Well....hmmm....like food, these things get exposed to oxygen and therefore, will "oxidize" over time, I suppose. They may not "go bad" as soon as they may lose nutritional value. Usually, FDA requires a biiiiiig safety net in terms of the expire date. In other words, if the expire date is March 1st - they're actually still "safe" well beyond that date.
Another thought I just had - iron is usually light sensitive as well, isn't it? So, there would be some add'l nutritional-loss issues to consider there as well...
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