In 1869, zinc was identified as a mineral essential for the growth of living organisms. Today, this trace mineral is widely known for its role in supporting immune function, especially as an effective preventative method against the common cold.1
Zinc is the second most prevalent mineral in the body and is required for the proper functioning of almost every cell. Its importance is reflected in the fact that it is a crucial component of some 300 metalloenzymes and 2,000 transcription factors (proteins) which are required for the regulation of lipid, protein and nucleic acid metabolism.2
Research has revealed the central role of zinc in DNA protection,3 reproductive and hormonal health, immune status, wound repair and cognitive function to name but a few. While it has been estimated that approximately two billion people are affected by zinc deficiency worldwide,4 this data is not inclusive of those who may be suffering from marginal zinc deficiency5,6 which can also contribute to issues such as impaired development and increased susceptibility to infection.7
Today we’re taking a deep dive into all things zinc where we will be exploring its origin and functions; key research, common supplemental forms and important considerations to make in order to maximise uptake in the body.
Origin and Functions
Zinc’s primary function is as a prosthetic group for several enzymes called metalloproteins which are involved in gene expression, cell proliferation and signal transduction. The superoxide dismutase (SOD) enzyme is one such protein; a powerful endogenous antioxidant which regulates levels of harmful superoxide molecules created as a by-product of oxygen metabolism. Without SOD, superoxide can cause significant cell damage.8 Sub-optimal zinc levels may reduce the activity of SOD, as well as other important enzymes in the body.
The best food sources of zinc generally come from animal origin; with animal tissues (lamb and beef), eggs and fish (especially shellfish) showing the greatest concentrations. With that said, plant-based options such as pumpkin seeds, legumes, cashew nuts, mushrooms and spinach are also available – just be sure to consider the dietary factors which influence zinc absorption, as outlined below, in order to maximise uptake into the body.
- Iron: the divalent metal transporter DMT-1 is involved in the absorption of both iron and zinc in the intestinal tract.9 Supplemental iron causes a dose-dependent decrease in zinc absorption, while zinc supplementation also has an inhibitory effect on iron absorption. This inhibitory effect was only observed when both minerals were taken at doses higher than 10mg.10,1
- Copper: clinical studies have indicated that taking doses of zinc up to 50mg daily can significantly affect the bioavailability of copper as it competes with copper to bind with metallothionein;10 the binding protein which brings zinc and copper into the intestinal cells. However, it appears that the ratio between zinc and copper may have a greater impact on health outcomes than copper concentration itself.11
- Magnesium: zinc and magnesium will also potentially compete for intestinal absorption, however this was only noted at a dose of 142mg of zinc.12.
- Calcium: intake of calcium also appears to affect the absorption of zinc. Doses of calcium between 600-1200mg reduced zinc absorption by as much as 50%.13
Studies into zinc supplementation have demonstrated several effects on various systems in the body:
Zinc affects multiple aspects of the immune system, from maintaining macrophage and neutrophil function, to supporting natural killer cell activity. As zinc is required for protein synthesis, it is needed for the production of immune antibodies. Zinc is essential for activation of the phagocytic function of granulocytes, as well as for stabilising plasma subcellular membranes. It further modulates the production of the inflammatory cytokines TNF-α and IL-6, while reducing the production of nitric oxide, an inflammatory mediator. In fact, the immune system cannot function properly without sufficient zinc levels.14
Approximately 6% of the body’s zinc is held within the skin cells. It is not surprising then that suboptimal zinc concentrations have been indicated in a number of common skin disorders such as acne, rosacea, psoriasis and eczema.15 For example, in one study patients suffering from acne vulgaris had significantly lower serum zinc levels than those in the control group,16 while oral zinc was effective for rosacea after 3 months of therapy.17 Furthermore, zinc has demonstrated a clear ability to prevent UV-induced damage, thus reducing the incidence of malignancies.18 For this reason, zinc oxide is still widely used as a physical sunscreen.
Histamine and the allergic response
Zinc can inhibit the release of histamine from basophils and mast cells,19 which may support a reduction in symptoms caused by histamine excess such as itchy eyes and a runny nose.
