October 3, 2019

RER (Respiratory Exchange Ratio): What Is It? And Should You Care?

by ironparadisefitness

RER (Respiratory Exchange Ratio) Determines Whether You’re Burning Carbs Or Fat For Fuel. But Is This Important? And Should You Care?

The Respiratory Exchange Ratio (RER) might be something you’ve never heard of. Simply put, it’s the mechanism determining whether fat or carbohydrate is being utilised for fuel at any given point in time.

You might think that if you could manipulate RER to predominantly burn fat, then you’re on to a winner.

Is this how you turn yourself into a fat burning machine and cheat the system?

Is fat loss heaven finally here?

Unfortunately, it’s not that simple. As you’ll discover in this article, RER is important, but not to the extent where you’ll be able to defy science. Sorry… it’s the truth.

What you WILL find out over the next few minutes is how RER works. And more importantly, you’ll be empowered with more knowledge in relation to fasted cardio, how the body burns fat, the importance of recovery, and more.

Here’s what this article covers:

  • What is RER?
  • How RER affects weight loss with high carb or high fat diets.
  • The truth about fasted cardio and why RER is further proof it is no more beneficial for fat loss.
  • How a lack of sleep could affect RER and result in muscle loss.

And if you want to listen to the audio version, click the play button below:

What Is The RER?

RER Iron Paradise Fitness

In simple terms, RER refers to the energy source your body uses at any given point.

Most activities you do throughout the day will use a mix of carbohydrate and fat.

However, there are exceptions to the rule. For example, when you are asleep, fat will be the substrate used. After all, you’re not exactly bouncing off the walls with boundless energy at that point (unless you’re having some weird dreams, or have a severe case of sleepwalking).

Conversely, bouts of intense exercise, such as sprinting are almost exclusively carbohydrate-fuelled.

RER is typically measured on a scale of 0.7 – 1 (weird, I know).

0.7 indicates sole fat utilisation. And 1 indicates sole carbohydrate utilisation.

RER Chart
RER Chart courtesy of MacNutrition Uni

But don’t be overly concerned with these numbers. They’re not important for helping you achieve your goal. Although, it’s good to know these things exist.

A number of factors affect the balance of carb and fat utilisation, including;

  • Exercise duration
  • Exercise intensity
  • Environmental conditions, ie altitude and temperature
  • Your fitness level
  • Food intake pre-exercise, ie fasted or fed?
  • Habitual diet

So, now you know what RER is, the big question is should you care? And should you be manipulating your nutrition and training to take advantage of this new-found knowledge?

The answer to that question is up next.

The RER (Respiratory Exchange Ratio) dictates whether carbohydrate and/or fat is utilised during a specific activity. 


Should You Care About RER?

RER Iron Paradise Fitness

The answer to this question is… PROBABLY NOT.

In reality, as long as you have the basics of nutrition and training in place, RER takes care of itself.

Over the years, the human body has become efficient at keeping you alive. All you need to do is fuel it correctly and show it some TLC.

When it comes to fat loss, research shows all methods of dieting, whether high carb, high fat, or otherwise can work, despite wild variances in substrate utilisation.

“Significant weight loss was observed with any low-carbohydrate or low-fat diet.” Johnson et al (2014)

“All diets resulted in significant loss of body fat and weight over six months. Groups did not differ significantly but loss of body fat and weight was greater in all groups compared with the control group.” Truby et al (2006)

“Weight loss at 1 year was 2.1kg for Atkins, 3.0kg for Weight Watchers, and 3.3kg for Ornish.” Dansinger et al (2005)

So, the takeaway point here is that RER has no material impact on weight and fat loss. Selecting a diet in an effort to be a fat burning machine is no better than any other approach.


Because weight and fat loss is primarily driven by a calorie deficit. The method by which you choose to achieve that deficit comes down to your individual ability to adhere to it. In practical terms, this means choosing a way of eating you enjoy and can see yourself following for a long period of time.

Don’t go chasing the hack. It doesn’t exist.

As TLC sang, “Don’t go chasing waterfalls. Please stick to the rivers and the lakes you’re used to.” 

