Cardiovascular training for fat loss is common practice within the fitness industry, typically used by those looking to lose excess weight for health benefits (obese/diabetics) or those whom are trying achieve extremely low body fat levels for competition purposes (Bodybuilders/fitness models). There are as we now know larger contributing factors to the fat loss equation such as caloric intake/macro and micro nutrient ratios, digestive health, hormone levels and who could forget genetics. But, if we are to use cardiovascular training as an additional tool in the pursuit of fat loss, which one is optimal?
For those unsure of what these two forms are here is a brief explanation: Note. (both are merely examples, there are of course many other ways in which either can be performed)
HIIT- repeated bouts of short, high intensity anaerobic exercise done using intervals. An example would be 30 seconds of sprinting followed by 90 seconds of walking or resting repeated for a time of usually 4-30 minutes. It is maximal effort exertion and uses glycogen as its main source of energy production. Heart rate zone roughly between 85%-100% HR max.
LISS- a continuous form of low intensity steady state aerobic exercise. An example would be 40 minutes of treadmill walking on a low level incline. It requires little effort and uses fat as its main source of energy production. Hear rate zone roughly between 55-70% HR max.
Based on those statements it would seems sensible to conclude that LISS should be used for achieving overall fat loss. Yes, LISS does burn more fat relative to glycogen, however HIIT uses far more total fat substrate during activity even though it has a worse fat/glycogen ratio [1]. To add, it is not the immediate effects of the activity that really matter here, it is the time after the activity whereby other processes contribute to further fat loss via metabolic adaptations. This is known as EPOC (excess-post exercise oxygen consumption), in simple terms it means that more energy (fat substrate) is used after the activity than what normally would be. This effect does not occur when LISS is used, if you go for a jog within the ‘fat burning zone’ almost no calories will be used after. [3]
Fig 1.[5]
HIIT does not only prove to be an extremely efficient way to burn fat but it has many other physiological benefits also:
• Increased Aerobic Capacity – The amount of oxygen your body can use (oxygen uptake) is increased, so your overall aerobic capacity can increase faster than with low intensity endurance exercise. [3]
• Increased Lactate Threshold – Your ability to handle increased lactic acid build up in your muscles increases
• Improved Insulin Sensitivity – Your muscles more readily suck in glucose, instead of the glucose going to your fat stores. [4]
• Anabolic Effect – Some studies show that interval training combined with consuming slightly more calories than you burn creates an anabolic effect, which helps you put on muscle. The opposite occurs with steady state cardio, which for long durations is catabolic (muscle wasting). [2]
Ok, so it’s been established HIIT is the most optimal way to ‘burn’ fat, but there is without doubt a need to use LISS in specific situations. The following is a list of reasons why you may perhaps opt for this method throughout your fat loss pursuit.
1. Utilise circulating FFA’s (Free fatty acids) in the blood post HIIT.
2. Minimalise stress response (Delayed onset muscle soreness/mental fatigue).
3. Reduce the chance of injury.
4. Improve accuracy in measuring energy expenditure.
References
[1] King, J.W. A comparison of the effects of interval training vs. continuous training on weight loss and body composition in obese premenopausal women (thesis). East Tennessee State University, 2001.
[2] Naito, et al. Satellite cell pool enhancement in rat plantaris muscle by endurance training depends on intensity rather than duration. Juntendo University, Japan. Acta Physiologica. 2011 Oct.
[3,4] Wilson, et al. Concurrent Training: A Meta Analysis Examining Interference of Aerobic and Resistance Exercise. University of Tampa, FL. J Strength Conditioning.
[5] Meuret, J.R, et al. A comparison of the effects of continuous aerobic, intermittent aerobic, and resistance exercise on resting metabolic rate at 12 and 21 hours post-exercise. Medicine & Science in Sports & Exercise 39(5 suppl) :S247, 2007.
Body Re-composition by Scott Mccreath 