Why Dandruff Shampoos Work (Even Though We Don’t Know the Exact Cause of Dandruff)

Skin Care


We’ve all seen it — whether on ourselves or others — the telltale white flakes that accompany dandruff. Dandruff, in particular dandruff caused by seborrheic dermatitis (SD) , is often caused by overproduction of sebum (skin oils) and skin cells in addition to irritation from malassezia (a yeast-like fungus). This fungus lives on all adult skin, but when it grows out of control, which researchers think may happen because of excessive shedding of dead skin cells, it can cause problems (Mayo Clinic, Journal of Medicinal Chemistry).

This fungus causes the inflammation that results in flaky, scaly patches and red, itchy skin (A.D.A.M. Medical Encyclopedia). In more severe conditions, sufferers may have excess oiliness, yellow crusts and fissures of the skin, and there’s concern about secondary bacterial infection. Dandruff/SD seems to be genetic, but can be triggered by weather changes, stress, skin disorders, fatigue, insufficient washing, oily skin, and obesity; certain neurological conditions and HIV have also been linked to increased cases.

Numerous studies have demonstrated the effectiveness of dandruff shampoos to treat these conditions, though they don’t cure them. Though we don’t understand the exactly why dandruff happens, we do know that these shampoos work.

Dandruff/SD and Too Many Dead Skin Cells

An excess of dead skin is definitely an important factor in dandruff and SD, but what causes it?
An excess of dead skin is definitely an important factor in dandruff and SD, but what causes it?

Dandruff shampoo primarily contains zinc pyrithione, which is an antibacterial and anti-fungal compound that also has cytotoxic activity against skin cells (Merriam-Webster, Food and Chemical Toxicology). In fact, this cytotoxic action may be an important part of how dandruff shampoos work.

Studies have looked into the scalps of SD sufferers to understand why malasezia, which is found on all skin, causes SD in some and not in others. In a small, single-center, double-blind, randomized, parallel group comparison study of 12 participants to determine whether the ultrastructure of a scalp with dandruff was different than a non-sufferer’s scalp (Journal of the American Academy of Dermatology).

Those with dandruff had an altered structure of the scalp. This primarily entailed an increased number of intercellular lipids. These appeared in and cause separation and curling of the corneocytes, which are what makes up the stratum corneum (outermost layer of skin). So, the researchers conclude that SD is caused both by altered keratinization and altered sebum production. The researchers found that these scalp abnormalities were greatly reduced by zinc pyrithione, meaning that whatever the cause of these abnormalities, zinc pyrithione helps.

An in vitro study on human cells found that zinc pyrithione might have an antimetabolic action on the skin, which decreases the cell proliferation (Journal of the Society of Cosmetic Chemists). This research shows that increased cell proliferation is an important part of SD, but doesn’t actually provide a root cause.

While researchers initially thought this meant that the fungus was a component but not a cause of SD, the introduction of azole anti-fungals, which proved to help people with SD, seemed to suggest that the fungus was complicit in causing SD.

Dandruff/SD and Fungus

Malasezia, a fungus found on nearly everyone's skin is a part of dandruff. But researchers want to know why only some people are affected.
Malasezia, a fungus found on nearly everyone’s skin is a part of dandruff. But researchers want to know why only some people are affected.

Some researchers have suggested that the increased proliferation of skin cells might actually be the body’s response to the presences of the fungus malassezia. People who are sensitive to this fungus might have their immune system provoked by its presence, resulting in the proliferation of cells (PLoS Pathogens). This is further backed by the increase in SD in people with AIDS, which seems to suggest that there are immunological factors at work (Clinical and Experimental Dermatology).

Some people with SD have also been shown to have an elevated number of activated lymphocytes in their system, which backs the idea that SD has ties to the immune system. Research as to whether malassezia raises the antibody count in SD sufferers has been inconclusive.

The fact that lesions improve with the removal of malassezia from skin seems to suggest that this fungus is directly correlated with SD. There are still numerous questions to be answered, but the currently theories about SD and dandruff include the presence of fungus. That’s a good thing, because Proctor & Gamble cracked the genetic code for the fungus and hope to create products to combat it even better.

And thanks to its antimicrobial capabilities, zinc pyrithione also helps to take care of malassezia. One study found that an important part of how it functions is inhibiting fungus by increasing copper uptake and activating key proteins that stop the growth (Antimicrobial Agents and Chemotherapy). Still, we don’t entirely understand it’s antimicrobial properties and more tests must be done.

Bottom Line

While we don’t know exactly what causes seborrheic dermatitis or the accompanying dandruff, we do know that dandruff shampoos seem to work to aid the problem, if not cure it altogether. That’s because the usual ingredient, zinc pyrithione, combats many of the components of SD and dandruff. As a cytotoxic agent, it helps to curb the excess proliferation of skin cells. As an antimicrobial agent, it curbs the growth of malassezia, the fungus that is a huge component and possibly the cause of SD. Future studies to understand exactly what causes SD and dandruff will help us to create products that might actually cure the condition one day.

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