Introduction to Human Nutrition

Minerals and Trace Elements 233

Table 9.21

Absorption, transport and storage characteristics of the ultratrace elements

Element

Major mechanism(s) for homeostasis

Means of absorption

Percentage of ingested absorbed

Transport and storage vehicles

Aluminum

Absorption

Uncertain; some evidence for passive diffusion through

the paracellular pathway; also, evidence for active absorption through processes shared with active processes of calcium; probably occurs in proximal duodenum; citrate combined with aluminum enhances absorption

<1%

Transferrin carries aluminum in plasma; bone

a possible storage site

Arsenic

Urinary excretion; inorganic

arsenic as mostly dimethylarsinic acid and organic arsenic as mostly arsenobetaine

Inorganic arsenate becomes sequestered in or on

mucosal tissue, then absorption involves a simple movement down a concentration gradient; organic arsenic absorbed mainly by simple diffusion through lipid regions of the intestinal boundary

Soluble inorganic forms,

>90%;

slightly soluble inorganic forms, 20–30%; inorganic forms with foods, 60–75%; methylated forms, 45–90%

Before excretion inorganic arsenic is

converted into monomethylarsonic acid and dimethylarsinic acid; arsenobetaine not biotransformed; arsenocholine transformed to arsenobetaine

Boron

Urinary excretion

Ingested boron is converted into B(OH)

and absorbed in 3

this form, probably by passive diffusion

>90%

Boron transported through the body as

undissociated B(OH)

; bone a possible 3

storage site

Cadmium

Absorption

May share a common absorption mechanism with other

metals (e.g. zinc) but mechanism is less effi cient for cadmium

5%

Incorporated into metallothionein which

probably is both a storage and transport vehicle

Germanium

Urinary excretion

Has not been conclusively determined but probably is by

passive diffusion

>90%

None identifi ed

Lead

Absorption

Uncertain; thought to be by passive diffusion in small

intestine but evidence has been presented for an active transport perhaps involving the system for calcium

Adults 5–15%, children 40–50%

Bone is a repository for lead

Lithium

Urinary excretion

Passive diffusion by paracellular transport via the tight

junctions and pericellular spaces

Lithium chloride highly absorbed:

>90%

Bone can serve as a store for lithium

Nickel

Both absorption and urinary

excretion

Uncertain, evidence for both passive diffusion (perhaps

as an amino acid or other low molecular weight complex) and energy-driven transport; occurs in the small intestine

<10% with food

Transported in blood principally bound to

serum albumin with small amounts bound to

L-histidine and

α^2

-macroglobulin; no

organ accumulates physiological amounts of nickel

Rubidium

Excretion through kidney

and intestine

Resembles potassium in its pattern of absorption;

rubidium and potassium thought to share a transport system

Highly absorbed

None identifi ed

Silicon

Both absorption and urinary

excretion

Mechanisms involved in intestinal absorption have not

been described

Food silicon near 50%; insoluble

or poorly soluble silicates ~1%

Silicon in plasma believed to exist as

undisassociated monomeric silicic acid

Tin

Absorption

Mechanisms involved in intestinal absorption have not

been described

~3%; percentage increases when

very low amounts are ingested

None identifi ed; bone might be a repository

Vanadium

Absorption

Vanadate has been suggested to be absorbed through

phosphate or other anion transport systems; vanadyl has been suggested to use iron transport systems; absorption occurs in the duodenum

<10%

Converted into vanadyltransferrin and

vanadyl-ferritin; whether transferrin is the transport vehicle and ferritin is the storage vehicle for vanadium remains to be determined; bone is a repository for excess vanadium

Reproduced from Nielsen (1999) in Sadler

et al. Encyclopaedia of Human Nutrition

, copyright 1999 with permission of Elsevier.