The essential trace element chromium exists in valences Cr0 to Cr+6. Chromium compounds of oxidation states below +3 have a reducing effect and those of oxidation states above +3 have an oxidizing effect.The most important compounds represent Cr+3 and Cr+6, with the naturally occurring trivalent chromium being the most oxidatively stable and showing the greatest biological significance in humans. Cr+6 is a strong oxidant and is rarely found in nature. Chromium-6 compounds are also very unstable and can be spontaneously reduced. For this reason, foods do not contain chromium in the hexavalent state.Because high energy is required to oxidize trivalent chromium to hexavalent chromium, hexavalent chromium compounds are virtually impossible to form in biological systems.
Resorption
Trivalent chromium present in foods is absorbed-bound to amino acids-through the mucosal cells of the small intestine, primarily in the jejunum (empty intestine). Absorption can occur either by passive diffusion or receptor-mediated, i.e., active transport.The overall absorption rate of orally ingested chromium is very low.Cr+3 is absorbed by only 0.5% and Cr+6 by about 2%. In addition, absorption is influenced by numerous factors:
- Intake amount – as the supply of chromium increases – 40-250 µg/d – the rate of absorption decreases to about 0.4%; however, at lower intakes – for example, 10 µg/d – absorption is 2%
- Chemical properties of the ingested chromium compound – while the absorption from chromium chloride is very low, chromium from chromium picolinate can be absorbed much better
- Type and amount of other food components present at the same time – the factors promoting absorption include vitamins, such as vitamin C – ascorbic acid – and vitamin B3 – nicotinic acid – as well as natural chelating agents, amino acids, oxalate, starch and iron and zinc deficiency; phytic acid (phytates) and the trace elements zinc, iron and vanadium, however, inhibit absorption.
Transport and storage
Following absorption, chromium is bound in the blood mainly to the transport protein transferrin. If the binding capacity of transferrin is saturated, chromium can also be transported to tissues in combination with albumin and beta and gamma globulin.
According to recent studies, the chromium content of serum and plasma, respectively, is about 0.01-0.05 µg/dl.Chromium is mostly stored in liver, spleen, bone, and soft tissues, such as kidney and lung. Chromium concentrations in these organs and tissues are approximately between 20 and 30 µg/kg and vary by geographic origin.With increasing age, both the absorption of chromium and the chromium concentration in most tissues and organs decrease. As a result, significantly less Cr+3 is incorporated into glucose tolerance factor (GTF), which has unfavorable effects on carbohydrate, protein as well as fat metabolism. In addition, the ability to form GTF decreases with increasing age. Finally, older people should pay attention to an adequate chromium intake through food. In addition, the intake of chromium-containing GTF molecules is recommended. For example, chromium yeast contains the factor already synthesized. In carbohydrate-rich plants – sugar cane, sugar plants – the GTF is also found. However, the GTF is lost in the production of refined sugar.
Excretion
Absorbed chromium is predominantly excreted in the urine via the kidneys. 80 to 97% of glomerular-filtered chromium is renally reabsorbed and made available again to the organism.Excretion of the portion of chromium that was not absorbed by the jejunum (empty intestine) occurs largely in the feces (stool). Small amounts are lost through hair, sweat, and bile.
