12-11-2012, 03:36 PM
EXPERT GROUP ON VITAMINS AND MINERALS
EXPERT GROUP.pdf (Size: 100.96 KB / Downloads: 43)
CHROMIUM
Chemistry and Geochemistry
The transition element chromium (Cr) belongs to Group VI of the Periodic
Table and has an atomic weight of 52.0. Chromium can exist in any oxidation state
from –2 to +6; however, oxidation states other than 0, +2, +3 and +6 are uncommon.
In biological materials chromium is most stable in the oxidation state +3, as
chromium compounds with oxidation states below +3 are reducing, and those above
+3, are oxidising. In acidic solutions chromium (III) is soluble, readily forming
hexahedral complexes with appropriate ligands, such as oxalate and sulphate ions. At
the oxidation state of +6, chromium forms chromates and dichromates, which have
strong oxidising potential.
Natural occurrence
Chromium is ubiquitous in nature, occurring in air, water, soil and biological
materials over a great range of concentrations. Almost all of the sources of chromium
in the earth’s crust are in the trivalent state, naturally occurring chromium compounds
in the hexavalent state are rare. Hexavalent chromium compounds are thus, man-made
products (WHO, 1988).
Occurrence in food, food supplements and medicines
Food
Most of the chromium ingested with food is in the trivalent form. Early data
(pre-1980) on the chromium concentration in foods are flawed, due to the difficulties
encountered in contamination control during sampling, sample pre-treatment and
analysis (Nordic Council, 1995). Recent data indicate that staple foods are particularly
low in chromium. Processed meats, whole grain products, pulses and spices are the
best sources of chromium, whilst dairy products and most fruit and vegetables,
contain only small amounts.
Most of the total chromium in foods derives from food processing in stainless
steel containers and processors, which typically contain 18% chromium.
Homogenisation of fresh meat in a food processor equipped with standard stainless
steel blades almost doubles the chromium concentration of the meat (Kumpulainen et
al., 1980). Thus, canned and other processed foods, particularly acidic foods such as
fruit juices, are clearly higher in chromium than fresh foods, with the exception of
refined sugar which is very low in chromium, compared to brown sugar and molasses
(Offenbacher and Pi-Sunyer, 1983). Some brands of beer are very good sources of
chromium and a half pint serving may contain as much as 20 μg or approximately
two-thirds of the normal dietary chromium intake (Anderson and Bryden, 1983).
Intake and exposure
Food
Information on dietary intakes of chromium is limited. Most of the chromium
ingested with food is in the trivalent form. Chromium is not included in the nutrient
databanks for dietary surveys. The most up to date information available is from
analysis of samples from the 1997 Total Diet Study (TDS)1,2. This showed that the
population average intake of chromium was 0.10 mg/day. This value is lower than that
of 0.34 mg/day obtained from the 1994 Total Diet Study1 but is consistent with intakes
from previous TDS prior to 1994. Chromium intake in 1994 was unexpectedly high
due to relatively high concentrations in the oils and fats, milk, dairy products and nuts
groups. It appears that this is unique to that year and is not part of a trend towards
increasing intakes. Table 1 shows the concentration of chromium in each of the TDS
food groups in 1997 and the intake from each group.
Mean and upper level2 (97.5 percentile) chromium intake for adults has been
estimated at 0.10 mg/day and 0.17 mg/day respectively using the 1997 TDS
concentrations combined with consumption data from the 1986/87 Dietary and
Nutritional Survey of British Adults. These figures are lower than those obtained from
the 1994 TDS (0.30 mg/day and 0.52 mg/day respectively), which were unexpectedly
high.