Fasted cardio for weight loss?

You may have heard that fasted cardio is beneficial for weight and fat loss.

Table of contents:

• Debunking the myths
• The science: fat versus carbs for fuel
• Fuel source preference versus fat loss
• What does the science say about fasted cardio for weight and fat loss?
• Does that mean that fasted cardio isn’t beneficial for weight loss and fat loss?
• Take aways

Debunking the myths

“Fasted cardio is better for fat loss and/or weight loss“

The reasoning behind this myth includes:

1. Insulin levels are low and insulin is responsible for repressing fat breakdown (lipolysis). Therefore doing cardio in a fasted state should release this inhibition and allow more fat to be burned during the session
2. Glycogen levels are low and the body will be more likely to turn to stored fat to supply energy needed to fuel the cardio session, meaning that you burn more fat
3. Research showing that when you fuel your body with more carbohydrate, it priorities utilising more carbs and less fat for fuel

What’s actually going on:

1. Insulin levels are low allowing more fat to be burned during the session

Firstly, fat loss takes place over a period of time rather than during a 45 minute cardio session. So this statement is redundant – period.

Secondly, fat loss requires an energy deficit to be created (which occurs when you lose weight or through body recomposition).

Thirdly, lowering levels of insulin does not equal fat loss.

2. Glycogen levels are low, so the body can turn to stored fat to supply energy needed to fuel the cardio session

A common misconception is that utilising fat for fuel to generate energy results in fat loss. They are not the same thing. Fat burning/oxidation is not the same as fat loss.

Additionally, fatty acids can be used to make glycogen!

3. Research showing that when you fuel your body with more carbohydrate, the body priorities utilising more carbs and less fat for fuel

Research also indicates that if you utilise more of one fuel source during a cardio session, then you will subsequently burn less of that fuel source over the next 24 hours (1).

Therefore if you utilise fats during the cardio session because you’re fasted, then post exercise you will utilise more carbs, and vice versa if you’re fed! The body is incredibly clever and will always do whatever it can to get back to balance.

The science: fat versus carbs for fuel

Both fat and carbohydrates can be broken down to generate energy (ATP). Whether the body predominantly utilises fatty acids (from fats) or glucose (from carbohydrate) will depend on both the intensity of activity and the available fuel – so whether carbs or fats are mostly available either from food consumption or being in a fasted/fed state.

During low intensity activity (<45% VO2 max), such as walking, yoga, a gentle jog or cycle, the predominant fuel for energy is fatty acids. At rest, the predominant fuel utilised is fats – however, note that this doesn’t mean that at rest you are therefore losing fat.

At a higher intensity of exercise, approximately 75% of VO2 max, carbohydrates become the predominant fuel source. This is known as the crossover concept. At higher intensities of exercise, the body generates energy more quickly and this takes place with less oxygen, which carbohydrates are preferential for.

FAT = fats

CHO = carbohydrates

The body also utilises what’s readily available / circulating in the blood stream, to generate energy, as again this is easiest for the body. Therefore, if you recently consumed a meal of mainly carbs, then the body will prioritise utilising that to generate energy.

If you are in a fasted state, at a low intensity of exercise, then the body will tend to draw upon fatty acids from fat stores.

Research in 1976 (Alborg and Felix) indeed showed this – that when you fuel your body with more carbohydrate, then during exercise your body will prioritise using more carbs and less fat for fuel. This led people to believe that therefore if you want to burn as much fat as possible during a cardio session then you should avoid eating carbs before and during a cardio session.

Fuel source preference versus fat loss

As mentioned above, a fuel source preference, is not the same as weight loss or fat loss. Fat burning or fat oxidation, or utilising fat for fuel, is not the same as loss of bodyfat stores.

For fat loss to occur, the rate of fat burning/oxidation/utilisation for fuel, has to be greater than the balance of fat storage.

This involves creating an energy deficit. Period.

What does the research say about fasted cardio for weight and fat loss?

