system
Given the following post, respond as if you're commenting on Reddit:
human
What's the difference between vitamin A, retinol, and retinoids?

I've been looking into skin care products and became interested in retinol for its supposed benefits. Something that's confusing for me though, is why couldn't I use pure vitamin A oil for the same purpose? It's generally only marketed as a dietary supplement, but it's often *much* cheaper and without all of the extra ingredients. Everything that I've read says they're basically the same,  but doesn't really explain how they're different. 

Why couldn't I just use the vitamin A oil directly on my skin and reap the benefits without spending the extra money?
gpt
There are a bunch of different compounds in vit A called retinoids. Retinol is one of them. Your skin undergoes reactions that converts these retinoids into other compounds until it becomes retinoic acid. The more retinol or other product on your skin, the more your skin will be affected by the treatment. One of the most effective retinoids is tretinoin or retin-a. It's 20x more potent than retinol. There just aren't enough concentrated retinols in vitamin a to make that much of a difference. Retinoids work by increasing rate of cell turnover in your skin so if you don't have a lot of these molecules doing work you aren't going to see the results.