When I first tested D levels many years ago they were low, around 30 and my immunity was very poor as a result. It was during that time that I kept getting one chest cold after another which likely led to Bronchiectasis. I tried a D3 supplement and after some months re tested. No improvement. Then I got a spray in the mouth D3 and when I next retested my levels had gone up. I continue to use either a spray or a liposomal D3. When I was using 4000 iu a day my levels went up to 82 which is actually good if not a bit high. I cut back and use 2000 to 3000 and recently tested 64. So anyway you need a supplement you can absorb. And sitting in the sun if you are older may not produce much D as you lose the ability to absorb it that way. Not to mention skin cancer risk. Does keeping my D levels up help? That is hard to say but I think I would be even worse off if I did not.