There won't be any sunlight to shine on you if you're underground.

That's right.

And you'll get vitamin deficiency if you don't have enough sunlight.

Does that mean Okuu's artificial sun isn't good enough? Should be 人工, not 人口.

And so, I made some bentou.

Don't you say things like "Well, since it's good for the body, nothing wrong with it tasting bad".

What a precaution you're taking.