The intestinal tract is protected by a layer of specialised epithelial cells which are held together by proteins called tight junctions. These tight junctions control what can pass between the bloodstream and the digestive system. If these proteins fail, intestinal permeability is increased. Heightened intestinal permeability or ‘leaky gut’ has been indicated in many gastrointestinal disorders. Following 8-weeks of zinc supplementation, patients with Crohn’s disease showed a reduction in their lactulose: mannitol ratio (an indicator of intestinal permeability); 83% of patients had normal intestinal permeability and no relapse of Crohn’s symptoms following the experiment.20 These results were mirrored in a similar study which also identified zinc levels as a factor in increased intestinal permeability.21
Zinc plays a central role in the production and regulation of the sleep hormone melatonin. Following an 8-week supplementation period which included zinc, magnesium and melatonin, improvements were observed in ease of getting to sleep, sleep quality, total sleep time, alertness and behavioural integrity in individuals experiencing long-term insomnia.22
Zinc is involved in gene transcription at various levels via participation in histone deacetylation reactions.1 Moderate increases in zinc supplementation resulted in heightened levels of proteins involved in DNA repair and, possibly as a consequence, decreases in leukocyte DNA breaks.3
Zinc is important for healthy neurological function. Low levels of urinary zinc have been linked to hyperactivity in children.23
Trace mineral concentrations are a key consideration in many forms of hair loss. Serum zinc concentrations have been shown to be lower in patients diagnosed with alopecia areata, male pattern hair loss, female pattern hair loss and telogen effluvium (temporary hair loss).24
Zinc has become well-recognised as an essential micronutrient for infant growth and development and is a standard component of parenteral nutrition for infants with gastrointestinal dysfunction or low birth weight. Zinc is not only vital to support the healthy growth, development and maintenance of bones,25 but 6 months of zinc supplementation increased both the weight-for-age and height-for-age score in infants and children with failure to thrive.26
Hormonal health and fertility
Zinc plays a crucial role in hormonal and reproductive health, with zinc deficiency linked to an impaired ability to convert cholesterol and lipid precursors into sex hormones.27 Furthermore, the steroid and thyroid hormone receptors essential for hormone binding to facilitate their effects are classed as zinc-finger proteins, meaning that they contain one or more charged atoms of zinc.15
- Males: zinc stores are positively correlated with serum testosterone levels,28,29 with animal studies demonstrating an increase in circulating testosterone concentrations following oral intake of 20mg/kg of zinc.30 It appears that zinc is a 5α-reductase inhibitor and so can reduce the conversion of testosterone into dihydrotestosterone (DHT).31 Increased levels of DHT have been observed in infertile men, making zinc an important consideration for fertility.32 Zinc also supports normal testicular development and aids sperm motility.33
- Females: increased oestrogen receptor expression may play a role in breast cancer34 and this heightened receptor expression was observed in rodents fed a zinc-deficient diet.35 A systematic review also linked low levels of zinc to premature births, prolonged labour and low birth weight.36
Zinc supplementation appears to be protective against cognitive decline.37 It can reduce free copper levels in the blood, which are correlated with cognitive loss and may be elevated in Alzheimer’s disease.38. Other protective effects are attributable to zinc’s antioxidant activity and restoration of neuronal zinc.
Patients with depression appear to have lower levels of circulating zinc;39 with treatment-resistant patients having serum levels that are even lower,40 suggesting that symptoms of depression worsen as zinc levels are reduced. A number of mechanisms have been suggested including zinc’s role as an antagonist at the NMDA-R (N-methyl-d-aspartate receptor) which has been found to possess an antidepressant effect.41
Signs of zinc deficiency
|· Diminished smell or taste
· Poor vision and/or night blindness
· Slow wound healing
· Loss of appetite
· Weight loss
· White spots of finger nails
· Apathy or low mood
|· Hair loss
· Behavioural changes
· Skin lesions
· Delayed sexual and bone maturation
· Increased susceptibility to infections
Who is most at risk for zinc deficiency?
While zinc levels should be considered in all individuals,6 there are certain factors which may lead to an increased risk of deficiency:
- Dietary insufficiency can quickly lead to low circulating zinc.
- Alcoholism is linked to poor absorption of zinc in the body.42
- Diabetics (both type I and type II) have increased rates of urinary excretion which appears to reduce cellular concentrations of zinc.43,44
- Pregnant and breastfeeding women have increased zinc requirements.
- Strict vegetarians/vegans or any group who have a high intake of foods rich in phytic acid such as grains and legumes.
- Elderly individuals (over 65 years) appear to have lower levels of serum zinc.
