I’ve read several triathlon and running articles which state that it is a good idea to do long-slow endurance work to train the ability to use fat as a fuel (nowhere have I seen this suggested to train the body to use stored muscle glycogen). The suggestion has been to improve ability to metabolize fat, but that’s tied in with optimizing training to *conserve* muscle glycogen – which I think is indisputably an important factor for triathletes (at least in races lasting longer than about 90 minutes).

The strongest poroponent I’ve seen for this is the alpine mountaineer Mark Twight, but I’ve seen related advice from Mark Allen and Tim Noakes.

Twight’s advice for best performance on 1-5 day alpine ascents is to eat protien/fat early and during the day use gels / cytomax etc to keep fueling as long as possible, bascially avoiding anything that requires diverting blood to the stomach for digestion as long as possible. In training he advocates training to metabolze fat as well as ‘depletion days’ bascially going out for a full day of training with minimal reserves and minimal intake to simulate what alpine mountaineers often experience — complete depletion of reserves.

This kind of mountaineering is substantially different from most endurance racing, you’re carrying all of your supplies along with you and the energy output is a lower average with short bursts of very high output, however the need to conserve glycogen and maximize efficiency are certainly true of most endurance sports.

I do a small amount of training for both climbing and triathlon-specific training without stored glycogen and with minimal — but not zero — intake to get a feel for how much I can do while burning principally fat. When I do this I use a low-concentration energy drink because I *don’t* want to be going into catabolysis — making carbohydrate by burning the body’s protien for fuel.

I don’t know for sure if these things help the metabolism to learn new tricks — people who ought to know say both yes and no. Even if not, I think there’s a valuable element of mental training. If the athlete needs to learn a given limit (like how fast can you do the bike leg without digging into your glycogen reserves which are critical to save for the run), then maybe ‘running on empty’ is the way to get there.

Assuming that your body weight isn’t too far off target (too little body fat can also cause problems), then you should eat when you’re hungry, and drink when you’re thirsty. During long / more intense exercise it can be harder to hear the messages, so I nibble & sip regularly, and look out for the other symptoms of problems – dehydration is the one that tends to creep up on me, in the form of a slight headache at the back of my head, and visual snow (see http://en.wikipedia.org/wiki/Dehydration).

If I get either of these (to which I’m more prone when skiing – too much mountain, too little time!), I slow down, and take the time to drink.

Karl.