An attempt to run a marathon using SCIENCE: part X (Rest)

My race number arrived!

This week I've been getting to grips with rest. In the practical and the academic sense - while winding down ("tapering") my training I've been devoting some time to theories about *when* to rest. Common sense dictates that your rest day should be the day after your long run. For me, that's usually a Monday, after a 20k or 30k run on a Sunday. However, there are some running bods who would advise me to delay my rest day till Tuesday. Why? Surely it would be madness to run again the day after a killer 30k?

 As I've been finding out, there are plenty of dubious reasons to delay your rest by one day - cleaning out clogged systems etc - but a few days ago I came across this post referring to Matt Fitzgerald's book Brain Training for Runners, which offers an immunologically-founded scientific explanation.

Now, I like immunology. I was such a nerd about it at university that I took *Advanced* Immunology as well as ordinary, boring, easy immunology. And I got top marks. YEAH. So as you can imagine I was pretty excited to hear this explanation.

Ben at Ben's Book Blog has helpfully copy and pasted page-referenced sections of the book if you're interested. The gist of it is that immune cells produce a molecule called IL-6, which is released during long, exhaustive bouts of exercise and is involved in the body's adaptation to endurance training - suggesting it would work to enhance your marathon performance. Therefore, running again while IL-6 levels are still high should offer some training benefit.

"IL6 is believed to facilitate many of the body’s adaptations to exercise training, ranging from increased fat burning to greater resistance to muscle damage, to improved cognition." (Fitzgerald)

Sounds like SCIENCE. Except... "believed"? Could it be just another dubious theory? What worries me is not so much that the theory could be wrong, it's that runners are being told "don't give yourself a well-earned rest the day after a really heavy training session, go back out for more punishment..." The IL-6 theory has been used to support a training strategy known as "bonk training" (ahem), supposedly practiced by elite athletes, but certainly by diet and exercise obsessives looking for quick-fix solutions.

"Bonking" approaches involve running before breakfast or within a few hours of a long run, to get your body - in its state of depleted energy supplies - to shift from burning sugars to its back-up fuel, fat. At the extreme end of the scale, bonking means pushing yourself through "the wall" to a hypoglycaemic state where you feel sick, dizzy and fatigued. As those who read my second marathon post will know, I experienced something like this a few months ago due to misjudging my nutrition needs on a long run and I DO NOT recommend it. It is neither pleasant nor safe, especially if you are running on your own.

Right. So let's break down this IL-6 business. What's IL-6? It's a signal protein that tells your body - primarily your immune system - when it should be doing certain things. We've known since the early nineties that IL-6 is released during lengthy exercise. We also know that it's released when levels of glycogen - a stored form of sugar that you keep in your muscles and liver - are low. Makes sense, right? Energy levels are going to get lower the longer you exercise.

There are studies, in both humans and mice, showing that IL-6 increases up to 100-fold during extreme exercise. Here's one showing that IL-6 levels increase after running a marathon. It's thought the molecule is released by contracting muscles, as well as the brain.

So what happens when you run on already low levels of sugar/glycogen? What is IL-6 telling your body to do? What's the training benefit? It's not so clear. Despite evidence suggesting IL-6 is involved in fat-burning and the body's response to muscle damage, there's still a lot we don't understand about this molecule. In a 2012 study, Danish researchers wrote:

"it is evident that exercise training has beneficial effects on [fat] tissue inflammation and overall [fat cell] function... but it is not known whether IL-6 contributes to such training-induced adaptations in adipose tissue."

In the same paper, after summarising the results of some experiments in mice, they went on to suggest that IL-6 mediates effects of exercise training on fat tissue. Regular training seems to result in *lower* resting levels of IL-6, potentially balanced by increasing levels of receptors producing greater sensitivity to the molecule. But what does it all MEAN? Does running on IL-6 teach your body to switch between fuels? Does it make you fitter? We don't really know. Not only is research pretty early stage, it is published post- the initial hype about bonking and Fitzgerald's 2007 book... Even in 2012, the science of IL-6 was still dodgy.

The picture gets even more complicated when we appreciate that a good deal of the research in the field of IL-6 and exercise comes out of the lab of a minor celebrity scientist at the University of Copenhagen whose reputation appears to have been somewhat blemished by accusations of scientific fraud. "Leading" exercise researcher Bente Klarlund Pederson has written several books (in Danish, I can't translate) about exercise but recently found herself under investigation after a paper she co-authored with colleague Milena Penkowa was retracted.

As far as I can see, there just isn't enough evidence yet to cement the link between IL-6 and any training benefit in endurance runners. It's just a theory. We seem to be back to the same problems I've been coming up against with most of the exercise research I've looked at in this series - the studies are too small and too few. And in this case, they're not necessarily in runners/humans. I was able to track down a relatively recent paper in the Journal of Sports Medicine and Physical Fitness (October 2012) focusing on a study in 16 male runners, which showed that IL-6 levels remained elevated up to six hours after a 90-min run - although I can't get at the full paper because of annoying PUBLISHING REASONS. Anyway, the authors didn't translate this result into training advice, which seems perfectly sensible given the thin evidence base.

Personally, I think it's silly - you might say BONKers - to suggest running on empty, especially if we're talking about only a few hours after a long run. Or to base your advice on one molecule... which might or might not be beneficial in this situation. I imagine there probably are benefits, adaptations, whatever, to this type of training (through the IL-6 route or otherwise), but there may also be risks - like keeling over and DYING. Which, I distinctly remember saying, was something I wanted to avoid.

Also, if running on IL-6 did have some training benefit, presumably it would depend on levels of the molecule remaining high until your next run. In other words, not replenishing your carbohydrate stores, which would bring IL-6 levels down again. So, what, I'm suppose to come in from a 30k run on a Sunday, eat bacon for dinner (or, I dunno, something else devoid of carbohydrate) and then head back out on Monday morning? Pah! Thus I am going to continue taking a whole day's relaxing break every Monday and use it to stock up on lovely, lovely carbs. So there.

As for rest in the more practical sense, the hardest part of my training is now over and, so far, tapering has involved eating almost an entire box of chocolates in one weekend (it was Easter) and getting into the studio at 10.30 this morning. (Despite what you might think, keeping lazy working hours is not standard practice for freelancers. And I haven't worked in my pyjamas *once* this year. Okay, maybe once.)

12 days to go!