by Barbara Berkeley, MD
The latest chemical substance to get scientists into raptures is irisin, a hormone that is produced in muscle cells but exerts its effect on fat. We've all figured out that exercise and weight are somehow tied together and that this means that muscle and fat must communicate. Unfortunately, we've showed a woeful lack of imagination in tryng to come up with the mechanism. For decades, we've persisted in trying to jam the same puzzle piece into a space that doesn't fit. We are determined to prove that the connection is calories. It seems so logical. If we burn calories through working out we must lose weight, right? And yet the evidence that this is so remains annoyingly equivocal.
While readers will occasionally write to me about great weight loss successes from becoming physically active, it is rare to observe this in clinical practice. The more common experience is to see people who have added exercise, but have not achieved anything more than a minor result. As I've mentioned in the past, I have taken care of many very athletic people, even marathoners, who were clinically obese. Could they have just been eating enormous amounts of food? Of course it's possible. But most of them were trying to lose weight by modulating their diet. One would think that these prodigious amounts of exercise would force weight loss, but in their cases it did not.
On the other hand, it is observationally obvious that maintainers who exercise have a higher rate of success at keeping weight off. This observation has been confirmed by data from the National Weight Control Registry, a large study which monitors those in maintenance. While the NWCR subject pool is limited to maintainers who volunteered themselves (and thus are probably highly motivated), the data shows that successful study maintainers are exercising a lot (on average, about 1 hour daily).
Irisin is a hormone that is produced from the breakdown of a protein that is produced when muscles are exercised. In rodents, it's injection appears to turn white fat into the more metabolically active brown type. Humans do not have very much brown fat, so it is difficult to speculate whether this finding is meaningful for us. Even rats who made more brown fat did not lose weight when injected with irisin. So why the flurry of interest? Because high levels of irisin appear to improve the way the body metabolizes food, making it less insulin resistant and more weight stable. The following is from a recent NY Times article on irisin:
But while irisin appears to have a critical impact on metabolism, it does not appear to play any discernible role in the effects that exercise has on the heart or the brain. And various issues remain unresolved. Why, for instance, if exercise increases levels of irisin and irisin increases the body’s stores of energy-burning brown fat, does exercise so rarely produce significant weight loss? The mice injected with irisin lost little weight. On the other hand, Dr. Spiegelman notes, they resisted weight gain, even on a high-fat diet, and their blood sugar levels remained stable. So it would seem that exercise, through the actions of irisin, can render you healthy, if not svelte.
As I have repeatedly said in this blog, research results only become important when they are repeatedly reproduced. The irisin story is obviously in its very earliest chapters. But the existence of such a hormone may well help to explain why exercise is so important during maintenance, and relatively ineffectual during weight loss. For those who are currently losing weight, I don't discourage exercise, but I ask you not to rely on it. Weight loss is about forcing the body to burn itself (which it would prefer not to do) through caloric restriction. Think of your exercise as a way of rebuilding and retuning your muscles for later use. Once you reach maintenance, these irisin producing factories will be fired up and ready to go--giving you a much improved shot at maintaining the body you've earned.
