Metabolic Misconceptions: Part 7: by Fred Dimenna, CSCS

Metabolism is based on rebuilding ATP, the transfer mechanism that lets us use the energy contained in the foods we eat to drive energy-requiring processes in the body. Aerobic (oxygen-dependent) ATP formation predominates most of the time. The two primary fuels used to reform ATP aerobically are carbohydrates and fat. The process involves degrading these ingested macronutrients into two-carbon acetyl groups attached to coenzyme A, thereby forming acetyl CoA, which enters into a series of reactions known as the Krebs Cycle. This sequence modifies acetyl CoA and strips its electrons, allowing the energy of their movement to be harnessed to reform ATP.

Fat is stored predominantly as triglyceride in adipose tissue situated throughout the body. It's an abundant source of fuel, containing nine calories of potential energy per gram. Our capacity to store this energy-rich substrate is large. Even a lean person has a considerable amount of energy packed away in this easy-to-accommodate form. There are, however, some drawbacks to metabolizing fat. Liberating it from its storage form and transporting it to where it is needed is a complicated process. In this case, the oxygen required to accept the electrons once their energy-transferring duties have been fulfilled is relatively high.

Consequently, when energy need per unit time is elevated, fat metabolism must take a backseat. Feeding acetyl groups liberated from fat or carbohydrate breakdown into the Krebs Cycle is similar to water from two tributaries combining to form a river. Imagine one stream providing a constant trickle of water with a virtually limitless supply. If the other feeder was arid, the small stream would account for 100ñpercent of the river's input and effectively keep it from running dry. But the amount of water traversing the main waterway under those conditions would be negligible, so a dynamo situated downstream would be hard-pressed to generate power.

Now, consider what happens if the other tributary suddenly accepts a large watershed and produces a swift torrent. The small stream's flow would still be present and its absolute contribution to the impending river would be unaltered. But on a relative basis, the smaller tributary's contribution is insignificant. To put fuel use into perspective, consider fat as the water comprising the small stream while carbohydrates feed the other affluent. When energy need per unit time is low, fat reliance will suffice and there is no need to put the more advanced supply option into effect. Once demand is heightened, the heavy artillery must be called into play.

Compared to fat breakdown, glycolysis (carbohydrate degradation) is much less complicated. The glycolytic pathway involves a series of reactions that take place right in the cytoplasm of the cell. Using glucose or glycogen as its precursor, these reactions swiftly provide substrate to keep the Krebs Cycle operating in high gear. But glycolysis is only facilitated when energy demand is high. The critical step in the sequence, which involves the enzyme phosphofructokinase (PFK), is activated by substances that increase in concentration when energy turnover is elevated (like ADP and inorganic phosphate, the breakdown products formed when ATP is hydrolyzed to transfer energy).

When ATP concentration in the cell is plentiful, the activity of this enzyme is inhibited, the flow of acetyl CoA to the Krebs Cycle from glycolysis is reduced and fat oxidation takes center stage.In addition to its convenient location, carbohydrate energy is more resourceful to access because of the way it is stored. When we eat foods containing carbohydrates (simple sugars like sucrose or complex versions like starches), the various forms are digested, absorbed into the blood and transported to the liver. Some glucose (the simple sugar to which all carbohydrates are broken down) remains in the blood, while the rest is either joined together (polymerized) into long-chain molecules known as glycogen or converted to triglyceride. Glycogen is primarily stored in skeletal muscle and the liver.

Liver stores provide reserves that keep blood glucose within the narrow physiological range required for normal function. Blood glucose provides substrate to keep all glucose-requiring cells fully stocked. This includes muscle cells, which will receive glucose when needed to restore their glycogen reserves. When glucose is transported into the cell, it is phosphorylated, an energy-requiring irreversible process that effectively traps it within the cell. It can then be used immediately to feed the glycolytic sequence or added to existing glycogen chains, which are packed away and cleaved when substrate for glycolysis is needed.

Fred DiMenna, a Certified Strength and Conditioning Specialist and Lifestyle and Weight Management Consultant is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him at or email: