With 12 days of CrossFit in full swung and holiday season on our doorstep it is a great time to chat about how to fuel your body:

During past challenges, a few people commented that they felt great for the first week or so, but then workout performance and overall energy levels had dropped. Why is that? The answer is all to do with carbohydrates, so let’s take a look at what’s going on.

Hybrid fuelling

There is only one direct fuel for muscles movement, which is a chemical called ATP. Muscles store a small amount of it, plus another compound, creatine phosphate (CP) that can be almost immediately transformed into ATP. Together, they give enough fuel for about 10 seconds of max effort, then fade out until full depletion at around 45 seconds into a sprint (longer if the effort isn’t maximal). ATP-CP reserves are replenished quickly, with a 5 minute rest allowing them to restore almost completely.

Say you’re trying to set a new 800m run PB, you’re about 30s in, ATP-CP supplies are fading; what happens next? The answer depends on what you’ve been eating. If your diet contains enough carbohydrate then you’ll have 200-400g carbs stashed away in your muscles in the form of glycogen, a large molecule which plays a storage role in animals in a similar way to starch in plants. Glycogen can be rapidly converted into ATP, giving you a way to continue sprinting, but at a slightly reduced pace compared to the early ATP-CP surge. The highest glycolytic power is anaerobic, but this process creates the acidic end-products that cause that familiar burning feeling and an overwhelming desire to drop the bar during Fran. Anaerobic glycolytic power kicks in from about 10s into a sprint and increases in importance as the ATP-CP system fades, reaching a peak around the 1-minute mark and itself fading out between 2-3 minutes in. After that you are left with your aerobic metabolism which runs on remaining glycogen plus fat. The rate of ATP production from these pathways is significantly lower, but can last much longer, making them very important for endurance athletes.

Unplanned low-carb

Crossfit is high-intensity work and burns up glycogen like few other activities. A single 30s sprint can deplete glycogen levels by as much as 25-35% in the active muscles, so a typical WOD can pretty much trash your reserves. Your liver can hold a further 50g and will dump that into your blood stream as glucose in response to hard efforts, so it can be shuttled to your muscles and replenish them. Even on a rest day you’ll use around 100g of carbs mostly to provide blood sugar to your brain, and on a workout day you could use more than double that.

Now, when we “eat clean” it is easy to end up consuming around 100-150g carbs a day by default. Including enough paleo starches to lift daily intake to 200g or above requires effort – it means eating nearly a kilo of sweet potato for example. After a week or two of this little budget deficit, reserves can dwindle and in workouts when ATP-CP is exhausted you switch straight to running on fat, missing out that middle glycolytic energy system, and potentially feeling the general ill effects of hypoglycaemia between times too. This is not a happy place for a Crossfitter to be.

Fixing it

If you experience this kind of creeping carb shortfall, there are two approaches to dealing with it. One is to go all-out low carb and force your body to adapt to fat burning. That is an interesting approach that we will be exploring in the future, but let’s set it aside for now. The obvious fix that will enable you to keep your expected level of workout performance is simply to increase your carb intake to an appropriate level. The more muscle mass you carry, the more glycogen you need, but as little extra as 50-100g per day might be enough to straighten things out, and there is some benefit in experimenting to find the threshold. That’s because if you eat just enough carbohydrate just in time to fuel your workouts, then you will get some low-carb benefits in the post-workout periods. You will become a better fat burner, see body comp improvements and decrease your reliance on carbs as your main fuel source.

The how-to

If you are in that unhappy depleted state, first of all have a carb re-feed day to get out of it. 250-300g total carbs should get you back up to power, ideally mostly from paleo starches but also include a couple of pieces of whole fruit. Then go back to your previous eating plan, but with extra carbs targeted to the two meals prior to your workouts. If you train in the morning then that will usually mean the previous evening’s dinner and then breakfast. For afternoon or evening workouts, it’s most likely breakfast and lunch, or possibly lunch and a substantial snack at least a couple of hours before workout time. You need to allow time for absorption and not consume too much in one hit, hence the need for at least two carb-rich meals. After training, keep carbs low until the next pre-training refuel.

(Notice, this goes against the conventional wisdom that says you should have high-GI carbs after training to restore glycogen as fast as possible and increase muscle gains. If you’re at a competition and trying to recover between WODs, then the former matters, but not if you’ve got a whole day or more before your next workout. The second point is a bit of bodybuilder lore and doesn’t stand scientific scrutiny – in fact you’re probably better having a protein and high-fat meal to slowly absorb the protein for best results.)