Yes, you would need "do" there, as without it, it would mean, lierally, that the supply of "greater amounts of iron to the diet" is being compared to a lesser supply of iron to the "foods of plant origin". - which is not what is meant. It is also more completely as: "Foods of animal origin generally supply greater amounts of iron to the diet than do foods of plant origin supply", however the final "supply" is often omitted as it is understood.
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