Wise words ladies…


A Word On Fasted Cardio And RER

RER Iron Paradise Fitness

But where does fasted cardio figure in the RER discussion?

Surely, the fact you’re training in a carb-depleted, fasted state means you’ll be using fat for fuel? You’ll be bending the RER rules in your favour, right?

Well, the simple answer is no. Because that’s only part of the picture.

And don’t call me Shirley.

While it’s true you’ll burn more fat in a specific bout of fasted cardio, the subsequent substrate utilisation will change.

In more meaningful terms, this means training in a fed state will utilise more carbohydrate during exercise and more fat afterwards. Whereas, training in a fasted state will burn more fat during exercise, but more carbohydrate afterwards.

This study analysed these conditions and came to the conclusion, “…when moderate endurance exercise is done to lose body fat, fasting before exercise does not enhance lipid utilisation; rather, physical activity after a light meal is advisable.”

Essentially, when calories are matched, training fasted or fed makes no significant difference to weight and fat loss. So choosing what you prefer is the overriding principle (AGAIN!).

“But Simon, if I’m on Keto I’m not eating carbohydrate, so won’t I continue to burn fat during the day?”

Yes. You will.

But here’s the thing. Burning fat DOES NOT equal fat loss!

Let me repeat that so it sinks in…

Burning fat DOES NOT equal fat loss!

You see, if you eat more fat, then your body will burn the dietary fat you consume.

Unless, you’re in a calorie deficit, your body won’t tap into your stored body fat. So, yes you will be a fat burning machine, but unless you’re in a calorie deficit, it won’t mean jack shit.


How A Lack Of Sleep Affects RER And Fat Loss

Sleep Deprivation Iron Paradise Fitness

Sleep is important for recovery. After all, being constantly knackered makes it hard to be your best when it comes to working out.

Likewise, sleep deprivation can cause increase feelings of hunger, exposing you to a higher risk of overeating, thereby undermining your fat loss efforts.

But there’s another aspect of sleep deprivation you should be aware of. And it relates to RER.

A study looked at the differences in weight and fat loss between participants averaging 5.5 and 8.5 hours sleep per night (both groups were in a moderate calorie deficit). Interestingly, the group with reduced sleep had an increase of 60% in the amount of fat-free mass they lost.

What does that mean?

It means they lost some gains (probably some water and other stuff too). In this relatively short, 2 week study, the sleep deprived group lost an average of 2.4kg in fat-free mass. So you can imagine the negative impact of a prolonged period of time in a calorie deficit, while sleep deprived.

But what’s this got to do with RER?

The study highlights that a lack of sleep may shift substrate utilisation away from fat when at rest. So the lesson here is make sleep a priority if you want to burn fat, rather than muscle.


The Bottom Line On RER And It’s Practical Application

RER isn’t something you need to concern yourself with on a daily basis. Sticking to the basics of managing calories, exercises, and recovery will ensure RER takes care of itself.

When it comes to RER, managing the basic of training and nutrition will always be the best course of action.

There’s no need to overcomplicate the process and disappear up your own ass by trying to manipulate substrate utilisation (unless you’re a pro endurance athlete).

In reality, if your goal is weight and fat loss, your focus should be on the basics; eat in a sustainable calorie deficit, exercise regularly, maximise sleep, and eat a diet high in nutritious, whole foods.

Do that, and RER will take care of itself and work in your favour. There’s certainly no need to train fasted or follow a high fat diet, unless you want to.


Need Help Getting The Basics Right? Here’s A Free Online Calculator To Help You Discover How Many Calories You Need To Achieve Your Fitness Goals

Online Coaching Iron Paradise Fitness

If you want to get on the fast track to a leaner, healthier, stronger body, I’ve got something for you. Simply click the link below and you’ll get access to my free online calorie and macro calculator. Designed to take the headache out of knowing how many calories you need to achieve your goals. Plus, you can grab my Lean Life Kickstarter Pack. In it you’ll find a free 4 week beginner’s training programme (complete with exercise videos and a workout tracker), free recipe packs, plus so much more.

So if you want all that, go here.

But for now, all I’ll say is… Keep living the Lean Life and I’ll see you soon.


fat loss, rer, respiratory exchange ratio, training, weight loss, workout

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