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Your body is a phone battery, not a car engine

A woman exhausted from cardiovascular exercise

I recently came across an excellent reel on Instagram by Jade Skillen, a Master Trainer for Hyrox, an infamous fitness race that takes place annually in various countries.

The reel is an excerpt from a presentation Jade gave on the timing of nutrition. During her presentation, she provides an excellent analogy that compares our bodies to a phone battery.

"You cannot just pull into the gas station, put your nozzle in for 2 minutes and you’re good to go. If your phone battery's completely dead, it takes a while to charge back to full. Your glycogen levels are like a phone battery. If you allow them to drop too low (~60-70 minutes into moderate to vigorous activity) it’s too late. You must catch it at the right time so you don't run on empty."

This analogy stuck with me and I decided to write this blog on the topic and add some more information. I do not doubt that a lot of what I have to say was covered by Jade elsewhere in her presentation, but sadly I wasn't able to find it online.

What is glycogen?

In my previous blog, "Carbohydrates: Friend or foe?", I explained where glycogen comes from and how your body uses it. However, I know some of you won't read that, so let's do a quick recap.

When you eat carbohydrates, they are broken down into glucose. The glucose then travels to cells throughout the body, where it's used to make energy. If your body has more glucose than it needs to fuel activity, it can store it in the liver and muscles as glycogen. Glycogen can be quickly broken down into glucose and used for energy whenever your body needs it.

Timing of nutrition

When you engage in moderate-to-vigorous activity, your body starts depleting its glycogen stores to fuel the active muscles. And that's a problem because glycogen is to your body what electricity is to a phone battery, they are both primary sources of energy. If your energy drops too low, it will take more than just a couple of minutes to recharge—it takes time to fully replenish.

That's why it's integral that you consume the right types and amounts of food before, during, and after exercise. It safeguards against your glycogen levels dropping too low, which may impact your performance. Let's dig into what that might look like (note that any recommendations I provide may differ depending on your age, gender, and conditioning).

Before exercise

The 2 main goals of nutrition before exercise are to optimise glucose availability and glycogen stores, which will provide the fuel you need to support exercise performance.

Fuelling could start up to a week before particularly strenuous endurance exercise, however, for most individuals’ performance goals (e.g. a 10-km race or a half marathon), adequate nutrition the day before or on the day of exercise will suffice.

Aim to eat a meal about 4–6 hours before exercise. This will leave enough time (approximately 4 hours) for the food to be digested and absorbed, which will increase glycogen levels and help ensure a more consistent and sustained energy supply during exercise.

A meal 4-6 hours before exercise should be high in carbs, low in fat and fibre, 400–800 calories, and tolerable.

In general, a meal before exercise should be:

  • Relatively high in carbohydrates (aim for 1–4.5 g per kg of body weight).

  • Low in fat and fibre.

  • Contain a moderate amount of protein.

  • Approximately 400–800 kcal.

  • Conservative! (Consume foods that you know your body can tolerate).

During exercise

The goal of nutrition during exercise is to provide your body with the essential nutrients it needs to maintain optimal blood glucose levels. This will help avoid things like “hitting the wall”, which is caused by extreme fatigue due to depleted energy stores.

As a general rule, exercise that lasts less than 1 hour can be adequately fuelled with existing glucose and glycogen stores (if you ate properly beforehand). You won’t need any additional energy supplements.

However, if exercise lasts longer than 1 hour, blood glucose levels will begin to dwindle. After 1–3 hours of continuous moderate-intensity exercise (RPE 5­­–6, above VT1 to just below VT2, or 65–80% VO2 max), muscle glycogen stores may become depleted.

At this point, if no glucose is consumed, blood glucose levels will drop, which is when performance will falter. If you wait until 60–70 minutes into the exercise to refuel, it might be too late. However, by refuelling from around the 40-minute mark, you catch your glycogen levels at the right time, just like charging your phone battery before it completely dies.

An energy supplement during exercise should be consumed from the 40-minute mark, be high in carbs, and be an energy gel, liquid, or chew.

In general, an energy supplement during exercise should be:

  • Consumed from around the 40-minute mark and then every 40–60 minutes.

  • Approximately 30–60 g of carbohydrates.

  • An energy gel, liquid, or chew, which can be quickly absorbed into the bloodstream.

After exercise

The goal of nutrition after exercise is to replenish glycogen stores and facilitate muscle repair.

For most individuals, a normal, healthy diet will suffice to facilitate recovery within 24­–48 hours. However, if you engaged in particularly strenuous endurance exercise, you will benefit from more strategic nutrition, which may start as soon as 30 minutes after exercise.

A meal after exercise should be high in carbs, contain a moderate amount of protein, and be consumed roughly 30 minutes after exercise.

In general, a meal after strenuous exercise should be:

  • Consumed around 30 minutes after exercise and then every 2 hours for 4­–6 hours.

  • Mostly carbohydrates (1.5 g per kg of body weight).

  • Contain a moderate amount of protein.

In conclusion

Of course, other important aspects of nutrition help ensure your body optimally performs during exercise, such as adequate hydration. Maintaining fluid balance prevents dehydration-related issues, which can have severe consequences, however, that's a topic for a different blog.

But, at the very least, you now know more about carbohydrates and how your body uses them to make energy (glucose and glycogen). I hope you also have a better understanding of what Jade meant when she likened your body to a phone battery, not a car engine.

Ultimately, when it comes to nutrition and performing at your best, failing to prepare really does mean that you’re preparing to fail.

If you'd like to stay updated on my workouts and nutrition, you can follow or contact me on Instagram.


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