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If you've ever tried to lose weight, you've probably had a lot of people trying to tell you the best way to do it, and there's a good chance you've heard people talking about fasted cardio. Fasted cardio is one of those forms of exercise that seems to generate strong opinions. Let's look at both sides of the story.

Fasted Cardio Definition

The name is pretty self explanatory, and for most people, fasted cardio refers to a moderate intensity workout, usually between 30 minutes and an hour in duration, performed before breakfast, on an empty stomach. For most people, this will mean a jog, a bike ride, or an aerobic session at the gym performed after a fast of 8-12 hours.

Benefits of Fasted Cardio In the Morning

The primary rationale behind fasted cardio is that some people feel the body burns fat more effectively in a fasted state. Insulin is the hormone that controls blood sugar, and it is present in the bloodstream at higher levels after a meal. Research has shown that raised levels of insulin in the bloodstream can inhibit lipolysis, or the breakdown of fat (1).

Also affecting fat breakdown is the body's stress hormone, Cortisol. Cortisol levels cycle during the day, and are highest in the morning after fasting. As this hormone exerts catabolic activity, many people believe that exercising when cortisol levels are highest encourages breakdown of fat.

The other big reason cited in support of cardio on an empty stomach is that the body has low glycogen stores after fasting, encouraging the body to utilise its fat stores for fuel.

Fasted cardio may influence food intake for the rest of the day. A study of food intake after fasted and non fasted exercise showed that exercise suppressed, and eating breakfast stimulated the appetite for the rest of the day, and the effect of food overrode the effect of exercise. The group with the lowest energy balance after an ad libitum lunch was the fasting exercise group (2).

There is also research indicating that regular fasted cardio may result in an adaptation that increases the body's ability to store muscle glycogen. For this reason, fasted cardio has been adopted as a strategic form of training by some endurance athletes (3).

Finally, there are many anecdotal accounts of fasted cardio's ability to shift small problem areas in people with very low body fat percentages, and a number of competing bodybuilders swear by fasted cardio in the days leading up to competition.

The Fasted Cardio Myth

For everyone who swears by it, fasted cardio has just as many, if not more detractors. While fans of the technique are quick to cite a study where fat was broken down at a 22% greater rate during exercise in fasting subjects, naysayers are just as quick to counter this by the notion that the breakdown of the fat molecule (lipolysis) is not the same thing as using fat for energy (oxidation). The same study shows there is no overall difference in fat oxidation during exercise between fasting and non fasting groups until the subject had been exercising for 80-90 minutes, which is longer than most people devote to a morning cardio session (4).

The idea that low glycogen levels mean the body must turn to fat for energy is the basis for low carb diets, like the Atkins diet, or the recently popular high fat low carb regime. The body is designed to use glucose as its first choice of fuel, and both these diets involve a period, where over many days, the body exhausts its stores of glycogen and transitions into metabolic ketosis. Ketosis is so named because the body is using its fat reserves to produce chemicals known as ketones, which the organs and tissues of the body use in place of glucose.

Moderate cardio is a great fat burner, but if the body is not producing ketones, normal body chemistry actually requires the energy of glucose to oxidise fat. Overnight fasting will lower glycogen stores, but generally not to the point of metabolic ketosis. What is more likely to happen when glucose is exhausted is something that endurance athletes call 'hitting the wall', a state of extreme fatigue that impedes performance and therefore fat burning.

While fasted cardio has suppressive effects on the appetite, studies have shown that people who eat before cardio actually burn more energy during the rest of the day. Measurements of fat oxidation for the rest of the day have shown that the afterburn effect is greatest when exercise is performed at a high intensity after eating, and that fasting with moderate intensity cardio produces the lowest afterburn (5).

Fasted Cardio and Muscle Loss

One of the most common arguments against fasted cardio is that it causes muscle loss. Muscle is naturally broken down during exercise, and where glycogen and blood glucose is low, may be catabolised for energy. Nitrogen excretion is a marker for muscle breakdown, and one study has shown that twice the nitrogen is released from muscle during a fasted workout than during one where the participant has eaten (6). Additionally, some people maintain that raised cortisol levels during fasting cause breakdown of muscle.

For this reason, many people believe in 'partially fasted cardio' and consume a low carb protein shake or BCAAs before or after morning cardio, which can be very effective in preventing muscle loss.

Fasted Cardio vs HIIT

The other type of exercise that gets a big rap for fat loss is high intensity interval training, or HIIT.

If you're comparing HIIT to Fasted Cardio, there are more issues to consider than fat burning during the session.

HIIT requires a high level of fitness, and people who aren't quite there yet will not see fat burning benefits from a poorly executed workout. That said, people who can handle it may choose HIIT for the short workout duration, but others who use exercise to wake up, wind down, or think, may prefer a longer, moderate intensity workout.

In terms of fat burning, HIIT comes out on top for a couple of reasons. Firstly, the aforementioned afterburn effect significantly boosts the fat burning for the rest of the day, to an extent that fasted cardio does not (7). Secondly, HIIT often necessarily incorporates strength exercises to maintain intensity, which can encourage the growth of metabolically active lean muscle and increase the rate at which the body uses energy in the long term.

Finally, it's important to mention that combining the two and doing high intensity exercise in a fasted state makes muscle breakdown very likely. Similar to semi-fasted cardio, a protein shake prior to high intensity exercise will make a big difference.

Fasted Cardio - Good or Bad?

For some people, fasted cardio will work wonders, and for others will yield poor results. Our advice to people is to get the best of both worlds and break the fast with a low calorie protein shake or amino acids before a cardio workout. This can help protect against muscle breakdown and may help to increase the body's fat burning post exercise. It's important to remember that no form of training is a one size fits all key to success, and the most important thing is to find the type that works for you.

(1) Choi SM, Tucker DF, Gross DN, Easton RM, DiPilato LM, Dean AS, Monks BR, Birnbaum MJ. Insulin regulates adipocyte lipolysis via an Akt-independent signaling pathway. Mol Cell Biol. 2010 Nov;30(21):5009-20.
(2) Javier T. Gonzalez, Rachel C. Veasey, Penny L. S. Rumbold and Emma J. Stevenson. Breakfast and exercise contingently affect postprandial metabolism and energy balance in physically active males. British Journal of Nutrition. Volume 110. Issue 04. August 2013, pp 721-732
(3) De Bock K, Derave W, Eijnde BO, Hesselink MK, Koninckx E, Rose AJ, Schrauwen P, Bonen A, Richter EA, Hespel P. Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake. J Appl Physiol (1985). 2008 Apr;104(4):1045-55.
(4) Horowitz JF, Mora-Rodriguez R, Byerley LO, Coyle EF. Substrate metabolism when subjects are fed carbohydrate during exercise. Am J Physiol. 1999 May;276(5 Pt 1):E828-35.
(5) Lee YS, Ha MS, Lee YJ. The effects of various intensities and durations of exercise with and without glucose in milk ingestion on postexercise oxygen consumption. J Sports Med Phys Fitness. 1999 Dec;39(4):341-7.
(6) Lemon PW, Nagle FJ. Effects of exercise on protein and amino acid metabolism. Med Sci Sports Exerc. 1981;13(3):141-9.
(7) Schoenfeld, Brad. Does Cardio After an Overnight Fast Maximize Fat Loss? Strength & Conditioning Journal: February 2011 - Volume 33 - Issue 1 - pp 23-25

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