Dude, you get vitamin D in your food, particularly in fish and butter. If you have a balanced diet, you do not need to take vitamin supplements - period.
Of course, spacefaring people might need to supplement their diet with various vitamins and trace elements to counteract a number of physiological problems caused by limited gravity, lack of UV exposure, et caetera. The simplest way to do it is to "spike" their food with higher than usual levels of vitamins.
Adding higher levels of carbs or stimulants to food is standard practice for many armies and navies around the world, so I don't see why a spacefaring navy wouldn't do it.
That is why I find the notion of "take these supplements to be okay on Ophion" extremely suspicious...
Then again, I'm a military man in an increasingly unfamiliar situation: my trust levels are getting pretty low...
Vitamin D in
butter: ~9IU per serving (not worth mentioning)
tuna/salmon: 300 - 600 IU per serving (worth mentioning)
herring/sardines: ~1680 IU per serving (serious) (specifically North Atlantic, Pacific herring are lower for some reason)
Recommended amounts: (note, the U.S. RDA is not how much you need to be healthy, just the bare minimum to avoid serious issues)
If you get lots of sun: 600 IU daily
Moderate amounts of sun: 1000 IU daily
*If you are diagnosed with low vitamin D levels: 2000 IU daily or a weekly shot of 50,000 IU (depends on your doctor)
Maximum that your doctor will likely recommend: 50,000 IU/weekly (if your levels are low)
Maximum tested safe dose: 100,000 IU weekly for up to 12 months
I can't imagine a ready supply of North Atlantic herring or sardines for the entire, spread across space Navy, but I can imagine regular supplementation (especially given that the requirements for our own bodies to make it, UV, can also lead to things like skin cancer. Thanks, evolution.)
That said, five days on a dusty rock, your regular supplements should be fine. Also, you don't switch up contraceptives like that. Both vitamin D and contraceptives need to accumulate in your system, and that's just not going to happen.
I tell you, I don't trust him at all. I
must get into that lab.
*Some of us (especially that live in places like the US Pacific Northwest) still have difficulty generating enough vitamin D to stay healthy. Optimal blood vitamin D is 40 - 59 ng/mL, adequate is 30 - 39, low is less than 30. My tested level was 12 ng/mL. So yeah, I take 2000 IU daily per doctor's orders and my tested levels have now been consistently between 49 and 53.