Table 106

Trace Element Kinetics

Chromium

Absorption is minimal (i.e., 0.5 to 2%). Chromium, not unlike iron, uses a transferrin vehicle for transport to target sites. Mean serum concentrations are 0.16 ng/ml. The excretion of chromium is 0.2 to 0.6 mcg/l urine, and diets high in simple carbohydrates may increase urinary excretion up to threefold.

Copper

Absorption is about 30 to 60%. Copper absorption is impaired by Zn supplements providing 22.5 mg/d, even when the supplement is taken independently of meals. Distribution is principally (94%) via binding to ceruloplasmin; however, there also is binding to transcuprein, albumin, and amino acids. Plasma concentrations are approximately 1 mcg/ml with principal excretion in bile (80%). Intestinal and urine excretion account for 16 and 4% excretion, respectively.

Iodine

Absorption is greater than 50% and up to 100% is from gut. Iodine is also absorbed from skin. Dietary iodized salt is a major source of this trace element. The distribution of iodine is mainly via triiodothyronine (T3) and thyroxine (T4). Plasma concentrations are about 60 ng/ml, and excretion is principally in urine with rapid turnover (i.e., two thirds of absorbed iodide is excreted within 2 to 3 d).

Iron

Absorption is approximately 5 to 15% as the ferrous ion. Once absorbed, it is converted to the ferric ion in the mucosal cells of the intestine and binds to apoferritin that forms ferritin (regulates absorption of iron); transferrin binds iron in the plasma and transports iron to the liver, spleen, and bone marrow (where it is stored again as ferritin). Nascent distribution is bound to transferrin and is influenced by interleukin I (leukocyte endogenous mediator). The plasma concentration is approximately 1 mcg/ml and is excreted principally in bile, but also via skin and urine.

Manganese

Absorption is about 3 to 4%; however, absorption may be decreased with iron, cobalt, calcium, and phosphate. The distribution of manganese is bound extensively to transferrin or transmanganin. There is a small amount bound to 1globulin. Plasma concentrations are about 0.6 to 2 ng/ml. Excretion is extensive in bile (greater than 99%), and excretion (not absorption) regulates homeostasis.

Molybdenum

Absorption is about 40 to 100% in duodenum. The plasma concentration is about 2 to 6 ng/ml, and although excretion is principally in the urine, great gastrointestinal losses can occur.

Selenium

Absorption is 35 to 85% in duodenum; however, absorption is dependent on selenium solubility and the ratio of selenium to sulfur. Plasma concentrations are approximately 100 to 130 ng/ml, and excretion is principally in the urine; however, as with molybdenum, great intestinal losses can occur. Patients with burns (greater than 20% of body surface area) will need 200 mcg selenium for at least 2 weeks.

Zinc

Absorption is about 10 to 40% with 2.5 to 4 mg absorbed per day in duodenum and proximal jejunem. Absorption will be diminished if given with copper; iron:zinc ratio of 2 to 3:1 and vitamin D may increase zinc bioavailability. Zinc metallothionein regulates zinc metabolism and absorption.

Distribution is via binding to albumin, transferrin, ceruloplasmin, and -globulin. Plasma concentrations are approximately 1 mcg/ml, and excretion in biliary and pancreatic losses may account for up to 25% of daily losses (12 mg/l fistula, 17 mg/kg stool). Sweat losses can be as high as 1 mcg/ml.

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