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Does Fasted Cardio Burn More Fat?

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Any opportunity we get to increase fat burning potential and we’re all over it – and one of the more interesting methods is fasted cardio.

But fasting, before training? Is that safe, and furthermore, does fasted cardio help you burn more fat?

It’s a fascinating topic, and there’s a lot of debate of how much difference it actually makes. We’ve done the researched, covered the main points regarding subject.

Here you’ll find out:

  • What is it?
  • The science – does it burn more fat?
  • Additional considerations
  • Final word

What is fasted cardio?

Essentially exercising on an empty stomach, this type of exercise is often performed in a morning before breakfast where a natural fasting period can take place. This is different to exercising after a meal which puts you in a ‘fed’ state which is referred to as postprandial.

As a training approach it has been around for quite some time now, and fasted cardio is often cited as an effective way to accelerate fat loss – it was first introduced as a training concept by Bill Phillips in his book ‘Body for Life‘. [1]

It is a proposed strategy to control body fat and mass, and improve metabolic adaptations to training used frequently in the physique and recreational gym-goer environment – this training method is usually low intensity cardio, but occasionally HIIT training is used as well.

The theory behind it is that by exercising on an empty stomach, there is a shift from carbohydrate utilization to fat mobilization – however it is not often accompanied with an explanation as to if or how it helps with fat loss.

So we’ve broken the research down for you. Read on to find out what it says…


Does-Fasted-Cardio-Burn-More-Fat

Key Point: Fasted cardio is exercising on an empty stomach in order to shift from carbohydrate utilization to fat mobilization


The science – does it burn more fat?

Studies have previously found that ingesting carbohydrates either just before or during exercise reduces fatty acid use. For example, Ahlborg [2] investigated the effects of ingesting 200g of glucose 90 minutes into a 4 hour low intensity exercise session and found that fat use fell by 60-70%.

This is where it all started – studies of this kind provided an initial base of interest and theories suggesting that if carbohydrates are taken out of the equation, then fatty acid use must go up.

Here is a breakdown of the studies looking at fasted cardio. Lets see if the theory holds up:

#Study 1: Deighton et al [3]

This study from the journal Appetite, used 12 healthy males to investigate the effect of fasted and postprandial exercise on appetite, energy intake and resting metabolic responses. The volunteers were put into one of three groups:

  • Group 1: 8 hour fast
  • Group 2: Exercise 4-5 hours postprandial
  • Control

Each experimental group was subjected to a 60 minute treadmill run at ∼70% of maximum oxygen uptake. At 1.5 hours post exercise, each group was offered a standardized breakfast and ad-libitum buffet meals as well, at 5.5 and 9.5 h.

The results showed that the 60 minute run induced a negative daily energy balance in both groups, but was no more effective when performed before or after breakfast – this means that fasted cardio did not change any parameter of fat loss. 

#Study 2: Enevoldsen et al [4]

This study by Enevoldsen and colleagues saw 7, healthy male subjects studies completing exercise for 60 min at 55% of peak oxygen consumption in a fasted or fed state.

Results showed that whilst abdominal fat tissue use during exercise was not influenced by pre-exercise food intake, and fatty acid mobilization was increased by only 1.5-fold during postprandial exercise, there was a fourfold increase during exercise in the fasted state. 

This led the researchers to conclude that exercise performed in the fasted state shortly before a meal leads to a more favorable fat metabolism during and after exercise than exercise performed shortly after a meal. 

#Study 3: Horowitz et al [5]

This study aimed to determine the fat burning response of 6 healthy, trained volunteers by training them at various intensities in either a fed or fasted state – either low or moderate intensity exercise. 

During the trials, the participants ingested a high sugar meal (glucose or fructose) or remained fasted for 12-14 hours prior to exercise.

The results suggested that in the low-intensity; fed state protocol, fat burning was suppressed by 22% in comparison to the fasted state – however, fat burning remained the same in both groups until the 80-90 minute training mark. Prior to that, fat burning was the same in both groups.

