Saturday, December 6, 2008

Peripheral vs. Ectopic Fat

I went to an interesting presentation the other day by Dr. George Ioannou of the University of Washington, on obesity and liver disease. He made an interesting distinction between the health effects of two types of body fat. The first is called subcutaneous fat (or peripheral fat). It accumulates right under the skin and is evenly distributed over the body's surface area, including extremities. The second is called ectopic fat. Ectopic means "not where it's supposed to be". It accumulates in the abdominal region (beer belly), the liver, muscle tissue including the heart, the pancreas, and perhaps in lipid-rich deposits in the arteries. Subcutaneous fat can be measured by taking skinfold thickness in different places on the body, or sometimes by measuring arm or leg circumference. Ectopic fat can be measured by taking waist circumference.

It's an absolutely critical distinction, because ectopic fat associates with poor health outcomes while subcutaneous fat does not. In
this recent study, waist circumference was associated with increased risk of death while arm and leg circumference were associated with a reduced risk of death. I think the limb circumference association in this particular study is probably confounded by muscle mass, but other studies have also shown a strong, consistent association between ectopic fat and risk of death, but not subcutaneous fat. The same goes for dementia and a number of other diseases. I think it's more than an epidemiological asssociation. Surgically removing the abdominal fat from mice prevents insulin resistance and prolongs their lifespan.

People with excess visceral fat are also
much more likely to have fatty liver and cirrhosis. It makes sense if you think of them both as manifestations of ectopic fat. There's a spectrum of disorders that goes along with excess visceral fat and fatty liver: it's called the metabolic syndrome, and it affects a quarter of Americans (NHANES III). We already have a pretty good idea of what causes fatty liver, at least in lab animals: industrial vegetable oils and sugar. What's the most widely used animal model of metabolic syndrome? The sugar-fed rat. What are two of the main foods whose consumption has increased in recent decades? Vegetable oil and sugar. Hmm... Fatty liver is capable of causing insulin resistance and diabetes, according to a transgenic mouse that expresses a hepatitis C protein in its liver.

You want to keep your liver happy. All those blood tests they do in the doctor's office to see if you're healthy-- cholesterol levels, triglycerides, insulin, glucose--
reflect liver function to varying degrees.

Abdominal fat is a sign of ectopic fat distribution throughout the body, and its associated metabolic consequences. I think we know it's unhealthy on a subconscious level, because belly fat is not attractive whereas nicely distributed subcutaneous fat can be. If you have excess visceral fat, take it as a sign that your body does not like your current lifestyle. It might be time to think about changing your diet and exercise regime.
Here are some ideas.

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