Metabolic Misconceptions: Part 5
by Fred Dimenna, CSCS

ATP is the transfer mechanism that allows us to use the energy contained in the foods we eat to drive energy-requiring reactions in the body. As the critical link between utilizing energy from food stores, ATP must be available at all times. Fortunately, the body has a number of mechanisms to ensure that an adequate amount is present whenever we need to cash it in. These include actual ATP stores (limited in quantity, but highly accessible), phosphocreatine (PCr) stores (which are used to rapidly rebuild ATP, but are also relatively small), anaerobic glycolysis (carbohydrate breakdown in association with the formation of lactate) and oxidative phosphorylation (ATP formation that uses oxygen as the final electron acceptor). Each of these options has advantages and, accordingly, will be preferred in specific circumstances.

The primary path ATP is made available is based on ATP hydrolysis (splitting, which puts the potential energy it contains into use). When ATP is hydrolyzed rapidly, as in the case when you’re sprinting, or demand increases abruptly (making a rest-to-activity transition), stored phosphagens (ATP and PCr) are relied upon to shoulder the burden. Because these stores are small, they are used up rapidly.

Consequently, if you’re sprinting as fast as you can, you can only maintain your best velocity for 10-20 seconds before your pace declines. On the other hand, if stored phosphagens are simply filling transitional voids (going from lower to higher levels of effort), other systems will be able to take over before their limited capacity begins to expire. If you continue to run as fast as you can once you can no loner rely on the phosphagens you have packed away, your next best option is to transfer energy by breaking down carbohydrates incompletely in a process known as anaerobic glycolysis. Carbohydrates are stored in the body in the form of glycogen, which accumulates in muscles, the liver and heart. The blood also contains readily available carbohydrate supplies in the form of glucose. Glucose in the blood is needed to replenish glycogen in muscle, while liver glycogen is liberated to maintain blood glucose in the narrow range necessary for normal function. This is essential because the central nervous system is highly dependent on carbohydrates in this form.

In general terms, glycolysis (glycogen breakdown) actually occurs under two diverse circumstances: aerobically and anaerobically. The major determinant of which process is in effect is the rate at which ATP is needed. If rapid hydrolysis is called for (as would be the case if you were continuing to sprint as fast as you could once your stored phosphagens were incapable of sustaining the effort), the anaerobic option would be preferred because it provides for ATP reformation more rapidly than the aerobic one. But with this advantage comes a drawback. Hydrogens liberated in the process cannot be utilized to their fullest energy generating extent. Instead, they are stored temporarily by forming lactate, a product that spells trouble if it accumulates. As a result, anaerobic glycolysis serves as an appropriate quick fix, but relying on it for extended periods is not feasible.

For this reason, aerobic ATP reformation (oxidative phosphorylation) predominates most of the time. This option involves the complete breakdown of carbohydrates and can also make use of the energy stored in lipids (including fatty acids that can be liberated from fat stores). In addition, even lactate and amino acids (proteins) can be used by this easy to-accommodate system.

Building ATP aerobically engenders virtually no limitations when this system predominates; you can maintain what you’re doing for extended periods. Consequently, regardless of whether you’re fast asleep, sitting at your computer or jogging a challenging, but sustainable pace, if you’re performing the effort comfortably, you can be rest assured ATP demands are being supplied this way. The fuel being accessed, however, will depend on a number of factors, the major one being the rate at which ATP is needed.

The two primary aerobic fuels are carbohydrates and fat. Both are utilized to provide acetyl CoA, the substrate that gets the aerobic ball rolling by entering a cyclical series of reactions known as the Krebs Cycle. The primary purpose of these reactions is to liberate hydrogens and accompanying electrons. The electrons are transferred by a series of protein carriers known as the electron transport chain, causing a flow that transfers energy through motion (much like the movement of water turns a water wheel to power a dynamo). This energy is used to add a phosphate group to ADP, the product formed when ATP is broken down. The rebuilt ATP can then be used to satisfy demands that are present, stored in the aforementioned small quantities or employed to reform PCr from the free creatine that has accumulated from prior PCr breakdown.

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: