The truth is that a lot (most) people have less than optimal vitamin D levels. 400IU is often not adequate. There isn't much of a risk in increasing the RDA for vitamin D. Also, the average person doesn't get 30 minutes of sun exposure every day.
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Especially in the winter and in northern latitudes, it's not as efficient. And like you said, I believe the need increases with age.
I agree with everything you said. People come into GNC pretty often and buy vitamin C supplements...they taken in a few grams of vit C a day. It's pretty ridiculous. On the other hand, there are lots of older women that come in and won't buy or use calcium because it's too much of a hassle to take a few capsules each day.....even though their doctors have recommended that they supplement with calcium. They say it's a "quality of life issue".