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Cardio Controversy: High Intensity vs Low Intensity, Fasted or Fed?

Typically, cardiovascular exercise is performed at a low intensity, a steady pace, and for an extended duration. This is often referred to as Low Intensity Steady State exercise, or ‘LISS’. Alternatively, cardiovascular exercise can be performed in brief, high intensity intervals performed repeatedly with short rest periods over a short duration, and this is known as High Intensity Interval Training, or ‘HIIT’.

HIIT has been pioneered and popularised Dr. Izumi Tabata, who created the ‘Tabata Protocol’ involving eight rounds of 20 second high intensity intervals with 10 second rest periods between intervals equating to 4 minutes of total work. Furthermore, Jamie Timmons research group at Loughborough University devised a similar HIIT protocol, involving three 20 second high intensity intervals followed by two minutes of active rest, which is performed three times per week. However, a form of HIIT exercise, in which repeated 200m sprints were performed with 30 seconds rest between sprints, was believed to be first utilised by Sebastian Coe as early as the 1970’s.

Traditionally in bodybuilding circles, and to this day, LISS cardio has been used as it is believed that not only is this the most effective method to burn fat, but that it also places less impact upon the joints and is less detrimental to recovery and strength. Furthermore, it is often considered that HIIT, particularly when combined with resistance training, may increase the potential for overtraining. However, more recent opinion has supported the benefits of HIIT, highlighting the potential for similar fat loss results with less time spent exercising, greater retention of muscle mass, as well as an increase in metabolic rate.

Additionally, it is commonly believed that performing LISS cardio on an empty stomach, typically in the morning and in a fasted state, is the superior method for burning fat. This article will review the scientific literature regarding LISS cardio, HIIT cardio and fasted cardio, will discuss the benefits and limitations of each, and will aim to identify the method that is most adequately supported by the data.


From the scientific literature, it is apparent that HIIT may be a valuable and efficient form of exercise. Furthermore, HIIT may improve a number of physiological and performance markers to a greater extent than LISS despite utilising a lower exercise volume.

HIIT and LISS cardio have both shown to result in improvements in cardiovascular fitness and VO2 Max, however HIIT has been demonstrated to result in significantly greater increases in VO2 Max compared to LISS1,2,3. Additionally, HIIT has been shown to induce greater improvements in peak power and cycle time trial performance4.

HIIT improves a number of cardiovascular system parameters. Acutely, heart rate is significantly higher during HIIT compared to LISS exercise5. Chronic cardiovascular responses to HIIT include increases in stroke volume, as well as greater increases in endothelial function, blood flow and maximum oxygen uptake6. Regular HIIT training may also improve heat tolerance, by increasing cutaneous blood flow and sweat rate, and maximal oxygen uptake6,7,8.

In terms of body composition changes, HIIT has been demonstrated to decrease total body mass, fat mass, and central abdominal fat to a greater extent than steady state exercise5. Also, when corrected for energy expenditure, skinfold measures were found to decrease 9 times more after HIIT training compared to LISS9. Furthermore, HIIT was observed to increase lean body mass to a greater extent than LISS, although this finding was not statistically significant1. Regional specific changes in body composition have also been proposed, with HIIT involving legs and core was linked to increases in lean mass and reductions in fat mass in those body parts5.

When compared with LISS, only HIIT cardio was found to significantly reduce fasting plasma insulin and increase resting muscle glycogen, therefore attenuating glycogen depletion during exercise5,10. Additionally, HIIT has been demonstrated to result in increases in hydrogen ion buffering, muscular oxidative enzyme expression, beta oxidation and fat metabolism, and mitochondrial enzyme activity, in particular PGC-1α which is a master regulator of mitochondrial biogenesis5,6,7,8,9,10,11. Furthermore, HIIT training has been shown to increase the oxidative capacity of type II muscle fibres to a greater extent than LISS training7. These factors combined may lead to greater improvements in endurance and increased time to fatigue after regular HIIT exercise.

In contrast to the majority of studies, Nybo and colleagues3 concluded that interval training did not reduce resting heart rate, body fat percentage, or total to HDL cholesterol ratio as much as prolonged cardio. Also, similar increases in mitochondrial markers of carbohydrate and fat oxidation, reductions in whole body carbohydrate oxidation, and increases in VO2 max have been reported after HIIT and LISS exercise programmes12.

