September 15, 2023 by Fuel & Fortify

Carb Loading for Endurance Explained: What You Need To Know

Carb loading – for most, this immediately brings to mind a big bowl of pasta the night before and that’s about as scientific as we tend to get with this entrenched, yet commonly misunderstood, pre-race ritual. While it is appropriate to give ourselves a license to go hammer and tong on the carbs, understanding how long prior and what type of carbs should be preferenced is often the missing piece for a lot of budding race goers out there.

Why Do We Carb Load?

When we do eat that bowl of pasta, the majority of carbohydrates get converted into glycogen and are stored in your muscles and liver. Glycogen serves as your body’s readily available energy source—yet it’s not the only one. More on that below.

In events where carbohydrates will be the primary fuel source, the rule of thumb is that carb loading is really only effective for endurance events lasting longer than 90 minutes. Moreover, the effects of carb loading have been proven to be amplified is less fit athletes.

Anything short of 90 minutes and the science doesn’t support an increased level of performance, even when compared with athletes adhering to what can be considered as a low carb diet.

What’s in our Fuel Tanks

To understand this more, we need to consider what sources of energy are called on when.

Let’s start with carbs!

The whole premise of carb loading is centred around topping up our glycogen stores. Specifically, in the liver and muscles. while Glycogen in the muscles remains confined to the specific muscle where it’s stored, and this proportional to the size of the muscle too - in other words, the more muscle, the more glycogen is able to be stored. For an averaged sized person this is estimated to be about 500g of glycogen. Liver glycogen however, serves as the body’s primary energy source, with a “well fed liver” containing about 100g of glycogen (6–8% of tissue weight).

Glycogen stored in the liver primarily serves as a reserve of glucose that can be released into the bloodstream when needed to maintain stable blood sugar levels throughout the body. When blood sugar levels drop, the liver can break down glycogen into glucose, which is then transported to various tissues, including the brain, to provide energy.

And then there’s fat…

While we certainly have a ceiling on how many carbs we can burn, with fat—it’s practically endless. Wishnofsky’s 60 year-old finding that one pound of fat is equivalent to 3,500 calorie still finds its way into the literature these days.

And if we take a conservative minimum of 20 pounds of fat for either man or woman, then we’re talking in the magnitude of 70,000 calories!

Fuel source exertion is typically represented as a percentage of one’s VO2 max. Put simply, the slower and gentler your pace, the more your body depends on fat as its primary source of fuel. Conversely, when you run faster and engage in more intense activity, your body relies more on carbohydrates for its energy needs.

Source: Rapoport

We can see from above that 65% of VO^{2 peak} represents somewhat of an inflection point. This is approximate however—we know that peak rates of fat oxidation can be sit anyway between 47% and 75% of VO^{2 peak} and this can be stretched even further on a low-carb diet and/or elite level athletes—and occasionally, without a hit to performance. This is BIG topic in the endurance circles though, and if you’d like to find out more, check out our blog post here.

How Long to Load?

Studies confirm that 24 hours is sufficient, in fact it’s very much a case of diminishing returns if we extend any scoffing beyond 36-48 hours.

10g/kg and High GI foods should be the aim here. Carb loading should be somewhat unpleasant, if done correctly. Keeping on top of your fluids can be a way to get those extra carbs in when eating really becomes a chore.

For someone weighing 70kg, the below could represent a day-before meal list.

As for the carbs we should be taking onboard during any training session or event spanning greater than 70 minutes, we’ve covered this is a separate piece which you can find here.

Importance of Gut Training

A common misconception is that the way our gut behaves is fixed and unalterable. That is far from the truth however, and like our muscles—is trainable.

Training your gut can certainly allow you to cope better with any of the potential symptoms.

• Gastrointestinal (GI) distress: It’s advisable to source your extra carbohydrates from easily digestible whole foods like rice or sweet potatoes. Overindulging in pizza, bread, or pasta can lead to bloating and excessive water retention.

• Electrolyte Imbalance: Excess water retention can result in your muscles excreting calcium and potassium, which may lead to cramping.

A note for Low-carb dieters—if you’re used to fuelling sessions beyond 60 minutes without carbohydrates, your body is more likely to primarily rely on fat for energy during longer workouts. This means that you might not benefit as much from the extra glycogen stores that carb loading typically provides and you may want to consider reducing the 10g/kg recommendation mentioned above.

Considerations for women

Female hormonal changes can play a significant role in glycogen metabolism. These hormonal fluctuations, particularly changes in estrogen levels, can influence the body’s capacity to store glycogen.

A recent study (Moore et al.) found that women may have a slightly higher capacity for glycogen storage during periods of elevated estrogen, such as the late follicular and mid-luteal phases (approximately 5 to 7 days before and 10 to 16 days after ovulation). Conversely, during phases of lower estrogen, like the early to mid-follicular phase (approximately 7 to 10 days leading up to ovulation), prioritizing carbohydrate intake in terms of quantity and timing may be necessary to enhance glycogen storage.

Summary

Remember, individual needs may vary, so it’s essential to experiment with carb loading strategies during training to find what works best for you. Working with with your coach and a nutritionist is always advised to understand and work towards a tolerable level of consumption pre and on race day!