Low intensity exercising in a fasted state is associated with utilising more fat for fuel, rather than carbohydrate for fuel to generate energy. This is shown in studies by measuring the respiratory exchange ratio (RER), the ratio of carbon dioxide production to oxygen uptake, which indirectly determines the relative contribution of carbohydrates and fats to energy expenditure.

A higher RER, with a value of 1, indicates that carbs are predominantly being used and a lower RER, approximately 0.7, indicates that fats are predominantly being used.  

However, research indicates that if you do utilise more of one fuel source during a cardio session, then you will subsequently burn less of that fuel source over the next 24 hours (1).  This was demonstrated in a study showing that pre cardio session, the fed group had a higher RER than the fasted group, implying a greater utilisation of carbs and less fat for fuel.

However, 12 hours later, the RER numbers between the fed and fasted group had reversed: the fasted cardio group were utilising less fat for fuel and the fed group more fat for fuel (1).

The authors subsequently concluded that exercising fed, rather than fasted, would be more effective for weight loss due to an increase in metabolism and reduced RER in the hours post training.

Whilst some studies on populations during the Ramadan have suggested positive effects on fat loss from fasted cardio, these groups were not matched for calories, and therefore the difference in fat loss is likely explained by a difference in calorie intake (2).

A study on 20 young women, exercising in a fasted versus fed state, in a 500-calorie deficit, showed that both groups lost weight and fat mass but that there was no significant difference groups (3). 

Additionally, a systematic review of 5 different studies, all of a 4-6 week duration, concluded that fasted versus fed exercising does not increase weight loss and fat mass loss (4).

The authors recommended that weight loss and fat loss from exercise is more likely to be enhanced through creating a meaningful calorie deficit over time, rather than exercising in a fasted state.

Does that mean fasted cardio isn’t beneficial for weight loss or fat loss?

Mechanistically, it is not superior for weight loss or fat loss. Just like a higher fat diet is not superior for weight loss versus a higher carb diet.

Weight loss requires an energy deficit to be created and consistently achieved. It will most readily and sustainably be achieved by choosing a strategy that works best for you and your individual needs.

Therefore, if exercising in a fasted state assists you with achieving a desired calorie deficit, then it will be beneficial.

However, if exercising in a fasted state makes you feel miserable and fatigued and you are less likely to exercise, let alone than at the intensity that you would when you were fed, then exercising in a fasted state is likely to contribute a reduced energy output, which will not benefit your calorie deficit!

Take aways

To effectively and sustainably lose weight: you need to follow a diet that allows you to consistently create an energy deficit that you can adhere to. This is most effectively achieved through following an individualised nutrition and exercise program that is continually progressed and adapted as you progress to your weight goals (to avoid plateaus and weight regain)!

To effectively and sustainable achieve fat loss: you need to follow a nutrition plan that creates an appropriate calorie deficit for your needs, includes enough protein and involves periodisation. You will massively benefit from resistance training.

To achieve body recomposition, that is the loss of body fat and the gain of lean muscle, again requires following an individualised nutrition and exercise program – focusing on appropriate energy intake, protein intake and distribution and resistance training.

Do what works best for you: whether you want to lose fat or weight, if you want it to be sustainable, then it has to be about what works for you. If this includes fasted cardio, then go for it! But realise that fasted cardio will not inherently cause you to lose weight or fat – what you eat and how you train will!

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Will fasted cardio benefit my endurance performance?

Intermittent fasting best for weight loss and body composition?

Do you do your cardio fasted?

References

1. Paoli et al. (2011). Exercising fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and excess postexercise oxygen consumption after bout of endurance training.
2. Trabelsi et al. (2012). Effects of fed-versus fasted-state aerobic training during Ramadan on body composition and some metabolic parameters in physically active men.
3. Scheonfield et all. (2014). Body composition changes associated with fasted versus non-fasted aerobic exercise.
4. Hackett and Hagstrom (2017). Effect of overnight fasted exercise on weight loss and body composition: a systematic review.