In the moderate intensity training protocol there was no difference at all between fat burning in either the fed or fasted state.

This study suggested then that you may be able to burn more fat, but only at a low intensity, and after 80 minutes. 

#Study 4: Febbraio et al [6]

This study in the Journal of Applied Physiology investigated the effect of carbohydrate ingestion before and during cycling exercise in 7 trained male volunteers.

Testing saw the volunteers cycle for 120 minutes at 63% peak power followed by an all-out intensity time trial. This occurred 4 times under different conditions:

  • A placebo before and during training
  • A placebo 30 minutes prior to training, then a carbohydrate drink every 15 minutes throughout training
  • A carbohydrate drink 30 minutes before training and then a placebo during exercise
  • A carbohydrate beverage both before and every 15 minutes during exercise

The results of the study found that pre-exercise ingestion of carbohydrate improved performance only when ingestion was maintained throughout exercise – and that fatty acid use was not impaired with ingestion of carbohydrates.


Does-fasted-cardio-lose-muscle

Key point: Evidence seems inconclusive but sides more with fasted cardio have minimal beneficial effect on fat burning when compared to fed.


An additional point to consider – will you lose muscle doing it?

One additional consideration to bear in mind is that of proteolysis – the breakdown of protein building blocks (amino acids) for energy. Your body cannot manufacture carbohydrate from fats but it can do it from protein – a process called gluconeogenesis. If there are no carbohydrates present then will your body turn to muscle for energy?

It appears so:

The higher the rate of proteolysis, the more muscle you will lose as you’ll be converting it to glucose for energy.

A study by Lemon [7] illustrated the effect of protein during unfed exercise on 6 volunteers by asking them to cycle for 1 hour at a moderate intensity either after carbohydrate loading, or after fasting.

The results found that after fasting, nitrogen loss doubled – and protein loss increased by over 10%.

If you consider that the more muscle mass you have, the higher your resting metabolism – a term called basal metabolic rate (BMR) – then this means bad news for your long-term fat loss strategy, or if you are a bodybuilder wanting to maximize muscle gains.


Summary – Does fasted cardio burn more fat?

The jury is out. When it comes to fasted cardio, some studies demonstrate that at a low intensity, there is an increase in fatty acid use, but only after a prolonged period of exercise. However most studies show no differences at all.

On the whole the evidence does not suggest the use of early morning, fasted cardio for additional fat loss. Particularly when you look at the bigger picture of throughout-the-day calorie turnover from fat.

Looking at fat burning during a workout does not tell a full story – what you need to look at is the effect of fat use over the course of hours or maybe even days. It seems as a general rule, if you burn more carbohydrate during a workout, you inevitably burn more fat in the post-exercise period and vice versa.

On a final note, the increased loss of lean protein from carbohydrate fasted cardio can’t be overlooked – in order to maintain muscle mass and BMR you may even wish to avoid it altogether. 


References

  1. Schoenfeld, B. Does cardio after an overnight fast maximize fat loss?. Strength Cond J. 2011; 33(1): 23-25
  2. Ajlborg, G et al. Influence of glucose ingestion on fuel-hormone response during prolonged exercise. J Appl Physiol. 1976; 41(5): 683-8
  3. Deighton, K et al. Appetite, energy intake and resting metabolic responses to 60 min treadmill running performed in a fasted versus a postprandial state. Appetite. 2012.; 58(3): 946-54.
  4. Enevoldsen, LH et al. The combined effects of exercise and food intake on adipose tissue and splanchnic metabolism. J Physiol. 2004; 561: 871–882.
  5. Horowitz, JF et al. Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. Am J Physiol. 1997; 273(4 Pt 1): E768-75
  6. Febbraio, MA et al. Effects of carbohydrate ingestion before and during exercise on glucose kinetics and performance. J Appl Physiol. 2000; 89: 2220–2226
  7. Lemon, PW et al. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appl Physiol Respir Environ Exerc Physiol. 1980; 48(4): 624-9.