The Fuel
The fuel your muscle cells burn is called ATP - adenosine triphosphate. It's derived from glucose in the blood. That glucose can be derived from a number of different sources:
- Fat
- Protein
- Carbohydrate.
Up to marathon distance
When you are exercising, things are a little different. You have enough glycogen to keep you running for about 25-35km (assuming you're pretty fit - this figure can vary, and you can increase it a little by long term training, and increase it a lot [in the short term] by planned carb depletion/loading. Search 'carb loading' if you don't already know about it).
Beyond marathon distance
Some people run far more than 35-40km without problems though (me included). That's possible because your body only burns glycogen when it has to. If your heart rate is above 60-70% of maximum you have no choice but to run on glycogen. If it's below that (but still raised above resting levels), your body will switch to burning fat. So the trick to long and particularly ultra distance is simply to slow down to the point where your body will burn fat.
Running for weight loss
Obviously a lot of people seek to run to lose weight. Very often it doesn't work, they gain weight instead, become disillusioned and give up. What is usually happening is they're exercising too hard - gaining muscle (which is heavier than fat) and not burning body fat. If you want to burn fat, slow down. Walking or a very slow jog will do you more favours than running. The fitter you get, the higher that fat burning threshold gets, and the more energetic you can be whilst still burning fat.
Getting it wrong: hypoglycaemia or 'The Wall'
If you run hard past the point where you run low on glycogen, a number of things happen (symptoms vary a bit from person to person, but the general gist is the same):
1) Tiredness and slowing down
2) Irritability/depression: you may get frustrated or upset/anxious/weepy
3) Clumsiness & disorientation
4) Sudden severe sweating: cold sweats and shivering
5) 'Bonking': suddenly your legs just won't do what you tell them any more.
3 can be risky because you can fall and hurt yourself or get lost, and 4 onwards can be properly dangerous. In off road situations you are at risk of hypothermia. If you got lost because of #3, and become hypothermic, you are in very serious trouble. Runners have died in that situation.
At this point, without a carbohydrate intake after a while your body will start to produce glycogen again, by glyconeogenesis. This involves burning protein (usually muscle tissue). This is probably not good news, it takes a lot of training to gain that muscle, you don't want to lose it. You sometimes get a flavour of acetone in your mouth when this is happening.
Fixing the problem
In this situation, just eating some sugary carbohydrates will fix the problem in the short term. Your blood sugar will rise, and your body will start to burn fat (it needs a little carb to 'power' the fat burning process).
Working out how long it takes food to actually do some good is a bit tricky. Sweet liquids take about 15 minutes to be absorbed. Solids generally take more like 2 to 4 hours, but soluble sugars dissolved in the stomach acid are probably absorbed as quickly as liquids. Certainly it feels like that, but as Bernd Heinrich found, just the taste of food can make your body tolerate lower blood sugar levels - sort of a 'sugar overdraft' as it knows there's food coming. (Attempting to diddle that by eating something that doesn't supply energy is pointless, and could be harmful).
Eating to maximise energy
There are two things to look at here: what to eat before running, and what to eat during running. There's a ton of advice out there on the former point, and it's mostly about your normal routine diet anyway.
The second, what to eat during running, can be tricky. I've tried eating sugary stuff during a run, and it disagrees with me (seems to bring on hypoglycaemia sooner, and makes me burp lots). Starchy food works better, but makes me a bit bloated and slows me down (cold jacket potatoes aren't very palatable, but are a very easily portable source of starch). It's only necessary if you're out quite a long time - I'd never bother if I was planning on being out less than about 3 or 4 hours. Very often even then I'll carry food, but not eat it.
The 'Nakd' cereal bars are the best thing I've tried so far. They're mostly date, so they're not intensely sweet. They seem to work really well for me. YMMV.
I've done better by training to improve my ability to burn fat, which is the next thing. Long fell races involve a much higher energy expenditure than road races (ascent is the main difference, but rough ground and with navigation to do concentration is a factor too), so some eating is necessary for most of us. I find gels unsatisfying, and the sticky goopiness is a nuisance (not to mention the wasteful packaging).
Bernd Heinrich's book 'Why We Run' is a good source for understanding the fat burning process, but by running slow when hungry/low on glycogen (such as training runs before breakfast) you can increase the ability of muscle mitochondria to burn fat. That will increase your range, particularly for distances over the 25-35km point. It's also a handy tip for getting rid of unwanted body fat - low intensity exercise when hungry will burn body fat, and also increase your body's propensity to burn body fat as a matter of course.
Training to maximise energy
Regular training will increase your glycogen storage capacity. Note that muscle can only use local glycogen - there's no point working out your upper body as a glycogen store for running, your leg muscles can't access that glycogen.
Glycaemic Index
The GI of a food reflects it's impact on your blood sugar levels. High GI foods hit your bloodstream fast, but can cause insulin spikes, whereas low GI foods provide slower release energy. It's not as simple as 'low GI before running, high GI during' though, as I've noted before if I eat very sweet food during a run it tends to bring on a sugar crash earlier.
Water & salt
During the winter I rarely carry water if out for less than 2 hours. As well as staying hydrated, you need to consume water if consuming food, and that's also the case for almost all energy gels. How much depends on individual and circumstances - in cooler weather I'll use fairly little - 1.5l will do me for 6 hours. In hot weather I can get through 4l in the same time and still need more.
If you get dehydrated, the initial symptoms are very similar to bonking. Later on after running you're likely to get stomach cramps and severe sweats/dizziness (especially if you eat too much when dehydrated). Dehydration can be fatal, you need to guard against it.
Unless it's extremely hot, you need to consume half a litre or so an hour during running (I tend to just drink when I feel like it rather than some kind of artificial regime). Salt tablets are not necessary for most people unless you're doing ultra distances, and even then probably only for really long runs in hot weather.
Conclusion
All you can do really is experiment to find what foods suit you. It helps if you understand a bit about how your body handles food and running though. As Richard Askwith points out in "Feet in the clouds" everyone has a different food to get them through the tough stuff. For him it was bananas mashed up with lucozade.
In summary, I recommend the following :
- regular slow hungry runs to improve fat burning ability
- using a heart rate monitor to understand what your body is doing at different levels of work
- trying different foods to see what suits you when running.
Links
http://www.medbio.info/horn/time%203-4/homeostasis1.htm
runnersconnect.net/running-training-articles/cience-of-bonking-and-glycogen-depletion
www.livestrong.com/article/415921-what-happens-when-your-body-runs-out-of-glycogen-during-a-long-workout
http://www.amazon.co.uk/Why-We-Run-Natural-History/dp/0060958707/ref=sr_1_1?ie=UTF8&qid=1419335206&sr=8-1&keywords=why+we+run
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