Zinc absorption in the intestines varies from 15 to 40 percent. Zinc can be absorbed by a special protein in the intestinal cells called metallothionein. As you may recall, this is the same protein that can bind copper. Zinc is held by metallothionein until needed in the blood. If the zinc is not needed, it may be eliminated in the stool when the intestinal cells slough off. Metallothionein also binds zinc in the liver until the zinc is needed. Some zinc is transported by the iron transporter transferrin.
Zinc enters the blood surrounding the digestive system and is commonly bound to albumin, a blood protein. Iron and zinc should be somewhat balanced in the diet so that one does not interfere with the other’s absorption. Considerable amounts of zinc enter the intestines in pancreatic juices and intestinal cell secretions, as seen in Figure 12-4. The intestines are given the opportunity to reabsorb this recycled zinc along with dietary zinc, or to allow the zinc to be eliminated in the stool. Both zinc recycled from the body and zinc from food may bind to dietary amino acids, peptides, nucleic acids, and phytates in the intestines.
Zinc also binds tenaciously to the protein casein in dairy products. Most of this bound zinc passes in the stool. Urine losses are usually under 10 percent of the total zinc eliminated. Small amounts of zinc are also lost from the body in skin cells, the menstrual cycle, sweat, semen, and hair.
Figure 12-4 Zinc is returned to the intestines.
Summary for Zinc
Main functions: in enzymes, with hormones, protein structure, and diverse functions. RDA: 2 to 6 mg for children and 8 to 13 mg for adolescents and adults.Toxicity: excesses may induce a copper deficiency and gastrointestinal disturbances. Tolerable upper intake level is 3 mg for infants ranging up to 40 mg for adults. Deficiency can cause growth retardation and susceptibility to infection in children.
Sources: meat, fish, poultry, whole grains, and Brazil nuts.
Forms in the body: found in zinc-copper superoxide dismutase.
Can be bound to metallothionein or albumin.
Certain vegetarians may need increased dietary zinc because of lower absorption.
Most vegetarians have adequate zinc to meet needs. Vegetarians or others whose major food staples are grains and legumes may consume enough phytates to reduce absorption by one-third. The consumption of dairy products also reduces zinc bioavailability because of the zinc-binding action of casein and calcium.
More about Zinc:
0 comments