Mildly Interesting

SLS also makes toothpaste foam, but gives dinner people canker sores. It's in most toothpastes.

He says he was using Head & Shoulders prior to going water-only. I'm not sure if H&S has SLS or not, but I've not known H&S to lather up that well, so I would assume it probably doesn't.

So unlike hand soaps etc, shampoo is not trying to sanitize your hair. Washing is to remove dirt and grease, not necessarily kill microbes.

Yes sebum is beneficial to the scalp. And yes stripping too much sebum from the skin (washing too often) can lead to dryness and irritation and dandruff symptoms.

But dandruff can also be caused by fungus and bacteria, see the "Causes - > Microorganisms" section of the Dandruff Wikipedia article. The fungus mentioned here specifically feeds on sebum, so not washing enough to remove that sebum can cause that fungus to thrive.

So yes washing too much can cause dandruff, but so can not washing enough (I linked this interview with a dermatologist in another comment, they say the same thing here.)

There isn't a one-size-fits-all answer here. But doctors do recommend washing your hair regularly - how regularly depends on a variety of factors.

That's how it works. There's another layer to it though. We have symbiotic relationships with microbes and bacteria that live on our skin and in our pores. They swim around in the oil and they eat our dead skin and they manage all the stuff on our skin. When a foreign pathogen or microbe shows up not only do they have to make their way through all the protective oily waxing stuff but they also have to fight our little army on our skin before it ever even gets to us. By using soap you're killing everyone on your skin and removing everything leaving your bare skin with zero defense. We evolved to have the oils and the bugs on our skin and it's a great system. I'm pretty sure soap salesman just started making up lies in the Victorian era and beyond and everyone believed it an internalized it.

As he points out in the video, people that use shampoo regularly will quickly get very greasy hair if they don't wash their hair with shampoo in a while. In my experience, it's directly related to how often you use shampoo- I only use shampoo about twice a week now, and my hair gets less greasy than it used to when I used shampoo more often.

Co-washing only (washing with conditioner and skipping shampoo) is highly recommended for folks with extremely curly hair.

I tried it with my largely straight and thin hair and it was a disaster for me. But different hair textures require different solutions.

Re: "all the after-products" Again, different hair types require different solutions. A heat protectant product (applied on damp hair after washing and before drying and styling) is really important if you're using heated styling tools for straightening or curling your hair. Even moreso if you're also coloring your hair, which can make your hair more susceptible to heat-based damage. Additionally, for certain hair types and cuts, styling products are absolutely necessary to achieve the look you're going for. My hair won't hold a curl without adding product to it before styling.

As I say on another comment on this thread, my hairdresser does recommend that I shampoo twice on wash days to ensure I remove all the dirt/grease/styling product. Shampooing once (with my SLS-free shampoo) wasn't getting my hair clean enough.

So yes, including after-care products in your routine may necessitate different cleaning steps compared to someone who isn't using the same products. But again, what you are doing with your hair and the type of hair you have also influences the types of products that are valuable for you to use (shampoo, conditioner, and aftercare.)

All that said, claiming that haircare is all advertising "with a bit of social dynamics and zero science behind it" is misleading, uninformed, and incorrect. 😘

Since you sound like you want science, here's an interview with a doctor of dermatology.