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I had a quick question. I've heard that you need a UVB bulb for the turtles so that they can produce Vitamin D3 (I believe). I was just wondering if the UVB bulb does anything else for them? What if you fed them vitamins with D3 in it? Does that do the same thing for them? Sorry if this was discussed somewhere already, but I didn't have a lot of time to search for it. Thanks!

I use pellets that have calcium and d3 in them but I also provide uvb. I'm just not up with all the science behind it.

They do get some Vit D3 from the diet, but I've read the vitamin is felt to be more efficiently utilized when it's synthesized through the shell. In the wild, this would be from basking in the sun. As turtlekeepers, we try to mimic this with UVB lights, which also help ensure they're getting what they need.

UVB is also used to mimic the day/night cycles. I have mine on a timer, 12 on, 12 off. I believe it also aids in stimulating metabolisim.

from http://www.austinsturtlepage.com/Care/care.htm
UVA . This is the visible wavelength. It is responsible for inducing normal behavior in reptiles such as feeding, climbing, mating etc. In other words, UVA helps with the mental well-being of animals. Zoos have been using UVA bulbs such as the BLB blacklights and have found that exposure to high levels of UVA lighting for 2 hours daily induced mating in many species.
UVB . One of the two non-visible wavelengths of light. This is what gives humans suntans. In reptiles, UVB allows for the synthesis of vitamin D3 which allows reptiles to process calcium in their system, thus preventing or reversing metabolic bone disease.

Read
http://www.austinsturtlepage.com/Articles/guidetolighting.htm
for a more in depth explaination of lighting, vitamins, and what's necessary.