Characteristics: Zinc is an essential trace element known to play an important role in all human cells. The human body contains approximately 2 g of zinc distributed in all body tissues and fluids, with 60% found in skeletal muscle and 30% in bone mass. Zinc belongs to the class of type II nutrients, which are considered cellular building blocks, and is therefore essential for the synthesis of any new tissue.

Absorption: Dietary intake of zinc in healthy adults is 6–15 mg/day; however, less than half of this is absorbed. Zinc absorption is affected by many factors. Foods with a high content of phytates (nuts, seeds, grains, legumes) significantly reduce the absorption of zinc due to the formation of insoluble complexes. Diets rich in calcium can cause zinc deficiency. Soy protein also has a negative effect on the bioavailability of zinc. In contrast, the amount of animal protein in food is positively correlated with zinc absorption, and the amino acids histidine and methionine, and various organic acids present in food, such as citric, malic, and lactic acid, may also increase absorption. As such, zinc, like iron, is best absorbed from animal food sources. These facts may partly explain why vegetarians are at risk of zinc deficiency. Several papers report that the bioavailability of zinc from dietary supplements is significantly better than from food. However, longer-term studies have revealed that this changes within a few days and becomes comparable to the absorption seen from food, as the body has compensatory mechanisms to keep zinc levels in the body in a delicate balance.

Dietary Supplements: Zinc sulfate and zinc gluconate are the most common forms of zinc found in commercially manufactured supplements. Other forms such as picolinate, zinc methionine and carnosine complexes are being investigated. The latter specifically to eliminate poor stomach tolerance. Animal studies indicate that zinc sulfate is approximately twice as bioavailable as zinc oxide (in dogs, horses, and rabbits). Methionine and zinc propionate are also better absorbed forms. Zinc absorption from supplements containing zinc citrate or gluconate is similar, approximately 61% in young adults, absorption from supplements containing zinc oxide is 50%.
Natural sources: Meat, liver, eggs and seafood are the best sources in terms of quantity and quality of zinc. Zinc is also found in nuts, legumes, and various seeds, but due to their high phytate content, they are a far worse source. Phytates can be reduced through fermentation or sprouting. Other dietary sources include miso, tofu, brewer's yeast, mushrooms, and green beans.

Effect: Zinc participates in countless chemical reactions that are important for the normal functioning of the body, such as carbohydrate metabolism, protein and DNA synthesis, protein digestion, bone metabolism and antioxidant systems. Zinc is involved in the proper functioning of both the innate and the acquired immune system. Lack of zinc adversely affects the proper functioning of the gastrointestinal tract. It is found in high concentration in the pancreas, where it helps release insulin and further regulates its action. Zinc affects mental health because it is an important regulator of neurotransmitters. The highest concentration of zinc is found in the prostate, while its decrease is associated with impaired fertility and higher oxidative damage. In humans, zinc is essential for the formation and maturation of sperm, for ovulation and for fertilization. A positive relationship between seminal zinc levels and sperm counts and motility has been demonstrated. As an antioxidant, zinc protects vulnerable sperm from reactive oxygen radicals, heavy metals, fluoride, cigarette smoke and heat. Zinc affects the metabolism of sex hormones and is considered a key element during pregnancy, necessary for the proper development of the fetus. Similarly, zinc is essential for the proper functioning of the mammary gland. Zinc limits oxidant damage in many indirect ways. It protects against the depletion of vitamin E, controls the release of vitamin A, affects antioxidant enzymes, limits the production of free radicals, and absorbs them itself. Zinc is an essential cofactor in wound healing and infection prevention, its deficiency slows epithelization and leads to delayed healing. Administered orally and externally, it has proven effective in acne therapy.

Deficiency: Since there is only a small supply of zinc in the body, deficiency symptoms develop quickly. In growing children, deficiency signs and/or symptoms may develop within a few days, while in adults it may take several weeks. Once zinc intake is increased, the condition quickly returns to normal. Young children aged 1–3 years, adolescent females and the elderly aged ≥ 71 years have the lowest percentage of "adequate" zinc intake and are therefore most at risk of zinc deficiency. Detecting clinical signs of deficiency is difficult because the ubiquity and versatility of zinc in cellular metabolism suggests that zinc deficiency can lead to impairment of many metabolic functions. Loss of appetite, impaired perception of taste and smell, slowed growth, hair loss, skin inflammation and impaired healing, diarrhea, impaired immunity, depression, erection problems are usually observed. Traditionally, zinc supplementation has been used to treat or prevent deficiency in conditions such as acrodermatitis enteropathica, anorexia nervosa, malabsorption syndromes, conditions associated with chronic, severe diarrhea, alcoholism and liver cirrhosis, diabetes, HIV and AIDS, recurrent infections, severe burns, post major surgery, Wilson's disease, and sickle cell disease.

Recommended daily dose: adult: 11 mg for men, 8 mg for women (11 mg during pregnancy, 12 mg during breastfeeding).

Adverse Effects: Mild gastrointestinal distress has been reported at 50–150 mg/day.

Interactions: Zinc has the potential to interact with certain medications. Both quinolone and tetracycline antibiotics should be taken at least 2 hours before or 4-6 hours after taking zinc. Zinc may reduce the absorption and effect of penicillamine, a drug used to treat rheumatoid arthritis and Wilson's disease (at least 1 hour apart). Thiazide diuretics increase the urinary excretion of zinc, which reduces its serum concentrations. Coffee, calcium, and iron (25 mg) reduce the absorbability of zinc (2 hours apart).

Pregnancy: safe in usual doses.

Breastfeeding: safe in usual doses.

Toxicity: Symptoms of intoxication are nausea, vomiting, diarrhea, fever, and lethargy and have been observed after ingestion of 4-8 g of zinc. Single doses of 225–450 mg zinc usually induce vomiting. Doses of zinc ranging from 100 to 150 mg/day interfere with copper metabolism and cause hypocuprinemia, red cell microcytosis, and neutropenia with long-term use. These doses of zinc can also interfere with the absorption of magnesium.


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