The "exercise hormone" irisin has generated controversy among scientists — some say it's produced when humans work up a sweat, and holds promise as a weight-loss treatment, but others contend that irisin doesn't even really exist in people.
Now, one research team says it has proven the existence of irisin in humans once and for all, using a technique that is more precise than those used in the past to identify the protein.
"These data unequivocally demonstrate that human irisin exists," the researchers wrote in the Aug. 13 issue of the journal Cell Metabolism.
Irisin was discovered in 2012, when researchers at Harvard Medical School found that both mice and people produced the hormone during exercise. Later studies found that, in mice, irisin improved blood-sugar regulation and led to weight loss, causing some people to speculate that a drug based on the hormone could be used as a treatment for obesity.
Recently, however, the existence of irisin was debated when another group of researchers, including scientists from Duke University, questioned the methods used to identify the hormone. They said that the antibodies used in the original study were not specific, meaning that they reacted to other proteins in the blood besides irisin.
Now, in the new study, the researchers who discovered irisin used a technique called quantitative mass spectrometry, in which a protein is broken down into smaller fragments. Researchers then use information about the mass of the molecules in the fragments to identify specific proteins.
The researchers identified irisin in blood samples from both sedentary individuals and people who underwent 12 weeks of aerobic training. The level of irisin was 3.6 nanograms per milliliter in sedentary people and 4.3 nanograms per milliliter in those who underwent the training.
"Our paper definitively confirms that irisin circulates and is altered with exercise in humans," said study researcher Bruce M. Spiegelman, a professor of cell biology and medicine at Harvard Medical School. [10 Medical Myths That Just Won't Go Away]
These measurements show that, although irisin is present in small amounts in the blood, its concentration is similar to that of other important hormones, such as insulin (which causes body cells to take up sugar) and leptin (which causes people to feel full after eating), the researchers said.
Keith Baar, an associate professor at the University of California, Davis who has studied the gene for irisin, said the new findings are convincing. "I do think that they show that the protein [irisin] can be found in a relatively decent amount in humans," said Barr, who was not involved in the new research.
However, the new study does not address the question of whether this hormone has benefits for the body when people exercise. A previous study in which Baar was a co-author found no link between the amount of irisin mRNA — which can be thought of as a precursor to the irisin protein — and positive health outcomes in people who exercised.
Future research will need to show whether irisin is "in some way related to the health benefits of exercise," Baar said.
Alisa Blazek, a graduate student at The Ohio State University's Department of Molecular, Cellular and Developmental Biology, agreed. "Other studies would need to be done to determine how important it is for health," Blazek said.
Blazek added that the study of irisin and other myokines — proteins secreted from muscle in response to movement and exercise — is an important area of research.
"There's so much that we don't know about the molecular mechanisms of exercise," Blazek said. "And if we could learn more about them, it can help us to design therapeutics, and learn how effective our exercise or physical therapy programs are."