Both calcium and phosphorous are very important nutrients for your iguana. The unfortunate thing is that if they are fed too much phosphorous they are less able to absorb the calcium from their food. For this reason, the correct amount of each that they should be eating has been calculated. Their diet should include twice as much calcium as it does phosphorous. In vegetables (and especially fruits) there is more phosphorous than calcium present, so to combat this iguana keepers sprinkle the plain calcium supplement into the food to boost the calcium level to something more acceptable.