“Muffin top” was Australia’s word of the year in 2006. Back then, it applied to skin showing over low-slung tight jeans when worn with a cropped top. Now, it’s the ubiquitous phrase for all the extras around the middle that we hide under our shirts. And why does it progressively worsen as we get older, despite our best efforts at the gym?

THE SHORT ANSWER: We accumulate fat because of insulin’s effect on the enzyme LPL (lipoprotein lipase).  LPL pulls fat out of the bloodstream into the cells. If the LPL is on the muscle cell it directs fat into the muscle for fuel; if it sits on a fat cell, then it makes that cell fatter. Both testosterone and estrogen (which are hormones) suppress LPL activity on abdominal fat cells. We lose testosterone and estrogen as we age, and therefore as Gary Taubes says we get “more activity on the fat cells of the gut, and so more fat.”

HOW IT WORKS

Insulin, Carbs, and Fat
First, a little background on the hormone insulin. The level of insulin in the bloodstream is mainly a function of the carbohydrates we eat. Carbs are what ultimately determine how much fat we accumulate. Cells will burn carbohydrates for fuel before they burn fat. If you eat a carbohydrate-rich diet your cells must burn all those carbs before they get to the stored fat. As we eat carbohydrates, blood sugar levels rise (because carbs turn to glucose when digested) the body secretes more and more insulin to keep the blood sugar under control. Insulin’s job is to tell the cells to get the excess glucose out of the bloodstream where some of it winds up stored as fat in the fat cells.

Fat Tissue–How your body uses it
In Gary Taubes’ best selling book, “Why We Get Fat and What To Do About It,” he lays out what endocrinology experts say about fat metabolism (references from Chapter 11).

1. “Insulin is the primary regulator of fat metabolism.”
2. Fat cells work as a place to put calories until you need them.
3. Contrary to popular belief, we store calories as fat even when we’re not eating more calories than we expend (thus, exercise alone may not actually lead to weight loss).
4. Insulin and hormones operate on the fat tissue to regulate storage or use of fat.

According to Taubes, fat tissue is like a wallet that holds money for when it is needed. Cash is replenished at the ATM and then the money is stored in the wallet until required.  The same goes for the calories we consume in our meals–they get stored in the fat tissue and then the body releases them into circulation as needed throughout the day.

How Fat Gets Stuck in Your Cells

Whether fat flows in and out of fat cells, or just gets trapped inside, is based upon the form of the fat:

        Fatty acids flow in and out of the cells and are used for energy;

        Triglycerides are too big to flow out through the cell membrane, and get stuck inside the fat cells. Triglycerides are made up of a bundle of three fatty acids but which are bound together with glyceride.

You will store fat and get fatter any time you put fatty acids into your fat cells where they can be bundled together into triglycerides and get stuck.

HSL –the enzyme that breaks down triglycerides and makes us leaner

The enzyme HSL (hormone-sensitive lipase) works to make us leaner. By breaking down the triglycerides into component fatty acids, fat can escape from the fat cells and be used for fuel rather than stored. Insulin, however, suppresses HSL and keeps triglycerides from being broken down thus trapping the fat in the fat cells.