Malassezia are lipid dependent fungi which require human fat to grow in our bodies. This is quite unusual for fungi: all other medically important fungal species can make their own lipids from sugars. Long ago, Malassezia species lost their lipid synthesis genes, probably because the animals they colonized always provided enough lipids for their needs.
Until very recently, Malassezia were mainly studied on the human skin, because it was believed that they were absent from internal organs. Dandruff and seborrheic dermatitis—two skin diseases caused by Malassezia—have been studied most of all. Seborrheic dermatitis develops mainly on lipid-rich skin regions, especially the scalp and face. Most studies of Malassezia inside the body report acute infections in patients fed with intravenous lipids, which resolved by discontinuing lipid injections (Shek 1989). Based on these observations, it is clear that Malassezia overproliferate when fats are abundantly available. Because of this, I’d expect diseases caused by Malassezia to be associated with body fat and obesity.
The two diseases for which we have the most evidence of Malassezia’s involvement are Crohn’s and psoriasis. Malassezia are a necessary factor in both, and many studies have shown that body fat and obesity increase the risk of these diseases (Mendall 2011; Suibhne 2013; Khalili 2015; Setty 2007; Gefland 2012; Romero 2014; Budu 2019).
Antibodies against fungi (ASCAs) are a biomarker of an immune response against Malassezia (Limon 2019). These antibodies correlate with body fat and systemic inflammation (Kvehaugen 2017), suggesting that moderate inflammation caused Malassezia occurs even in individuals not affected by overt “autoimmune” diseases.
Myriad factors can lead to body fat accumulation and observational studies can’t distinguish if disease risk is controlled by fat itself or by confounding factors correlated with body fat. Similarly, lipids have been implicated in many inflammatory mechanisms at the cellular level. This means it is very difficult to establish the main mechanisms through which excess fat is detrimental to our health.
That said, we now know that a lipid dependent fungus (Malassezia) is the main cause of Crohn’s and psoriasis. As such, the simplest mechanism to explain the link between these two diseases and body fat is: “Excess availability of fats in the body allows Malassezia to overproliferate.” I expect this to apply to all diseases caused by Malassezia.
This post was largely based on Celis 2017.
Animated Malassezia and lipids video: https://youtu.be/u1pckKEkwF0
