The research has deepened our understanding of how different forms of exercise affect the body.
A significant finding is that endurance exercise increases a metabolic hormone called fibroblast growth factor 21 (FGF21), whereas strength training decreases another called fibroblast growth factor 19 (FGF19).
The effects of various forms of exercise on better-known hormones such as insulin and adrenalin are already well-understood, says senior study author Christoffer Clemmensen, who is working as an associate professor in the Novo Nordisk Foundation Center for Basic Metabolic Research in Copenhagen.
The new insight from their findings, he explains, is "that strength training and cardio exercise affect FGF hormones differently."
The study — which features in the Journal of Clinical Investigation: Insight — also confirmed some known effects of other metabolic substances.
Metabolism and exercise
The word metabolism comes from the Greek phrase for "to change," but we use it to refer to all the processes that extract and use energy to sustain life.
These range from breathing, digestion, and temperature regulation to muscle contraction, keeping the brain and nerves working, and getting rid of waste through feces and urine.
Metabolism changes over the lifespan. As we get older, we burn fewer calories and our digestion alters. We also lose lean muscle and — unless we take care of our diet and exercise regularly — gain weight.
Eating too much and having a lifestyle that is mostly sedentary can hasten these age-related changes.
To protect against this, experts advise us to follow guidelines on how to eat a healthful diet and remain physically active.
In the United States, the physical activity guidelines for adults advise a combination of muscle-strengthening and aerobic, or endurance, exercise.
Exercise and metabolic hormones
However, while there is a lot of evidence outlining the various benefits of exercise on health, "the underlying mechanisms remain incompletely understood," Clemmensen and his colleagues note in their study paper.
So, they investigated this further by examining the impact of two forms of exercise on metabolic hormones, which are the chemical messengers that regulate the processes of metabolism.
They recruited 10 healthy young men and randomly split them into two groups. In one group, the men did cardio training first, and then strength training about a week later. In the other group, the men did the strength training first and then the cardio.
All the exercise sessions lasted for about an hour and were intensive. In the cardio session, the men cycled at 70 percent maximum intake of oxygen. In the strength training session, they put all major muscle groups through a regime comprising five different exercises repeated between five and 10 times.
During the 3-hour recovery period following each exercise session, the researchers took blood samples from each man immediately after exercise and then at intervals thereafter.
They used the blood samples to measure changes in levels of: blood sugar, lactic acid, several hormones, and bile acid.
Different effects on metabolic hormones
The results showed that blood levels of FGF21 rose significantly during the cardio or endurance sessions, but not in the strength training sessions.
The effect of cardio on FGF21 was so marked that the researchers believe that it warrants further investigation. Of particular interest is whether the hormone is directly involved in the health-promoting effects of cardio exercise.
The results also showed that levels of FGF19 fell slightly after strength training. This was a surprise to the researchers who were expecting it to rise, since animal studies suggest that the metabolic hormone helps with muscle growth.
FGFs are active in many different biological processes, in addition to helping regulate metabolism. These include, for example, cell growth, embryonic development, tissue repair, and tumor formation.
FGF21 is produced in several organs and is active in weight loss, glucose control, and reducing inflammation.
FGF19, which is produced in the gut, is described as an "atypical" member of the FGF family. As a hormone, it helps regulate bile acid production and metabolism of glucose and lipids.
Animal studies have shown that, as well as helping with muscle growth, FGF19 can aid weight loss, reduce fats and glucose levels in the liver, and improve use of insulin.
The team now plans to further investigate the links between metabolic hormones and exercise. One limitation of the new study was that it only looked at changes in the 3 hours following the exercise session. It is still not clear what happens in the longer-term.
"FGF21's potential as a drug against diabetes, obesity, and similar metabolic disorders is currently being tested, so the fact that we are able to increase the production ourselves through training is interesting."