Fat, I love you
Fat is great. Fat is the best. Fat is Fan-fat-tastic. Thick rolls of it under skin. Dimpled cheeks. Body fat has a lot of detractors. Fat and its portrayal in the public eye is a complex; too bodyfat much can be unappealing and unhealthy whilst not enough is unappealing and unhealthy. Perhaps the relationship with fat isn’t very complicated. We don’t like it, except when we do.
The mythos if fat is bad began recently within the last century or so. Barely 3 generations back we were all Sir Mixalot fawning over the fuller figure. This admiration was mostly female focussed. There is less evidence that the fluffy male was seen as overwhelmingly attractive as the rubenesque woman.
What is Fat
An organ is a collection of tissues, working together for a particular function. A tissue, in the biological sense is a collection of cells with the same function. This means that Fat is an organ. Forget skin on the modern European fat is likely to be their biggest organ. It is easy to think of fat as simply big cells full of fat, adipocytes, and not much else but there is a lot going on in fat. There are adipocytes which are the major fat stores but there is also nervous, immune and connective tissue all present in fat in dynamic communication with those adipocytes creating that glorious mass, Fat.
There are at least two major types of fat: Subcutaneous and Visceral. The are others such as the yellow fat of bone marrow and the intramuscular fat but those are not as contentious as their more popular brethren. Subcutaneous fat is the fat under the skin; it keeps us warm, protected and causes cellulite. Visceral fat is the fat around organs. This is the bad sibling of subcutaneous fat, this is the fat that increases the risk of major diseases yet is vital for its protection, its energy and the plethora of endocrine functions it provides.
The location of body fat is important but insufficient amounts can have detrimental effects on fertility, mood, immune system function and appetite to name a few systems. Visceral fat can be relatively high in a thin individual meaning that superficially they look fine and low risk for the disease linked to high body fat, when their visceral fat is measured they are at a much higher risk than weight matched peers.
Sexy Fat Factories
Fat is not a slouch. The plethora of endocrine functions carried out by fat are wide ranging on the systems they influence. Fertility. Immune system. Appetite. Mood.
All sex hormones are derived from steroids which are fat based molecules. The adrenal (small glands on top of your kidneys) and gonads (ovaries and testes) are the primary source of steroid hormones. Further metabolism is done by adipose tissue. Fat metabolises both oestrogen and testosterone and does so in a mass dependant manner. Visceral fat produces more of these conversion enzymes that subcutaneous fat.
The sex based distribution of fat is linked to these hormones and their metabolites. Post menopause the female and male hormone profile are more similar than previously, their fat distributions are also more similar with post menopausal women and men having increased upper body and visceral fat. Premenopausal women tend to have a greater amount of subcutaneous and lower body fat. Lower fat distribution is partly controlled by oestrogens which are produced by fat converting androgens, which includes testosterone, into oestrogens via the aromatase pathways.
Fat Energy and Fat Defence
Not content with contributing to the sex steroid hormonal profile fat also produces steroid hormones that orchestrate the immune system and energy metabolism. Fat produces signalling molecules such as the mostly inflammatory IL6 and TNF α. It also produces Tissue Factor which prevents premature clotting on blood vessel walls.
TNF α is mostly described in textbooks as a proinflammatory cytokine but the truth is more complicated. It does have a role in amplifying inflammatory signals but it also has a significant role in energy homeostasis. TNF α interferes with the insulin pathway, causing less insulin sensitivity, more cholesterol production and decreased glucose intake. TNF changes what energy source is delivered to the body shifting tissues away from using glucose and towards using fat derived molecules.
IL6 is a molecule that shares some of TNF α’s behaviour although it differed in critical areas. TNF α seems mostly to shift immediate energy supply IL6 seemed to correlate negatively with obesity but only when central nervous system concentrations were analysed. IL6 has different effects on energy in different body locations. IL6 is also a proinflammatory molecule pushing the immune system towards a particular immune response profile.
All of this should build a mighty confusing picture. All this talk of IL6 and TNFα and inflammation might seem like it has nothing to do with fat, but that is the point. Fat is involved in so many process and not all of them are immediately obvious, such as fat as an immune modulating organ, fat as a sex hormone factory. While this article has looked some of the endocrine features of fat we have glossed over the most studied: Appetite
There is a great deal of research on fat; so much that it deserves its own story. So click here for the second part (Posted 25/3/18) and find out more about fat and its relationship to energy and appetite.