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 From The Desk of Clarence Bass



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“We found new metabolic signatures that clearly distinguish more-fit from less-fit individuals during exercise. These results have implications for the development of optimal training
 programs and improved assessment of cardiovascular fitness….”      
 Gregory Lewis, MD, Massachusetts General Hospital Heart Center (ScienceDaily, May 26, 2010)

Brief Peak Exercise Has Continuing Metabolic Effect

Prolonged Exercise Burns Muscle

Fit Burn More Fat

New technology is allowing scientists to explore the effects of exercise in ever-increasing detail. “Using a new mass-spectrometry-based system that profiles more than 200 metabolites at a time…, [a] Massachusetts General Hospital (MGH) based team analyzed blood samples taken from healthy participants before, immediately following, and one hour after exercise stress tests that were approximately 10 minutes long,” ScienceDaily reported.

From that basic design, the MGH study branched into several areas of inquiry, all fascinating and meaningful to anyone interested in learning how exercise works.

The first part of the study focused on the metabolic impact of short-term exercise.

In another part of the study, blood samples were taken from locations in the upper and lower body to determine where metabolic changes occur, locally or throughout the body. 

The researchers also analyzed how fitness level affects metabolic change.

As a separate experiment, pre- and post-race blood samples were taken from 25 runners who competed in the 2006 Boston Marathon. One of the issues examined was muscle breakdown in prolonged exercise.

Lead author Gregory Lewis, MD, and his colleagues at MGH and Harvard reported their findings in the May 26, 2010 issue of Science Translational Medicine.

Interested? Read on. You won’t be disappointed.

Wide and Lasting Effects

We know that exercise improves fitness and reduces health risks, “but how and why these effects occur [is] not entirely clear,” the researchers tell us in introducing the study. The purpose of the study was to measure and analyze the metabolic effects of exercise.

Whereas previous studies have focused on the relatively few metabolites measured in hospital labs, this study looked at more than 200 biochemical factors before and after exercise.

They began by obtaining blood samples from 70 healthy subjects with normal exercise capacity before, immediately after, and 60 minutes after exercise treadmill testing to peak effort (about 10 minutes). They found more than 20 metabolites that changed with exercise, producing compounds involved in burning sugars, fats, and proteins as fuels as well as the use of ATP, the primary source of cellular energy.

“Ten minutes of exercise has at least an hour of effects on your body,” senior author Robert E. Gerszten, MD, told reporters. Important metabolic changes that appeared after 10 minutes to exhaustion on the treadmill were still present 60 minutes after exercise. 

Some changes weren’t known until now. One change not previously associated with exercise was a large increase in niacinamide, a vitamin derivative known to enhance insulin release and control blood sugar; leaner participants had double the increase seen in those with above average fat levels. (More about the impact of fitness level below)

Some metabolites when down with exercise and others went up. For example, concentrations of allantoin, a product implicated in oxidative stress, fell after the maximum stress test. Breakdown of carbohydrate stored in the liver and muscles increased.

While technical in nature, these results support previous findings that brief bouts of exercise have powerful effects. Recently documented benefits of short, hard intervals are an example.

Most Benefits in Muscles

Leonard Schwartz, MD, the inventor of Heavyhands, would love this next finding.

A second part of the study involved eight subjects tested on a stationary exercise bike. Blood samples taken from different locations in the body indicated that most metabolic changes were generated in the exercising muscles, while only a few appeared throughout the body. “We compared samples from the superior vena cava (SVC), which contains blood from the nonexercising upper body, with samples from the pulmonary artery (PA), which reflects the venous effluent from the exercising lower extremity skeletal muscle as well as cardiac muscle,” the researchers explained. “At peak exercise, most metabolites showed significantly larger changes in PA plasma [lower body] than in SVC plasma [upper body]…”

Looks like Dr. Schwartz was “spot on” about the importance of aerobic exercise for the entire body, arms and legs.

On both the treadmill and the bike, some metabolic changes continued for a least an hour after exercise—including lipolysis, the breakdown or destruction of fat.

Fitness and Fat Burn

“We’re only beginning to catalog the metabolic variability between people,” said Dr. Gerszten. As noted above, they tested the changes in people with normal exercise capacity, as well as marathon runners. As you’ll see, the benefits of exercise grow with fitness level. We’ve already seen that leaner people benefit more in blood sugar control.

