoxide dismutase), synthesis of alkaline phosphatase and carbonic anhydrase, immune function, protein synthesis, carbohydrate, and lipid metabolism, wound healing, regulation of inflammatory cytokines and oxidative stress, and production of insulin and thyroid hormone.

However, excess zinc can have negative consequences as well including suppression of immunity and copper depletion.

Revoredo, C. M., et al. "Zinc Status of and its Association to Cardiovascular Risk Biomarkers." Int J Cardiovasc Sci 29.5 (2016): 355-361

Olechnowicz, J et al. “Zinc status is associated with inflammation, oxidative stress, lipid, and glucose metabolism.” The journal of physiological sciences : JPS vol. 68,1 (2018): 19-31.

Prasad, Ananda S. “Zinc in human health: effect of zinc on immune cells.” Molecular medicine (Cambridge, Mass.) vol. 14,5-6 (2008): 353-7.

Shankar, A H, and A S Prasad. “Zinc and immune function: the biological basis of altered resistance to infection.” The American journal of clinical nutrition vol. 68,2 Suppl (1998): 447S-463S.

Read, Scott A et al. “The Role of Zinc in Antiviral Immunity.” Advances in nutrition (Bethesda, Md.) vol. 10,4 (2019): 696-710. 

Gammoh, Nour Zahi, and Lothar Rink. “Zinc in Infection and Inflammation.” Nutrients vol. 9,6 624. 17 Jun. 2017.

A functional, comprehensive approach is always best and should include levels of zinc in serum and/or RBCs along with serum alkaline phosphatase and evaluation of zinc deficiency symptoms including a reduced sense of taste and smell, dermatitis, poor wound healing, and compromised immune function.