- Increased sweating i.e. athletes or menopausal women experiencing night sweats may have increased zinc losses.45
Common forms of supplemental zinc
There are many forms of zinc compounds available:
- Zinc carbonate
- Zinc chloride
- Zinc citrate
- Zinc bisglycinate
- Zinc sulphate
- Zinc picolinate
- Zinc ascorbate
Research regarding the efficacy of one form of zinc over another is limited and may be subject to many individual variables. With that said, a small randomised, double-blind crossover study found that the bioavailability of zinc citrate was greater than that of zinc gluconate and zinc oxide in those receiving 10mg of elemental zinc.46 When compared with zinc picolinate and sulphate, the citrate form of zinc was better utilised in rodent models.47 Zinc bisglycinate also appears to be well absorbed when compared to zinc gluconate.48
Further comparative studies are required to establish clear indications of superior bioavailability between the available forms. Chelated forms of zinc are being investigated for their bioavailability, however the research is still very limited.
Dietary factors that influence zinc absorption
As with certain nutrients, fractional absorption of zinc appears to decrease with increasing total amounts in food. Other factors such as phytic acid and protein content also appear to impact absorption and so should be considered when designing meals for boosting zinc levels:
- Phytic acid: phytate or phytic acid found in many plant foods such as corn, rice, legumes and spinach has an inhibitory effect on zinc absorption.49 Phytic acid can bind to minerals such as zinc and form insoluble complexes. As the human gastrointestinal tract has limited phytase activity (the enzyme which breaks down phytic acid), this can have a significant impact on absorption. Fermentation, soaking and/or sprouting of foods prior to consumption can lower the levels of phytic acid present.50
- Protein: the quantity of protein in a meal is positively correlated with zinc absorption, however the source does matter. Animal protein, such as beef and eggs, appears to counteract the inhibitory effect of phytate on zinc, while the milk protein casein can impede zinc absorption.51
- Chlorogenic acid: found mostly in caffeine, chlorogenic acid appears to decrease the absorption of zinc.52 For this reason, zinc supplements should be taken away from caffeinated beverages.
How much zinc can be supplemented?
Researchers have recommended that women get 17mg and men 20mg of zinc daily for optimal health;6 however, the UK Recommended Nutrient Intake (RNI) is 9.5mg per day for men and 7mg for women. As the average UK diet provides approximately 9mg, most people are achieving the UK RNI which may be sufficient to prevent an overt deficiency but lower than the level needed for optimal health. The European Food Safety Authority suggests an upper limit of 25mg daily for adults,53 while researchers have used up to 50mg of zinc for 90 days without observable adverse effects under tightly controlled study conditions.54
Precautions when taking zinc
- Diabetics: zinc has beneficial effects on glycaemic control.55 Therefore, high dose zinc-containing products should be used with caution in people taking medication for diabetes.
- Drug interactions: zinc may reduce the efficacy of certain antibiotics and HIV medications. Those who are currently receiving drug therapy should speak with a qualified practitioner before taking zinc at levels higher than in a multivitamin and mineral supplement.
- Zinc is the second most prevalent mineral in the body and is required for the proper functioning of almost every cell in the body.
- It is a crucial component of over 300 enzymes which are required for metabolic activity.
- Research has revealed the central role of zinc in DNA protection, reproduction, hormone synthesis, immunity, gastrointestinal and cognitive function to name but a few.
- The best food sources of zinc generally come from animal origin such as meat, fish and eggs, however plant-based options include pumpkin seeds, legumes, mushrooms and spinach.
- Certain nutrients such as iron, magnesium and calcium can affect the absorption of zinc, while high zinc intake can impact copper levels within the body.
- Dietary factors such as protein and phytic acid content in food may affect the body’s ability to absorb zinc.
- Certain demographics are at a greater risk of zinc deficiency such as the elderly, pregnant or breastfeeding women, athletes or people who sweat frequently, as well as vegans and vegetarians.
- Check for contraindications before supplementing high dose zinc and be aware that zinc supplementation can reduce levels of other trace minerals in the body, such as copper and iron. We would usually recommend a good multivitamin/mineral supplement as a foundation. Supplementing with additional zinc is usually only recommended on a short- or medium-term basis.
If you have any questions regarding the topics that have been raised, or any other health matters please do contact me (Tracey) by phone or email at any time.
[email protected], 01684 310099
Tracey Hanley and the Cytoplan Editorial Team
Cytoplan related products
Wholefood Zinc: providing 7.5mg of zinc made from hydroponically-grown brassica (a cruciferous vegetable, similar to broccoli).
Zinc Citrate: providing 30mg of elemental zinc in citrate form.
Zinc & Copper: 15mg of Food StateTM zinc is combined with 1mg of copper grown on a food base of inactive Lactobacillus bulgaricus.
CoQ10 Multi: All-encompassing Wholefood vitamin and mineral formula which contains 7.5 mg of zinc per capsule.
References available upon request.