Thursday, April 11, 2019

What About Sugar? Part 2

In my blog, "What About Sugar?" we learned that sugar in its most basic form, glucose, is not a bad thing. In fact, our red blood cells rely strictly on glucose as they lack mitochondria to process fats and proteins. And the preferred fuel source of our brain is glucose. It's only when we take in too much sugar does it become a problem. So, let's understand exactly what happens when we eat too much sugar.

When we take a bite of that sugary doughnut, enzymes in our saliva begin the process of breaking down sucrose into glucose and fructose. These molecules travel all the way down to the small intestine, where they traverse the intestinal wall and enter the portal vein to be carried to the liver. Once the molecules reach the liver, they either stop there and are converted to their storage forms, or they go into circulation to be used for energy by our cells. Our muscle cells use only about 35-40% of the fructose eaten. Glucose is the preferred fuel of our muscles as it is absorbed more rapidly in the body and 
utilized more efficiently.

But, what if our muscles don't need to use glucose and fructose immediately for fuel? They then need to be stored for later use. Glycogen is the storage form of glucose. If fructose is not used immediately for fuel, it needs to be converted to glucose by the liver and then sent back out to the muscles to be stored as glycogen. The more muscle we have, the more storage depots we have. Typically, for an average female, 30-35% of her body weight is muscle, 36-40% or a little more for a more muscular woman. For an average male, 40-45% of his body weight is muscle, and up to 55% for a more muscular male. For every kg of muscle tissue we have, we store about 15 grams of glycogen. For example, I have 47% or 62 lbs of muscle tissue, so I can store roughly 420 grams of glycogen (62 lbs / 2.2 = 28 kg x 15 grams). The take home message here: the more muscle we have, the more carbs we can eat!

Very rarely do we deplete our muscle glycogen stores; 60-90 mins of intense exercise uses approximately 40% of our muscle glycogen tank. So, what happens if glucose arrives at the muscle but the muscle doesn't need to use it immediately and our glycogen tanks are full? Glycogen can also be stored in the liver, but not very much. Typically 75-100 grams for women and 100-125 grams for men. So, here's the problem! The remaining glucose is taken up by fat cells and easily converted to fat because they have the same building blocks; carbon, hydrogen and oxygen. But, excess fructose has to go to liver to be converted to triglycerides, put back into the blood (elevating total cholesterol), and then taken up by fat cells. When large quantities of fructose 
(50-100 grams) are consumed (sugar is 50% fructose), the liver may decide the amount of triglycerides is too much to be released into the blood, so it chooses to store it instead. This can lead to a disease called, "Non-Alcoholic Fatty Liver Disease," which is basically the same thing as cirrhosis. The liver becomes diseased and can no longer function the way it is supposed to. The take away message here: stay active so the muscles and liver need to store glycogen!

The more muscle we have, the more carbs we can eat!

Whitney | Rolfes. "Understanding Nutrition." Cengage Learning, 2011.
Comana, Fabio. "Energy Pathway, Ketones and Nutrient Timing." 2019 IDEA Personal Training West.