Zinc Digestion and absorption
The digestion of zinc entails the decomposition process known as hydrolysis, which is facilitated by the acidic environment of the stomach. Then the small intestine takes over to handle the task of absorption. However, zinc is not one of the more easily absorbed minerals. Even in healthy individuals, zinc absorption is usually only about 20-40% of the amount ingested. In others, the zinc absorption efficiency can be much less.
The bioavailability of zinc is affected by a combination of many factors. In fact, some of the same ones that that affect the absorption of iron can also influence the absorption of zinc. For example, the high phytic acid content of foods like legumes, seeds, and cereals, interferes with the absorption of both minerals. The phytates bind to the zinc, just like they do with the iron, to form insoluble compounds that can make the zinc difficult to absorb.
Other important food sources of zinc that can cause problems with the zinc’s absorption are those high in oxalic acid (like spinach, chard, and chocolate) and coffee and tea, which contain polyphenols. Even other essential minerals such as calcium and iron compete with zinc for absorption, with the unfortunate result that the high absorption of one nutrient may be at the expense of another.
The presence of calcium, phosphorus, copper, iron, folic acid, lead, and cadmium all inhibit the absorption of zinc and vice versa. The high zinc content in milk is somewhat negated by the fact that milk is also high in calcium. However, the zinc in breast milk is easier to absorb than the zinc in cow’s milk. The zinc in high fiber foods like bran and other whole grain products may be difficult to absorb due to the binding properties of the fiber. On the other hand, the greater ratio of zinc to the toxic mineral cadmium in whole grain foods as compared to processed foods decreases the likelihood of absorbing the cadmium.
On the other hand, a few substances such as glucose, lactose, soy protein, and even red wine improve zinc absorption. The amino acids in a high protein diet chelate to the zinc in a form that makes the zinc easier to absorb.
As a rule, zinc that comes from animal foods like meat is easier to absorb than zinc that comes from plant sources. Zinc is also more readily absorbed in an acidic environment.
Another very important set of factors in zinc absorption includes those relating to the health of the individual. Some people are born with certain conditions that interfere with normal gastrointestinal functioning. Other conditions are acquired later in life.
Normally, secretions from the bile and pancreas into the gastrointestinal tract serve as a source of reusable endogenous zinc to supplement the ingested zinc. This is one way in which the body can maintain serum zinc level homeostasis. But in individuals with reduced biliary and pancreatic secretions, this protective mechanism is lacking.
Zinc malabsorption can occasionally be caused by a very severe genetic condition called Acrodermatitis Enteropathica. The anomaly is already observed in the first few months of life and characterized by extremely poor absorption of dietary zinc. Infants with this disease fail to grow properly and have severe dermatitis, including reddened and scaling skin around the scalp, eyes, and feet. They also have chronic diarrhea. Approximately 1 in 500,000 babies are born with this disease. Survival depends on lifelong treatment with zinc supplements.
Various other diseases and physiological conditions can also impair zinc absorption. A few examples are irritable bowel syndrome, inflammatory bowel disease such as Crohn’s disease, and celiac disease characterized by an inability to tolerate gluten.
Because there are so many different factors entering into the equation, the 20-40% absorption rate mentioned as the average can actually vary considerably, not only in different individuals, but even from time to time in the same individual. In healthy individuals, a compensatory mechanism is in effect to help offset either taking in too little zinc or too much zinc. When zinc intake is low, more is absorbed; when zinc intake is high, less is absorbed. This up and down adaptation of the body helps most people maintain zinc homeostasis even with a less than ideal diet.
Zinc absorption is largely dependent on two proteins. The protein albumin enables the zinc to be transported from the plasma into the enterocytes. Another protein, metallothionein, binds to zinc so to makes the zinc unavailable. When the body is in a state of zinc deficiency, metallophionein production decreases, so more zinc is available for absorption. When the amount of zinc in the blood is excessive, metallophionein production increases. Excess zinc that is bound to metallophionein is excreted in the feces.
Ordinarily, 80% of the unused zinc is eliminated in the feces. Some zinc is also lost in the urine and sweat. Starvation, diabetes, alcoholism, nephrosis, and cirrhosis of the liver are all associated with increased amounts of zinc appearing in the urine. Laborers and professional athletes, on the other hand, who tend to sweat profusely, are likely to lose appreciable amounts of zinc via sweat. Only small amounts of zinc are lost in hair, semen, and menses. Stress, burns, surgery, and weight loss can also contribute to zinc loss.
Zinc supplements are often prescribed, either to treat an existing deficiency or to prevent it in someone who is considered at risk. The supplements are available in a variety of forms—tablets, lozenges, throat and nasal sprays, and nasal gels. The chemical makeup also varies. Zinc chloride, zinc sulfate, and zinc acetate are more easily absorbed than zinc carbonate and zinc oxide. Side effects from zinc supplements are common and include abdominal pain, dyspepsia, diarrhea, nausea, and vomiting.
It is best to take zinc supplements on an empty stomach, either before breakfast or two hours after meals. Doing so will minimize the likelihood of other minerals like calcium and copper, or the phytates and fibers in foods, making the zinc difficult to absorb.
Taking high doses of supplementary zinc could cause a deficiency of other vitamins and minerals because, just like the presence of some of them inhibits zinc absorption, the presence of a lot of zinc can inhibit their absorption as well. Thus, for example, people taking zinc supplements sometimes develop a copper deficiency, which in turn adversely affects iron metabolism, possibly resulting in anemia.
In other words, it is important to maintain a proper balance of all of the vitamins and minerals, not an excess of any one at the expense of the others. For this reason, for some people, a multi-vitamin/mineral supplement might be preferable to one that contains zinc exclusively.