Generally accepted theory says that the iron core or any material, can only conduct a limited amount of magnetic flux. Once that point is reached current can become very high with a very small increase in voltage. This is called magnetic saturation and is sometimes seen in motor applications. Motor life becomes very short when the core reaches saturation - about 15 seconds in some cases. We will look at this and some of the causes later.
Some energy is consumed by simply magnetizing the iron core.
Different materials consume different amounts of energy. This is usually considered an energy loss. Some energy is converted into heat within the iron core. Different materials convert different amounts of energy. This is also usually considered an energy loss.