Does Fasting Cardio Work? A Scientific Review
By Sam Love
This article will tackle the question- does fasting cardio work? Fasting cardio is a method traditionally used by bodybuilders in preparation for a show to help them lose as much body fat as possible. The idea of fasting cardio is to take part in cardio first thing in the morning before you break your fast with breakfast. The sciency thought behind this is, first thing in the morning glycogen stores in the muscles are depleted through the energy burned during sleep, meaning the body will be in a lypolytic state. It is also due to cortisol (lypolytic hormone) levels being higher first thing in the morning, meaning the chance of lipolysis is greater. Several studies have found after consuming carbohydrates before low intensity aerobic exercise (up to 60% V02max) can blunt fat oxidation (Ahlborg G and Felig P 1976, Horowitz JF Et al 1997). Although, not only is this research outdated, it can be attributed to an insulin-mediated attenuation of lipolysis, an increased glycolytic flux and a decreased expression of genes involved in fatty acid transport and oxidation(Civitarese AE Et al 2005, Horowitz JF Et al 1999). These are all things which, you may find in somebody who has type 2 diabetes. There is also a substantial amount of research disproving fasting cardio as a practice actually works
Lipolysis – is the breakdown of lipids and involves hydrolysis of triglycerides into glycerol and free fatty acids
Proteolysis – this is the breakdown of proteins or polypeptides into amino acids by the action
Thermic effects of exercise- Exercise, in any form, contributes to approximately 20% of total energy expenditure (TEE).
Glycotic flux- Glycolysis is a fundamental metabolic pathway, is critical for the production of energy. Glycolytic flux, or the rate at which molecules proceed through the glycolytic pathway, is tightly regulated in response to the cellular environment
EPOC- Excess post-exercise oxygen consumption (EPOC, informally called afterburn) is a measurably increased rate of oxygen intake following strenuous activity intended to erase the body's "oxygen deficit".
HIIT- high-intensity interval training, is a training technique in which you give all-out, one hundred percent effort through quick, intense bursts of exercise, followed by short, sometimes active, recovery periods. This type of training gets and keeps your heart rate up and burns more fat in less time
Hypoglycaemic- low amount of blood sugar or blood glucose, this can result in lack of energy, clumsiness and foggy brain.
One study was done on moderately trained individuals who consumed a high glycaemic food during different intervals of training (30,60 and 90 mins) along with individuals who had fasted for 12-14 hours prior to exercise. The results of this test showed there were no differences between the fasted and fed group’s oxidation levels until 80-90 minutes of training at which point the fasted subjects had a greater oxidation level than their fed counter parts, also showing the lipolysis of the fed group was decreased by 22% compared to the fasted group (Horowitz JF Et al 1997). In the fasted group it showed there was an effect on lypoisis and fat oxidation after 80-90 minutes, showing fasting cardio to be beneficial in terms of oxidising fat it has to be down for longer than a regular person performs this method. Furthermore it showed, lipolysis was greater, but it is worth mentioning, if free fatty acids (FFA) are not oxidised once they have been broken down into the blood stream, they are re-esterfied into adipose tissue, defeating the purpose of any lipolysis occurs during fasting.
Lee YS et al (1999) carried out a study on 10 male college students, showing the effects of glucose milk (GM) on exercise. This was done for the group of students during: long duration, low intensity exercise, with and without the consumption of GM and in high intensity, short duration bouts of exercise, with and without the consumption of GM. The conclusion of this study found, use of GM resulted in a greater post exercise oxygen consumption (EPOC) compared to when in a fasted state. EPOC is also the state, which is achieved from high intensity interval training (HIIT). HIIT has been found to be the best method of burning fat and is much better than its low to moderate steady state counterparts (Gibala MJ Et al 2006, Schoenfeld and Dawes J 2009) although it has been found, during a HIIT session blood flow to the adipose tissue decreases(Romijn JA Et al 1993) which is thought to have a negative effect on the usage of FFA as energy, but it is the reaction of EPOC allowing HIIT participants to continue to burn more calories throughout the day. This adds to the thought, burning fat is a 24hr process and it cannot just be achieved in a 30 minute fasting cardio session.
It is thought the location of adipose tissue mobilised during training needs to be taken into account. During low to moderate steady state cardio it is presumed fat is equal to 40-60% total energy expenditure (TEE). However, only 50-70% of this fat is taken from the plasma FFA (Van Loon LJ 2004). The other 30-50% comes from intramuscular triglycerides (IMTG). IMTG are a lot like muscle glycogen stores because they can only be oxidised locally within the muscles. Lipolysis of the IMTG stores are usually stimulated when exercising at an intensity of 65% V02 max (Romijn JA Et al 1993) although, IMTG have no bearing on personal health or aesthetics it is merely intramuscular fat. This fat allows for more energy should it be needed. IMTG is typically higher in trained endurance athletes. It is the subcutaneous fat sometimes known as cellulite, which has an impact on aesthetics and wellbeing.
Bad news for bodybuilding as research has shown exercise in a fasted state has an impact on proteolysis. During a fast there is evidence of a loss of nitrogen in the body while exercising. Nitrogen being a key component of protein synthesis, which is essential to building lean muscle. The research showed nitrogen loss was more than doubled when compared to somebody who had sufficient glycogen stores during exercise (Lemon PW and Mullin JP 1980).
