The issue of fat burning is often misunderstood. Perhaps the most emphatic and helpful summary is provided by the global marathon coaching guru Renato Canova in his marathon brochure produced on behalf of the International Association of Athletics Federations, the world governing body for athletics. When you run at about your lactate threshold pace you derive about 75% of your energy from carbohydrates and 25% from fatty acids. When you run at about your aerobic threshold – which in elite runners is very close to their marathon pace, so roughly the intensity of a two-hour run at full effort – the proportions are much closer to 50% carbohydrates and 50% fatty acids. At shorter endurance races between 1500 metres and 5000 metres your energy is provided almost entirely by carbohydrates. By contrast, as you slow down to something gentler than marathon pace you will use fatty acids as the source for most of the energy consumed.
Do bear in mind, though, that the usual summaries about how many miles we can run before our glycogen supply is depleted are implying – or assuming – that we are running at an even and optimum pace for the distance. The more we deviate from this by heading off too fast, the earlier we will become depleted. You can do some ballpark maths and calculate from the stats above that a change in pace of about 30 seconds a mile has a huge impact on the glycogen rate. If your marathon target is 3hr 30min (eight-minute miles), even a few 7min 30sec miles early on will be hugely unhelpful further down the road.
So the better you can train yourself so that your fastest running pace, when using a higher percentage of fatty acids, is close to your race pace when relying primarily on carbohydrates, the better prepared you are to ‘convert’ your 10k or half marathon pace to a strong marathon result. Conversely, you have to ensure that during the marathon you avoid shifting too close to your lactate threshold pace, or, regardless of the carb-loading you have done beforehand and whatever you take on board during the race, you will run down your muscle glycogen at too early a stage to maintain the overzealous pace using exclusively fatty acids. (The typical time lag from taking about 25g of quick-release glycogen to having that energy available in your working muscles is about 25 minutes.)
There are two particular types of repetition session that, backed up by long runs to develop muscular endurance and some capacity to metabolise fatty acids at a steady speed, seem to be of benefit – and may be different from what many readers have tried before. The two sessions are structured like this:
1. Some standard interval training of efforts at about 5k race pace, close to VO2 max, to rapidly deplete muscle glycogen followed by a further bout of running at marathon pace, to enable the runner to practice this pace in a relatively depleted state.
2. Mid-length efforts (between five and seven minutes) of between 10k pace and threshold pace, alternating with similar-length efforts at slightly slower than marathon pace. This teaches the body to slightly shift (ie increase) the speed at which it is juggling the utilisation of mainly glycogen with mainly fatty acids. The focus on these sessions, which should last between 60 to a maximum of 90 minutes, is about gradually pushing the pace of the slightly slower fat-burning efforts, and not so much about the pace of the faster stints. If you do this session for about 80 to 90 minutes, the average pace, for a three-hour marathon type, should be pretty close to target marathon pace.
While there are no magic sessions or shortcuts, and of course there is more than one way to skin the long-distance cat, these sessions seem to be very effective in helping well-trained and sensible runners to optimise their marathon performances in the context of their 10k and half marathon times. It’s also useful to do these sessions on the sort of surface, terrain and gradient that is similar to the marathon course for which you are preparing.
NB. Do be aware that you won’t actually ‘feel’ anything specific that indicates where the balance of glycogen and fat usage lies; it’s not like an on and off switch.
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