The HIIT Nature Of Crossfit : Why Crossfit Burns So Much Damn Fat!

Few sports have taken the fitness industry by storm quite the way Crossfit has! Since its official inception by founder Greg Glassman back in 2000, Crossfit has grown exponentially. Crossfit was developed from gymnastics, so you only have to think back to the 2012 Olympics or indeed the recent world championships to appreciate the powerful, explosive, functional moves that encompass Crossfit. So why is it then that the high intensity power moves that many strive to master result in such impressive amounts of fat loss too?

High Intensity Interval Training (HIIT) 

HIIT training is characterised by high intensity, intermittent activity patterns i.e. short periods of intense activity with slightly longer periods of low intensity activity (although these are relatively few and far between in Crossfit). HIIT training has been seen to develop the metabolic functions of our muscles, this enhances the delivery of energy substrates such as glucose, and the breakdown of glycogen to a point that improves high intensity performance. These improved metabolic functions mean that the intensity of exercise increases as well as our ability to train harder, for longer, meaning our bodies fat oxidation rates increase i.e. our bodies use fat as an energy substrate!

Although HIIT training usually lasts between 4-30mins, the reason this range is so broad is because research seems to show that it doesn’t take as much time to induce fat burning during interval training…hence the benefit of the short burst, compound power nature of Crossfit (Talanian, Galloway, Heigenhauser, et al. 2007).

The intermittent nature of HIIT training necessitates the person to have a good balance between aerobic (oxidative) and anaerobic (non-oxidative) energy systems. The main energy substrate during HIIT is glycogen, this is broken down to release free glucose which then fuels your muscles. A study by Krustrup, Mohr and Steensberg, (2006) investigating energy stores of athletes performing HIIT exercise found that either all or most of the athletes muscle glycogen stores had been depleted, meaning the next energy source in line is…FAT! The short bursts of activity results in the rapid release of glycogen for energy, similar to how you might use up more petrol in your car by putting your foot down compared to a nice, steady constant speed.

The Box Magasine explains how Crossfit places a lot of emphasis on specific weights, specific distances and specific movements over a specific time frame…pretty specific right? The benefit of such a specific structure is that it allows competitors to gauge performance more easily. Although the root of Crossfit lies in gymnastics, it has developed into a sport that incorporates weightlifting, sprinting and high intensity work in one. This structure makes Crossfit a HIIT powerhouse, with a fat burning machine that’ll help get you into shape!


Krustrup, P., Mohr, M., Steensberg, A. et al. (2006). Muscle and blood metabolites during a soccer game: implications for sprint performance. Med Sci Sports Exerc. 38: 1165-1174

Talanian, J L., Galloway, S, D., Heigenhauser, G, J., Bonen, A, & Spriet L, L. (2007). Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women. J Appl Physiol. 102: 1439-1447


About the Author

Job Role Qualified Dietitian and Sports Nutritionist Qualifications BSc (Hons) Sports Science | BSc (Hons) Dietetics Tom has always participated in sport both recreationally and competitively which led to an unquenchable thirst for information on anything health, nutrition and fitness. After leaving school Tom went on to play for a football academy during which time he studied Sport and Exercise Science. From here he went on to study a BSc (Hons) Sport Science at UEA followed by his second BSc (Hons) degree, this time at the University of Hertfordshire studying Dietetics. Tom has worked in the fitness, educational and clinical nutrition industry starting out at David Lloyd Health and Leisure Clubs. He then went on to work as a Dietitian (RD) in the NHS, during which time he conducted clinics for healthy eating, weight loss and weight gain, as well as specialised consultations on Diabetes, IBS and Coeliac disease to name a few. He has vast amounts of experience at devising diet plans and supplement regimens, as well as working in the community with schools and competitive athletes. As Head Nutritionist and Supplement expert at Discount Supplements Tom is here to provide current and evidence based health and nutrition information to help you reach your health and fitness goals!
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