Interesting question! I hypothesize the following:

Vitamin A is distributed throughout the body, so only a small amount actually benefits the skin. The body tightly regulates how much retinol is converted into retinoic acid, meaning that even if you consume a lot of vitamin A, your skin won't necessarily receive much retinoic acid. Topical retinoids, on the other hand, deliver a concentrated dose directly to the skin, providing targeted benefits that dietary vitamin A alone can't achieve. Even people with sufficient vitamin A levels in their bodies can still see significant skin improvements from topical retinoids due to their localized effectiveness. Also, consuming large amounts of vitamin A for additional benefits isn't advisable due to the risk of toxicity.

As a crude comparison, just drinking water doesn't automatically give you well-hydrated skin, while a moisturizer directly targets your skin's hydration. 

Oh, I'm no expert! Just a skincare nerd who loves research-based information. I apologize if my wording was confusing.

I'm not quite sure why there's so much concern in the comments. Yes, oral retinoids, such as Accutane, do carry significant health risks for humans, but that's not the focus of this post. While high doses of oral retinoids have been associated with an increased risk of cancer in some animal studies, these studies typically involve doses much higher than those used in topical treatments.

When it comes to topical use, only trace amounts are absorbed, with adapalene having the lowest (<0.25 ng/mL). I tried really hard to find evidence of health concerns associated with topical retinoids, but they seem to be limited to skin irritation and a small potential risk of birth defects during pregnancy. In my opinion, adapalene is the best overall - it's more accessible, less irritating, has additional anti-inflammatory benefits, and is more chemically stable than tretinoin while offering comparable results. 

As for sunscreen, you've opened up a whole can of worms there, haha! I'd recommend checking out Dr Michelle Wong, aka Lab Muffin Beauty, who is a fantastic cosmetic chemist with great knowledge on myths like hormone disruption and on retinoids as well. You can find her at https://labmuffin.com/ or on YouTube. I can imagine you're a busy man. If you have specific questions and my comment gives you a good feeling about my standards, feel free to ask – I'd be happy to look them up for you.

But to quickly summarize, there's tons of misinformation out there about the dangers of sunscreens that's not at all in line with the scientific consensus, made worse in the recent years by the "clean beauty" marketing / corporate opportunism movement. Sunscreens are rigorously tested and have huge safety margins (at least 100 times higher than the No Observed Adverse Effect Level or NOAEL), which take into account absorption through the skin and potential endocrine and other long-term effects. 
 

No, retinoids work by normalizing skin function and usually take at least a few weeks of consistent use to show results. If you need an acute treatment, consider benzoyl peroxide or acids like AHA and BHA.

Hi Gene! As a fellow research-based skincare enthusiast, I’m thrilled to see this discussion here.

I've been using adapalene for about two years now, and I've experienced all the benefits it's known for: a dramatic improvement in my skin texture, brightness, and acne.

Adding to the sun protection advice, I think this point is so often emphasized in conjunction with retinoids because reversing signs of skin aging is a major motivation for people using them, and UV radiation is a major factor in breaking down collagen and elastin, accounting for up to 80% of skin aging — this figure is cited by both the Skin Cancer Foundation and the American Academy of Dermatology. While retinoids can help reverse some sun damage, it's smart to prevent the primary cause of degradation in the first place. In addition to sun avoidance, I highly recommend Korean sunscreens for their advanced cosmetic technology and comfortable textures, and many of them are very reasonably priced.