In line with reader interest, the Associated Press zeroed in on fitness and fat burn: “Checking a metabolite of fat breakdown [glycerol], the team found people who were more fit—as measured by oxygen uptake during exercise—appeared to be burning more fat than the less fit…The extremely fit—25 Boston Marathon runners—had tenfold increases in that metabolite after the race.”

The study looked at  participants tested on the treadmill as a group and based of exercise capacity. As a unified group, fat burn increased up to 100% at peak exercise. The upper half in fitness, however, produced more glycerol than the less fit. “The magnitude of the change in glycerol was most closely related to VO2 max [exercise capacity],” the researchers reported.

Fair or not, fitness matters when it comes to fat burn. The Boston Marathon runners proved this in spades. As the AP article stated, the marathoners on average were producing 1000% more glycerol at the end of the race than at the start. Runners finishing under four hours were producing almost 15 times more glycerol. 

Keep in mind that top marathoners run the 26+ miles in just over two hours. Imagine the fat-burn capacity of runners in the lead pack!

Bad news for “panting newbies,” the AP opined. Perhaps—but encouragement for those just getting started and good news for those already dedicated to exercise.

*  *  *

The last part is a bit tricky. Please hang with me. Keep in mind that exercise breaks down complex substances into simple compounds for use as energy. We all want to burn carbs and fat. Query whether we want to burn muscle as well?

Marathoners have an urgent need for glucose, from carbs or protein (amino acids), to keep them burning fat throughout the race.

Marathoners Burned Amino Acids

One more important metabolic response affected by fitness level—and duration of exercise—is the breakdown of muscle tissue, as measured by amino acids in the blood. “In marathoners amino acid levels also fell significantly [during the race], reflecting their use of amino acids as fuel to maintain adequate glucose levels during extended exercise,” ScienceDaily reported.  

In the short-term exercise treadmill test group, “methionine excursions [abnormal changes] were greater in the less fit subgroup,” the researcher wrote. “Changes in glutamine concentrations were also greater in the less fit group, likely reflecting skeletal muscle release of ammonia during exercise.” (Methionine and glutamine are amino acids.)

The graphs in the report show that methionine and glutamine levels were lower in the more fit subgroup—and higher in the less fit subgroup—at peak exercise and 60 minutes after exercise. Amino acid levels went up in both subgroups.

As the researchers explained in the “discussion” portion of the report, “fatty acids and lipids are preferred substrates for exercising muscle.” There’s a catch, however. Glucose must be present. 

“In order for your muscles to burn fat…, carbohydrate [glucose] has to be present,” Chris Carmichael explained in Food for Fitness. “In conditions where your body is depleted of carbohydrates [glucose], the rate at which you burn fat decreases, and your capacity for high-intensity disappears.” Marathon runners and other endurance athletes call this “hitting the wall” or “bonking.”

As ScienceDaily indicated, muscle tissue can also be broken down to maintain glucose levels during prolonged exercise. 

Muscle breakdown can become a critical issue for marathon runners, who must conserve glucose for the final push at the end of the race. When glucose levels—from carbohydrate and amino acids—are depleted they “hit the wall,” a very distressing situation with more miles to go.

The average time for the Boston Marathon runners tested was 247 minutes. The researchers used 240 minutes (four hours) as the dividing line between the more and less fit runners. We’ve already seen that the more fit runners, those finishing under four hours, burned more fat than the slower runners. 

The team also looked at amino acid “catabolism,” which reflects muscle tissue breakdown for energy. “In contrast to our findings after [short-term] exercise, we saw a reduction in gluconeogenic amino acids,” they wrote. At the completion of the 26.2 miles, nine amino acids in the blood had declined by up to 45% on average. (Gluconeogenesis refers to the formation of glucose from amino acids rather than carbohydrates.)

Taking a clue from ScienceDaily, I interpret this to mean that the reduction reflects the burning of amino acids as fuel to maintain adequate glucose levels during the race. This suggests that muscle tissue was broken down.

That may explain why elite marathoners require a long time—weeks or months—to recover from an all-out effort. 

Previous studies have shown that prolonged endurance exercise destroys muscle tissue.

 Muscle builders beware!

 (Dr. Gerszten is an associate professor and Dr. Lewis is an instructor in medicine at Harvard Medical School.)

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