Fat is a difficult issue. You need it as an energy store, to keep yourself warm, to pad your bones and provide an attractive, comely shape to some of us. However, most of us constantly strive to have the least amount of body fat possible (though whether that is achieved is another thing!)
For some time there has been evidence that an increase in body fat (or visceral fat) is associated with an increase risk in conditions such as diabetes and heart disease, part of what we know as metabolic syndrome. It therefore makes complete sense to try to avoid obesity and central body fat.
So what has this got to do with dermatology?
Well, don’t forget that we don’t just have visceral fat. We also have fat in the skin, so called subcutaneous fat. And these two types of fat, visceral and subcutaneous have very different roles.
What are these roles?
There have been recent studies showing that an adequate layer of subcutaneous fat may play a pivotal role in controlling hormonal and metabolic pathways while excess visceral fat may have a role in exacerbating or encouraging the development of metabolic syndrome.
[Hamdy O, Porramatikul S, Al-Ozairi E. Metabolic obesity: The paradox between visceral and subcutaneous fat. Curr Diabetes Rev 2006; 2:367-73.]
There is a very interesting article in the February edition of the British Journal of Dermatology demonstrating a link between ultraviolet radiation and fat cell function.
[Kim EJ, Kim YK, Kim S et al. Adipochemokines induced by ultraviolet irradiaton contribute to impaired fat metabolism in subcutaneous fat cells. Br J Dermatol. 2018; 178: 492-501.]
But surely subcutaneous fat is unaffected by ultraviolet radiation?
It was originally thought that as UV rays cannot penetrate the deeper layers of the skin, then subcutaneous fat (which is even deeper) must be unaffected by UV rays. However, UV radiation has been shown to alter the metabolism of subcutaneous fat in human skin.
[Kim EJ, Kim YK, Kim JE et al. UV modulation of subcutaneous fat metabolism. J Invest Dermatol 2011;131:1720-6.]
How does that happen?
There are factors called adipokines which are secreted by fat cells. These adipokines are now viewed as the important mediators of inflammation and metabolism, locally in the skin and around the body.
This current study by Kim et al showed that UV irradiated human subcutaneous fat produces a particular range of adipokines which leads to loss of subcutaneous fat (and contributes to photoageing of the skin). These adipokines also lead to an increase in a particular type of cell called an adipose tissue macrophage which is similar to those seen in obesity.
What do these macrophages do?
These adipose tissue macrophages then lead to the generation of further adipokines which encourage inflammation. This chronic inflammation may induce resistance to insulin, lead to the development of metabolic disorders and may even lead to the development of chronic skin conditions such as psoriasis.
[Chirocozzi A, Raimondo A, Lembo S et al. Crosstalk between skin inflammation and adipose-tissue derived products: Pathogenic evidence linking psoriasis to increased adiposity. Expert Rev Clin Immunol 2016;12:1299-308.]
So sun exposure will make me fat?
I think the authors have shown that there is a dynamic interplay between the skin and adipose tissue that goes in both directions.
It has consequences when considering light treatment for patients with chronic inflammatory skin conditions. Also in those of us that still seek the sun it shows yet another reason to moderate sun exposure.
Dr Sandy Flann, Consultant Dermatologist