Which Came First, the Exercise or the Skinny?

Years ago, I read a really stupid "study" that claimed to demonstrate that running caused women's breasts to shrink. Why? Because the folks doing this ridiculous piece of "research" had looked at a group of serious female runners, and discovered that most of them wore a B-cup or smaller bra.

As a girl who was the first in her fifth grade class to wear a bra, who was a C-cup by freshman year, and is wearing a D as she writes this, I found this the most enchantingly bass-ackwards piece of reasoning I had ever run across. Um, folks? Girls built like me don't run.

It hurts. It chafes. It makes us embarrassingly conspicuous. Since becoming a bona fide health nut at 19, I have done many kinds of exercise -- step aerobics, weight lifting, breathing exercises, Pilates, Heavyhands Walking (my all-time favorite) -- but all attempts to take up running have been short-lived.

Very simply, running doesn't make women small-breasted, small-breasted women are more likely to run.

I thought of this when I saw Gary Taubes (Good Calories, Bad Calories) speak at the Mizzou School of Life Sciences last month. Among other important concepts, Taubes spoke about the commonly-accepted idea that exercise causes weight loss. Taubes contends that this idea is just as mixed up as the notion that running causes flat-chestedness. Instead, he says, thin people are simply more likely than fat people to exercise.

Why? Because of something called "fuel partitioning." In other words, how our bodies decide to allot the calories we eat. In people who gain weight easily, the body may well take, say, 70% of the fuel consumed and immediately stick it into storage -- ie, turn it into fat. This leaves only 30% of those calories to be burned for energy. As a result, these people are chronically tired. And tired people don't feel like working out.

On the other hand, in really skinny people, only a small fraction of calories will be stored -- maybe 10-20% -- leaving the rest to create energy for immediate use. These folks are bursting with energy -- the foot-tappers, the fidgeters, the can't-sit-still-for-a-minute folks. So they exercise! They're never happier than when they're doing something active. But they're not skinny because they exercise. They're driven to exercise because they're skinny.

There appear to be a few things that influence fuel partitioning. Genetics plays a big role, as does fetal environment -- things that are out of your control. But another major influence on fuel partitioning is insulin.

So long as there is much in the way of insulin circulating in the blood stream, fat will not be released from storage to be burned as fuel. Conversely, dropping insulin levels signal the fat cells to release fat into the blood stream to give you the fuel you need.

What this means is that if you are one of the people who genetically partition a lot of incoming calories to fat storage, so long as your insulin levels are elevated -- ie, so long as you eat a high carb diet -- you will have very little fuel available to burn. You will be tired and sluggish, and hungry most of the time. Tired, sluggish people find it very difficult to exercise. But it wasn't the lack of exercise that made you fat -- it was the fact that your body shunted your incoming fuel supply into storage that discouraged you from exercising.

Conversely, when you go low carb, and therefore reduce your insulin levels, the fuel stored in your fat cells becomes available for fuel. After a week or two of adjustment, you feel far more energetic, because you have all of that stored fuel available to burn. All of a sudden, a walk sounds a lot more appealing. And since regular exercise improves insulin reception, your blood insulin levels eventually drop still further, increasing your energy even more.

Doesn't mean you'll get to the point where you're partitioning fuel like a genetically skinny, athletic person. But you can certainly tilt the odds in your favor, not just of losing weight, but of having vastly more energy. Since the most common complaint to doctors in America today is simple fatigue, that's no small thing.