Dramatic Health Benefits After Just One Exercise Session

Researchers from the University of Michigan discovered that a session of aerobic exercise increases storage of fat in muscle, which actually improves insulin sensitivity. Low insulin sensitivity, or insulin resistance, is an impaired ability of the body to take up sugar from the blood, which can lead to high blood sugar and diabetes.

Jeffrey Horowitz, associate professor in the U-M Division of Kinesiology, and his former doctoral student, Simon Schenk, now a post-doctoral fellow at the University of California, San Diego, conducted the study, which appeared online May 17 in the Journal of Clinical Investigation.

"It's long been known that exercise can greatly improve insulin sensitivity," Horowitz said. "But how exercise improves insulin sensitivity in obesity, and how much exercise is necessary for this effect is not clear."

Impaired insulin sensitivity is particularly a problem in obese people because of the excessive amount of fatty acids released from their body fat stores. This overabundance of fatty acids is taken up by tissues like muscle and liver where they interfere with the ability of insulin to regulate sugar metabolism.

In muscle cells, fatty acids can be burned for energy, and they also can be stored as intramuscular triglyceride, or IMTG. IMTG is a reservoir for fat storage, and high IMTG levels correlate with insulin resistance in obese people and those with type 2 diabetes. Partly because of this correlation, many researchers assumed IMTG is somehow involved in the development of insulin resistance. Yet, people who exercise regularly also have high IMTG levels, but they are actually very sensitive to insulin.

With that in mind, U-M researchers set out to test their novel hypothesis: that increasing the capacity for fat storage in muscle after one session of exercise can actually increase insulin sensitivity. They suspected that for several hours after exercise more fatty acids entering the muscle will be stored as IMTG, thus keeping them from turning into more harmful metabolites that are known to cause insulin resistance. Essentially, this means that exercise may cause you to store more fat in your muscles, but in doing so your insulin sensitivity improves.

Researchers studied eight lean female subjects and infused fat into their bloodstream to increase fatty acid to levels commonly found in obesity. The subjects were admitted to the hospital for this two-day procedure on two separate occasions. On the first day of one hospital stay, they exercised for 90 minutes at 75 percent of maximum heart-rate; on the other visit, they remained inactive.

With all other conditions being equal, researchers found that during the nonexercise visit, the fat infusion reduced insulin sensitivity to levels commonly found in obese people.

However, they found that during the exercise visit, not only did the exercise prevent the impairment in insulin sensitivity, but it increased insulin sensitivity by about 25 percent over their base levels. The researchers also found that the exercise session had diverted more fatty acids to be stored as IMTG than without exercise, and as a result fewer fatty acids were available to become the harmful metabolites known to impair insulin sensitivity.

"We believe this describes a primary mechanism for how exercise improves insulin sensitivity in obesity," Horowitz said.

The study findings also highlight the important metabolic health benefits of a single exercise session.

"Some of the key health benefits of exercise are not related to improved fitness but instead, the residual effects from the most recent exercise sessions are most important," he said.

If this is correct, then getting a regular so-called dose of exercise may be much more important than your level of physical fitness. How hard the exercise dose must be in order for an obese person to reap the benefits, and how long the effects last remains unknown. Horowitz and his research team are addressing these issues.