Introducing Irisin: the exercise hormone

Wouldn’t it be great to never have to worry about exercising again – just take a pill or get a shot, and receive all the health benefits by activating the molecular pathways directly, no sweat involved? 

There is decidedly a demand for this type of treatment, and now it seems researchers might be one step closer to realizing the dream. In a paper published in Nature last week, Boström and colleagues claim to have found the “exercise hormone,” naming it irisin, after the messenger goddess Iris, for its role as a molecular messenger between muscle and other tissues, particularly fat. In it lies the potential for miracle treatment of the aforementioned kind, yielding both weight loss and improved glucose tolerance.

It is almost universally acknowledged that fat is undesirable. But all fat is not created equal. There’s white fat (the normal kind) and then there’s brown fat. While the presence of the latter in hibernating animals and newborns is well-established, the discovery of brown fat also in human adults was initially quite controversial. In many ways more similar to muscle than white adipose tissue, the primary purpose of brown adipose tissue is to generate heat. It does this by disrupting the normal function of mitochondria – the cell’s powerhouses that generate ATP, the body’s main fuel – with a protein called uncoupling protein 1. As the name implies, this protein uncouples the normal process by redirecting the energy that would have been used to make ATP to generate heat instead, a process known as thermogenesis. But how does brown fat tie into exercise?

In a 2010 review, Enerbäck suggests that brown adipose tissue has “evolved to safely dissipate large amounts of chemical energy.”  Moreover, Xu and colleagues showed that exercise training increased brown adipose progenitor cells compared to sedentary controls, both in mice on normal and high-fat diets. The exercising mice also had enhanced expression of uncoupling protein 1, and the researchers concluded:

“Exercise ameliorates high-fat diet-induced metabolic disorders and vascular dysfunction, and increases adipose progenitor cell population in brown adipose tissue, which might thereby contribute to enhanced functional brown adipose.”

In short, exercise is good for brown fat, which in turn is good at getting rid of excess energy. As Enerbäch points out, this can be very useful to the modern human, who wishes to remain healthy despite living a lifestyle with an overabundance of calories.

And now we can finally return to Boström’s discovery. Irisin, a previously undescribed protein hormone, is secreted from muscle into the blood, and then stimulates thermogenesis in adipose tissues. They also found that irisin has a powerful browning effect on some types of white adipose tissues, and even a moderate increase in irisin blood levels stimulates a massive increase in uncoupling protein 1. The researchers gave irisin to obese mice on a high fat diet for 10 days, and found that even this relatively short treatment significantly improved glucose homeostasis and caused a small weight loss. In other words, administering irisin seemed to counter some of the important symptoms of metabolic syndrome.

Therapeutic potential
By now the significance of the discovery should become apparent. This study only looked at adipose tissue, but clinical studies have shown exercise to be effective treatment in many other disorders than just metabolic syndrome. It would be interesting to explore the effects of irisin on other tissues too.

And even focusing just on adipose tissue, the results are impressive. Delivering peptide hormones to the blood by a shot is nothing new – ask any diabetic taking insulin – so presumably irisin could be administered fairly easily too. Naturally, further experiments are required in order to determine if the weight loss would continue with longer treatment, and what the potential side effects/benefits might result from higher doses, but at least this is an indication that treatment might work.

Nevertheless, for the time being more data is needed, and the process of confirming the results and developing a reliable treatment plan takes time. So for now, you’re better off sticking to your exercise plan than waiting around for the miracle cure!

Written by Hanna Sofie Ellingsen at CERG.

This entry was posted in Cardiovascular disease, Exercise, Fitness, In English, Obesity, Public health, Research by CERG. Bookmark the permalink.

About CERG

The Cardiac Exercise Research Group (CERG) at the Norwegian University of Science and Technology (NTNU) seeks to identify the key mechanisms underlying the beneficial effects of physical on cardiac health in the context of disease prevention and treatment. Named the K.G. Jebsen Center for Exercise in Medicine under Professor Ulrik Wisløff's leadership in 2011, CERG uses both top-down and bottom-up approaches to combat lifestyle-related disease.

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