That overdose thing is a pretty common misconception. Sunlight or artificial UVB needs to be actively turned into vitamin D by the animal's liver. Cholesterol is broken down and combined with calcium and other stuff to form cholecalciferol, or vitamin D3. If the liver has enough of a store of D3, it stops making more. There's no more risk of UVB tubes producing an overdose than natural sunlight. The body just stops making vitamin D when it gets enough.
The plus side of the liver actively having to make vitamin D is that cholesterol is changed into a substance that does not clog arteries and organs to the point where disease happens. Not only will UVB exposure promote the formation of a life-giving biochemical, it also reduces the risk of fatty liver and cardiovascular disease.
Supplements don't have a shutoff valve hard-wired into the body and just build up over time if given in excess. If you're going to choose one over the other, lighting is definitely far safer and more effective than the supplements. You are no more likely to get an overdose of D3 when supplements and lighting are combined, the body just doesn't make its own vitamin D if it's getting what it needs from the supplements. But you can easily underdose or overdose the supplements as the body can't regulate storage and usage from dietary sources.
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