Exercise, Weight Loss, Glycogen, Oh My!

For those of you following along on my great Twitter adventure know, I recently purchased a new bike and am trying to get out and get some exercise from time to time. Prior to leaving for Argentina I was actually going to the gym about four or five times a week, but didn't really get much exercise (other than massaging my liver from time to time) while I was away.

Despite what many people think, exercise really only has a moderate affect on weight loss, if any at all. In fact, almost every proper study ever done that tests the hypothesis whether or not exercise leads to weight loss is inconclusive. Yes, if you starve yourself and put yourself through four hours or crazy routines every day (like on the Biggest Loser), you will lose weight. But it's almost always short lived.

I'm one of the believers that calories in and calories out are generally irrelevant. If you eat less, your metabolism slows down. If you eat more, your metabolism speeds up. It's like a furnace, and you control what happens based on how much wood you toss on the fire. That's why people on prolonged diets often show low thyroid levels - their metabolism has simply downshifted based on the current energy inputs.

That said, exercise contributes to weight loss in other less known ways. First, after any period of exercise the muscles that were used became extra sensitive to the hormone insulin. Since the muscles require less insulin to perform the same functions, the pancreas outputs less of the hormone, and the mean level in the blood is reduced compared to a pre-exercise state. Since insulin is the dominant hormone involved in weight gain, a lower level makes it harder to gain weight and conversely easier to lose weight.

The other affect involves glycogen. The body actually keeps a resolve of carbohydrates in the liver to help with short-term energy needs. This energy is stored inside the liver as glycogen, and is broken down and utilized whenever the body needs more energy or blood sugar falls.

When glycogen levels are high the body has no need to access fat stores since it can obtain all of its energy from glycogen. When glycogen stores are depleted the body has no other choice but to start breaking down fat (or possibly muscle) to fuel the body. Often when people talk about "hitting the wall" in any particular activity they are referring to the moment in time when their glycogen stores are depleted. Also when people "carb up" before a workout or a game they are basically filling up their glycogen stores.

So you see, the lower the glycogen levels are kept, the more likely it is that the body will be forced to switch to burning fat in a given day. That's why periodic (or daily) exercise can help with weight loss - it keeps the average level of glycogen in the body lower. It's also one of the methods that low-carb diets work (the other being the prolonged lowering of insulin levels) - without carbohydrates in the diet, the glycogen stores will be depleted in a day or so, causing the body to switch to burning fat.

Exercise is fun and helps keep the body and cardiovascular system in shape, but the affects on weight loss are varied, and not all related to simple burning of calories as I pointed out above. You're now up to date in the world of glycogen.