From the available scientific literature, it is apparent that further research is required in order to optimise the HIIT protocol, provide data over a longer trial duration than is currently available (6 weeks), and to examine the recovery between HIIT bouts. However, with regards recovery after HIIT bouts, it can be recommended that a taper after a number of weeks of repeated HIIT exercise bouts be undertaken. Furthermore, it must be acknowledged that HIIT may be unsuitable for overweight individuals, that high subject motivation is required to perform exercise at such intensity, that inter-individual variation in response to HIIT may exist, and that differences in physiology or performed when comparing HIIT and LISS may often be negligible.

Fasted vs Fed Cardio

The concept of performing cardiovascular exercise after an overnight fast for increased fat burning was first popularised by Bill Phillips in his book ‘Body for Life’, and he proposed that 20 minutes of fasted morning cardio has greater effects upon fat loss than 1 hour of cardio in post-prandial, or fed, state.

Although scientific literature has been published in which the concept of fasted cardio is supported, there is also a substantial body of data which contradicts this hypothesis, and therefore there is currently a lack of conclusive evidence to support either fasted or fed cardio.

The consumption of glucose either pre- or during exercise has been found to increase plasma glucose, reduce plasma free fatty-acids (FFAs), blunt epinephrine response to exercise, maintain or increase Respiratory Exchange Ratio (RER), and maintain the rate of carbohydrate oxidation13,14,15. Furthermore, it has been demonstrated that these metabolic responses to exercise in a carbohydrate-fed state are similar in men and women16.

The findings of Febbraio and colleagues17 may highlight the benefits of intra-exercise carbohydrate consumption, indicating that carbohydrate consumption during steady state exercise, as well as when consumed both pre- and during-exercise, maintained plasma glucose after 80 minutes of activity. However, when carbohydrate was not consumed during exercise, plasma glucose was shown to decrease and improvements in performance were not maintained.

These changes after carbohydrate consumption have therefore been found to culminate in the postponement of fatigue13. Specifically, carbohydrate consumption has been demonstrated to facilitate the performance of an additional hour of exercise with little reliance on muscle glycogen14.

It has also been proposed that fed training results in an increased thermic effect of exercise and therefore a greater Excess Post-exercise Oxygen Consumption (EPOC)18. Exercising in a fasted state has been found to result in over double the nitrogen loss, which indicates protein breakdown, compared to training in a fed state19. Additionally, exercising in a fasted state is likely to affect energy levels, exercise intensity and thus performance. Therefore, fasted training may not be favourable for individuals seeking to gain muscle mass.

However, despite the substantial volume of evidence in favour of fed cardiovascular training, a number of studies have demonstrated positive effects of fasted cardio. It has been proposed that fasted cardio may burn up to 20% more fat than fed cardio20. Furthermore, it has been reported that fasted subjects exhibited greater increases in VO2 Max, attenuated increase in heart rate due to increased plasma norepinephrine, greater increases in muscle glycogen post-training, greater increases plasma FFA, and greater increases in fat oxidation21,22,23.

Furthermore, in contrast to fasted cardio, carbohydrate-fed exercise was not shown to influence genes involved in carbohydrate oxidation but was shown to negatively affect genes involved in fatty acid transportation and oxidation23. Additionally, no significant differences in glycolytic or beta oxidation enzyme activity, involved in carbohydrate and fat oxidation, were observed between fasted and fed training groups21,. Despite these proposed benefits, gender may influence responses to fasted cardio, with men responding more favourably to fasted cardio than women21.

However, it must be noted that examining and comparing fat burning during a single bout of exercise is short-sighted and that this should be considered over multiple days. Also, individual training status and exercise intensity may mitigate the effects of pre-exercise feeding upon fat oxidation24.


In conclusion, HIIT should be recommended for most individuals as the most efficient training method and due to possible greater improvements in a number of physiological and performance markers. However, for certain individuals, for example obese or elderly individuals, HIIT may be considered overly intensive. Additionally, further research is required to determine the most effective HIIT protocol.

With regards exercising in a fasted or fed state, fasted exercise may not offer significant improvements over fed training and may affect energy levels and performance. However, for individuals in contest preparation, fasted exercise may be useful for reducing body fat when individuals are already at a low body fat percentage. Furthermore, if exercising fasted, in the morning, fits best with an individual’s schedule, this should not be discouraged.


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