Intense exercise or ischemia (insufficient blood flow) may cause significant loss of nucleotides that leave the cell starving for energy. Obviously, when this happens, the function of the cell may be at risk. Weightlifters may not be able to generate the powerful contractions they demand, and the weight they were able to lift only a day or two ago may be impossible to move. Your heart may not pump enough blood to supply your tissues with adequate amounts of oxygen.
Under conditions of maximal exercise, there is a substantial decrease in the total ATP ADP and AMP pools in skeletal muscle cells. In fact, research has shown that decreases in these nucleotides can be as much as 20 to 28 percent after periods of high-intensity exercise. Both fast-twitch and slow-twitch muscle fibers use ATP to provide energy for contractions. During strenuous exercise ATP is broken down, rapidly causing AMP to build up in the cell. Some, but not all, of the AMP formed by either fast-twitch or slow-twitch fibers can be converted to other compounds that give the cell the ability to conserve energy-producing molecules to continue to fuel its work. A substantial portion of the AMP, however, is broken down and washed out of the cell. It is this breakdown AMP, and the loss of breakdown products, that deplete the cell of its energy-producing compounds.
How Much ATP Is Lost Due to Exercise?
Is the amount of ATP, ADP and AMP loss during exercise really significant? The simple answer to that question is yes! Skeletal muscles are very efficient at conserving energy. However, when they are called on to perform maximum work, the number of energy-producing or energy-conserving molecules that are lost can be significant. As long as there is a sufficient amount of oxygen present in the cell for aerobic metabolism, muscle cells are able to recycle energy virtually without losing any adenine nucleotides. Nevertheless, during periods of very hard supramaximal work, when there is not enough oxygen absorbed into the bloodstream to supply the demand of the cells, a large percentage of the total pool of ATP ADP and AMP can be lost.
A considerable amount of research has shown that the loss of adenine nucleotides by skeletal muscle can be severe during periods of intense exercise or ischemia. One very important study was performed at the Karolinska Institute in Stockholm, Sweden, which is one of the foremost centers for skeletal muscle research in the world. In this study 11 healthy male volunteers underwent six weeks of high-intensity training three times per week, followed by one week with two training sessions per day. A second group of nine healthy volunteers rested for the first six weeks but trained twice a day along with the first group during the final week. A small amount of muscle tissue was taken from the thighs off each of the subjects to analyze the amount of adenine nucleotides present following these periods of exercise and for three days following the final day of training. Muscle tissue samples were also taken before exercise began, as a point of comparison.
This research showed that ATP levels in the thigh muscles of the first group dropped 13 percent during the six weeks of training. ATP levels in the muscle tissues of this group did not decrease further during the final week of training. This shows that after a period of intense exercise ATP, ADP and AMP all dropped to well below pretraining levels. More significant, however, is the fact that even after three days of rest following the last exercise bout, ATP levels in the first group still did not recover to pretraining levels. In fact, ATP levels in the muscle cells of these subjects were still almost 10 percent below their pretraining levels. In other words, even after the three-day rest period the thigh muscles were not able to fully replace their lost ATP
In the second group the effect was even more dramatic! This group did not have a period of training before beginning high-intensity exercise in the final week. In fact, the subjects went from being sedentary to performing two exercise bouts per day for one week. In this group ATP in the thigh muscles dropped by 25 percent immediately after the last exercise bout. Even after three days of rest this group still had an ATP pool that was 19.5 percent less than it had initially! Think of it. The total energy currency in the thigh muscles of this group dropped by 25 percent after intense exercise and was still almost 20 percent lower than normal even after three days of rest!
These dramatic results show that energy-producing compounds are lost from the cell during exercise and do not recover even after three days of rest. In their paper the researchers concluded, "Repeated high-intensity intermittent exercise caused a decrease in resting levels of skeletal muscle adenine nucleotides [ATP] .... The decrease was greater when exercise was more frequently repeated."
In a similar study Dr. Stathis of Victoria University in Australia worked with other researchers to show that thigh muscle ATP levels fell by 19 percent after seven weeks of sprint training. These researchers concluded, "We observed reductions in resting ATP [and TAN, or total adenine nucleotides] after training and attribute these primarily to the inability of muscle to completely restore the purine base lost as a result of high ATP-turnover rates during training sessions." In other words, because of exercise, these muscle cells lost the molecules necessary to recharge their energy.
How Does Poor Circulation Cause Loss of Energy in the Heart?
Maintaining a healthy heart means that we have to be aware of its needs. Among those needs is maintenance of adequate energy supplies to allow the heart to perform normally. When the heart is starved of energy, it is not able to perform its work properly. Energy is required for the contraction, or pumping, that provides blood flow to the body. Surprisingly, maximal levels of ATP are also required for relaxation. If the heart is not allowed to relax properly, it cannot fill with enough blood and the total blood flow through the heart is decreased.
Heart cells show even higher losses of energy charge during ischemia than skeletal muscles. In one study done at the University of Minnesota, researchers showed that 15 minutes of ischemia in the heart led to a decrease in ATP by an amount greater that 50 percent! This study was done in dogs and was performed to mimic the effect of stopping the heart during surgery. Previous research has shown that the action of ATP metabolism in dogs and other animals is similar to that in man, so the comparison is a good one. In the University of Minnesota study ATP and total energy charge did not recover even 24 hours after blood flow was returned to the heart. It was reported in this study that it takes seven to 10 days for ATP levels to recover totally following severe ischemia.
These study results are reinforced by the similar results of many other studies. In 1984 Dr. Heinz-Gerd Zimmer, who is now the head of the physiology department at the Carl-Ludwig Institute at the University of Leipzig, Germany, reported a similar finding. In his study temporary ischemia of 15 minutes caused about a 45 percent decrease in ATP content. He showed that ATP was slightly recovered immediately after ischemia as the ATP content of the cell attached to additional phosphate groups to recycle ATP However, even after three days ATP levels in the heart did not recover to the levels that were present before ischemia occurred. In another study Dr. Herbert Ward and his associates showed that even after 48 hours of full blood flow following ischemia, ATP levels only recovered to 53 percent of preischemic levels.
How Can You Prevent the Energy Loss?
The bottom line is this. No matter how hard you try to stop it, no matter what foods you eat or supplements you take (except ribose), if you are actively exercising, then you are losing the compounds necessary to maintain maximum levels of energy in your muscle cells. If you have poor circulation to your heart or other muscles, a metabolic imbalance or a muscle cell enzyme deficiency, you are losing energy compounds. Certain things may be done to reduce the number of these compounds lost from your cells, but in the end, if you are using your muscles for strenuous, high-intensity exercise, your energy pools will decrease. The best you can hope for is rapid replacement of lost energy-producing compounds so that your muscle cells' energy charge can return to its peak efficiency.
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