Why is it unclear whether chromium is essential for the body?

Chromium was designated an essential mineral after several case reports of chromium deficiency in patients receiving total parenteral nutrition (TPN) (nutrition delivered intravenously).^{[1][2][3][4][5][6]} These patients demonstrated insulin resistance and glucose intolerance that seemed to be corrected by the administration of chromium. However, the validity of these reports has been called into question. The content of chromium in the TPN solutions was not always disclosed, and when it was, the solutions actually contained chromium in amounts exceeding what would normally be consumed in the diet given the low bioavailability of oral chromium.^[7] Additionally, researchers have failed to induce chromium deficiency in mice; results of these mouse trials suggest that chromium may be beneficial as a pharmacological agent but not a nutritional necessity.^[8] Lastly, in humans there are no definitive symptoms of chromium deficiency and no validated ways to measure chromium in the body, making it challenging to identify a true deficiency.^[9] Both blood and urine levels, which are frequently used in clinical trials, are not considered to be accurate representations of chromium stores in the body.^[10] While chromium may have small benefits when taken in supplemental doses, further research would be required to establish chromium as an essential dietary nutrient.