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Fasted vs. Fed Steady State Cardio: Is One Method Superior For Fat Loss?

20/09/2017
JD
Josh Dyson

Josh Dyson

“Beep. Beep. Beep!”

The dreaded morning alarm pierces around the room.

With your eyes half open you somehow navigate your hands to locate what appears at the time to be the world’s smallest alarm device.

You find it, but are hit with an instant dilemma.

OFF or SNOOZE

It then dawns on you.

Fasted cardio this morning!

After dragging yourself out of bed you pose yourself a simple question. 

“Is it really worth it?”

Today is the day we answer that question.

More specifically, is fasted cardio superior for fat loss than compared to fed cardio later in the day?

Fasted steady state cardio exercise is a fat loss strategy that has been utilised for many years, with its use particularly prevalent within bodybuilding and weight making sports, although such a strategy has frequently been utilised by recreational trainers looking to enhance their fat loss efforts.

The rationale for fasted steady state cardio is based upon that during low intensity exercise fat will be oxidised (burnt) for energy, over carbohydrate, with many people of the belief that body fat will therefore be lost.

However, is this necessarily the case?

Although, training in the fasted state has shown the ability to enhance aerobic metabolic adaptations, as well as improved glucose tolerance and insulin sensitivity, this article shall concentrate on whether such a strategy is a superior approach, in comparison to fed steady state cardio, for those looking to optimise body composition via increased fat loss.

Fat Oxidation and Exercise

Following an overnight fast due to the lower glycogen and insulin levels fat will be the primary energy source at rest resulting in the oxidation of mobilised fatty acids (FA) from adipose tissue via a process referred to as lipolysis. Alternatively, if an individual were to consume a meal (e.g. breakfast) prior to exercise the elevation in insulin would favour carbohydrate oxidation for fuel, therefore suppressing lipolysis.

Fat is the predominate fuel for energy at low to moderate exercise intensities, with peak fat oxidation rates appearing between 45-65% V02 max. Such fat oxidation rates are dependent upon exercise intensity, duration, gender, training status and dietary intake. However, it is also important to consider the contribution of the fat source oxidised during varied exercise intensities.

Adipose tissue provides the primary site of fat storage within the body via triglycerides (TG) within subcutaneous and visceral fat, whilst smaller quantities of stored TG are also found within muscle as intramuscular triglyceride (IMTG), or within the blood as lipoproteins or free fatty acids. The primary focus for individuals looking to enhance their body composition is to achieve a decrease in fat mass derived from subcutaneous and visceral fat stores i.e. adipose tissue.

24 Hour Fat Oxidation

Recent research has highlighted that steady state fasted cardio exercise has shown the ability to enhance 24 hour fat oxidation rate (how much fat is burnt for energy over 24 hours). Exercise performed after an overnight fast increased 24 hour fat oxidation via an increased transient energy deficit within both males and females, than when incorporating fed exercise following breakfast. However, it is important to note that there was no difference in 24 hour energy balance between groups, which is the key determinant of weight loss based upon the laws of thermodynamics where a consistent energy deficit (energy intake is less than energy expenditure) is required to achieve weight loss.

In fact the authors state that:

“the effect of a single bout of exercise on 24-h fat oxidation cannot be extrapolated to the reduction of body fat with chronic exercise” .

Further research would therefore be required to investigate over a prolonged time period, whilst also incorporating body composition measures (fat mass and fat-free mass) both pre and post-intervention to inform real-world applicability. Furthermore, with no data regarding the source of fat oxidation leaves further questions as to the influence upon subcutaneous and visceral fat stores. As exercise intensity increases the contribution of fat sources will vary, as at very low intensities (25% V02 max) energy is primarily via oxidation of plasma FA and only a small contribution from plasma TG and IMTG. However, as the intensity increases to moderate intensities (65% V02 max) IMTG energy provision increases. It is also important to consider that IMTG use is greater within trained individuals, which could provide insight as to why trained individuals have shown higher 24 hour fat oxidation rates.

Individuals with fat loss goals often become solely focused upon the source of fuel use during a narrow time period (e.g. during the exercise bout) without an appreciation of the dynamic nature of the human body and its ability to adjust substrate for fuel over the course of the day. For example, when conducting either fed or fasted steady state cardio (65%HR max) a compensation was found for greater fat utilisation (decreased respiratory exchange ratio), as opposed to carbohydrate, later in the day (at 12 and 24 hours post-exercise) during the fed exercise condition, with the reverse true for the fasted condition. Furthermore, the consumption of nutrients pre-exercise may provide an increased thermic effect of exercise due to the elevation in post-exercise oxygen consumption, which may therefore balance out the enhanced fat oxidation rates of fasted exercise.

Fed vs. Fasted: Influence on Body Composition

One such study was the first study to investigate the direct effects upon body composition comparing fasted vs fed steady state aerobic exercise when utilising a planned calorie deficit.

Over 4 weeks 20 healthy female volunteers were divided into a fasted steady state cardio group or a non-fasted steady state cardio group (60mins x 3 per week at 70% maximal heart rate), with all training sessions supervised and customised dietary plans created by a nutrition professional.

What did they find?
(1) Body Mass: A significant decrease in body mass from pre to post, with no significant difference between groups.
(2) BMI: A significant decrease in BMI from pre to post, with no significant difference between groups.
(3) BF% (ADP): No significant difference between groups, with a trend for decrease % pre to post.
(4) Waist Circumference: No significant difference between groups, with a trend of a decrease pre to post.
(5) Fat Mass: A significant decrease in fat mass pre to post, with no significant difference between groups.
(6) Fat-Free Mass: No significant difference between group and no significant effects of group or time.

The authors therefore concluded that there was no difference in any body composition outcome when utilising either fasted or fed steady state cardio.

However, it is important to acknowledge the limitations of this study:

  • Short study duration
  • Small number of subjects
  • The utilisation of female university students as subjects. The training status or body composition was therefore not that of competitive bodybuilders (aesthetic population) or athletes and results may be only specific to female non-obese population
  • Female subjects and menstrual cycle influence. Although testing was 1 month apart there was potential for irregular menses to influence body composition (fat-free mass and water contribution)
  • Self-report energy intake leaves questions regarding reporting accuracy (e.g underreporting of dietary intake)
  • Air Displacement Plethysmography via Bod Pod is not the gold standard of body composition measurement
Summary:

Based upon the research discussed there appears to be no additional benefit regarding fat loss when implementing fasted steady state cardio over fed (non-fasted). However, there could be a small undetected benefit that can’t be ruled out and further research is required.

For individuals concerned with fat loss their focus should be upon consistently maintaining a calorie deficit (energy intake less than energy expenditure) and opting for a training method (fasted or non-fasted) which suits their personal preference and schedule.

Thanks for reading. Got a question? Post this in the comments section below.
I am always happy to help.

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