In a paper researched by (Antonio Paoli Et al 2011) concerning exercise in fed and fasted individuals, it was discovered fat oxidisation after exercise was greater in fed individuals rather than in fasted individuals by showing, there was a decrease in RER and an increase in V02 in the fed group. A combination of decreased RER and increased V02 can be credited to a maintained increase in protein synthesis after training, which is beneficial to building lean muscle.
The literature points towards fasting cardio being negative to lean muscle gain or muscle maintenance and HIIT being superior. This is due to fasting during training actually has a negative effect on nitrogen balance, which in turn has a negative effect on protein synthesis. This can hinder muscle growth, however fasting cardio does in fact aid in fat burning although only after a period of 80-90 minutes does become more beneficial than eating before training. This leads us to believe, if you’re trying to gain or maintain lean muscle, fasting cardio isn’t the way to go in terms of fat loss. This is all evidence towards fasting cardio at a steady state, however while performing HIIT this is a different matter.
It is generally found in the literature, High Intensity Interval Training (HIIT) is a better method of fat loss than steady state cardio. HIIT causes a state of EPOC, which allows the body to burn calories throughout the day, rather than in steady state cardio where calories only burn up to an hour after exercise. In a study carried out by (Antonio Paoli Et al 2011), where they discover there was fat oxidation in a fasted subject while performing high intensity exercise through testing RER and V02 max, the RER was less after 12-14 hrs in the fed group compared to the fasted group, showing evidence of lipid utilisation occurs by consuming food before exercise. This is backed up by (Lee YS Et al 1999) who also found EPOC was greater after consuming food before high intensity bouts of exercise. Although, fasting does work in conjunction with HIIT, performing this in a hypoglycaemic state, will cause a decrease in performance, therefore leading to a decrease in the amount of calorie burnt long term.
From the research we find, if you are looking to burn fat by fasting cardio it isn’t necessarily the best way to do so. Due to the research it should be recommended for people to choose High Intensity Interval Training over steady state cardio and while performing HIIT training it shouldn’t be carried out under fasted protocol. Although, these test where primarily done using carbohydrate beverages and carbohydrate feeding. This could be a poor choice to bodybuilders who are leading a no carb approach when getting ready to compete, as the bodybuilders taking this approach may not have the energy to perform HIIT, having little energy from the lack of carbohydrates they are eating. Although, it is said HIIT has found beneficial to people in a hypoglycaemic state, in spite of this energy levels for training are imperative to high performance and in turn calories burnt. Furthermore, the use of carbohydrate allows for greater EPOC meaning more calories are burnt when you aren’t in a hypoglycaemic state more carbs on your way to a competition as it is allows for greater EPOC and in turn greater calories burnt and lipids oxidised throughout the day.
Ahlborg G and Felig P. Influence of glucose ingestion on fuel-hormone response during prolonged exercise. J Appi Physiol 41 : 683-688, 1976.
Horowitz JF, Mora-Rodnguez R, Byerley LO, and Coyle EF. Lipolytic suppression following carbohydrate ingestion limits fat oxidation during exercise. Am J Physiol Endocrinol Metab 273: E768-E775, 1997.
Horowitz JF, Mora-Rodriguez R, Byerley LO, and Coyle EF. Substrate metabolism when subjects are fed carbohydrate during exercise. Am J Physiol 276(5 Pt 1): E828-E835, 1999.
Civitarese AE, Hesselink MK, Russell AP, Ravussin E1 and Schrauwen P. Glucose ingestion during exercise blunts exerciseinduced gene expression of skeletal muscle fat oxidative genes. Am J Physiol Endocrinol Metab 289: E1023-E1029, 2005.
Lee YS, Ha MS, and Lee YJ. The effects of various intensities and durations of exercise with and without glucose in milk ingestion on post exercise oxygen consumption. J Sports Med Physical Fitness 39:341-347, 1999.
Schoenfeld B and Dawes J. High-intensity interval training: Applications for general fitness training. Strength Cond J 31 : 44-46. 2009.
Gibala MJ, Little JP, van Essen M, Wilkin GP, Burgomaster KA, Safdar A, Raha S, and Tarnopolsky MA. Short-term sprint interval versus traditional endurance training: Similar initial adaptations in human skeletal muscle and exercise performance. J Physiol 15(pt 3): 901-911, 2006.
Romijn JA, Coyle EF, Sidossis LS, Gastaldelli A, Horowitz JF, Endert E and Wolfe RR. Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity. Am J Physiol 265(3 Pt 1): E380-E391, 1993.
van Loon LJ. Use of intramuscular triacylglycerol as a substrate source during exercise in humans. J Appl Physiol 97: 1170-1187, 2004.
Lemon PW and Mullin JP. Effect of initial muscle glycogen levels on protein catabolism during exercise. J Appi Physiol 48: 624-629, 1980
Antonio Paoli, Giuseppe Marcolin, Fabio Zonin, Marco Neri, Andrea, and Quirico F. Pacelli. Exercise fasting or fed to enhance fat loss? Influence of food intake on respiratory ratio and exercise postexercise oxygen consumption after a bout of endurance training. Int J Spo Nuti and Exer Metabolism. 21, 2011, 48-54, 2011