Suppose you are an overweight individual who has just fronted up to your local gym or has just enlisted the services of a personal trainer. Naturally, your main objective is to lose body fat and reduce your overall weight. What exercise protocols do you suppose you are most likely to be given? The average person might expect it to closely resemble one incorporating regular steady state exercises such as walking or jogging. Perhaps if you have a more adventurous or innovative personal trainer they might give some more interesting variations on jogging, such as stair running, trail running, or obstacle-based courses. But if you are confined to doing your exercise in a gym, then your aerobic-based exercise options are a bit more limited, with popular options including: treadmills, rowing machines or stair climbers. But what if steady-state/aerobic-based exercise was not the best type of exercise for fat loss? Well it seems that may in fact be the case; judging by the results of a series of studies comparing the efficacy of different types of exercise on fat loss. Such studies have provided increasing evidence that high-intensity intermittent exercise (HIIE) has the potential to be an economical and effective exercise protocol for reducing fat of overweight individuals.
What is High-Intensity Intermittent Exercise (HIIE)
What is HIIE you might ask? Well the protocols vary but they typically involve repeated bouts of brief sprinting at an ‘all-out’ intensity immediately followed by low intensity exercise or rest. In studies, sprints are most commonly performed on a stationary bicycle, called an ergometer, which measures power output. Sprints are typically performed for 30 seconds. Subjects then rest for between 3 to 4 minutes and repeat the sprint again. Usually, there are only 4-6 sprint efforts per session, which means the subject is only really exercising for 2-3 minutes, while the whole session lasts between 18 and 27 minutes. In scientific circles, this protocol is commonly referred to as the Wingate test. In contrast the average aerobic exercise session which can last between 30 to 60 minutes. Thus, one of the characteristics of HIIE is that it involves markedly lower training volume making it a time-efficient strategy compared to traditional aerobic exercise programs.
HIIE vs Aerobic Exercise for Fat Loss
So what results have studies shown when comparing the efficacy of HIIE versus more traditional aerobic exercise for fat loss? One study compared HIIE and steady state aerobic exercise and found that after 24 weeks subjects in the HIIE group lost more subcutaneous fat, as measured by skin folds, compared to a steady state exercise group when exercise volume was taken into account1. A more recent study conducted an HIIE program for 15 weeks with three weekly 20-minute HIIE sessions in young women2. HIIE consisted of an 8-second sprint followed by 12 s of low intensity cycling. Another group of women carried out an aerobic cycling protocol that consisted of steady state cycling at 60% VO2max for 40min. Results showed that women in the HIIE group lost significantly more subcutaneous fat (2.5 kg) than those in the steady state aerobic exercise program who had negligible fat loss2.
There have also been studies which have only measured the effect of a HIIE program on fat loss, without comparing it to an aerobic exercise program. One such study examined 32 middle-aged metabolic syndrome men and women who performed 16 weeks of HIIE three times per week. VO2max increased by 26% and body weight was reduced by 2.3 kg3. Yet another study examined ten overweight males aged 32 years after two weeks of HIIE consisting of 6 sessions of 4–6 repeats of an Wingate test. VO2max increased (8%) and significant change in waist circumference (2.4cm) and body mass (1.0kg) was also found4.
Drawbacks with High-Intensity Intermittent Exercise
One of the drawbacks of HIIE is that it is very hard in nature and subjects performing it must have the psychological motivation to undergo considerable physical discomfort for short periods. On the other hand, some individuals may naturally gravitate to this type of exercise as they get a ‘high’ from the pain or the ‘burn’. However, for the elderly and grossly obese, this type of exercise may simply not be practical. There is also the unfounded notion that it’s just not healthy or safe to exert oneself in that manner. These are ultimately issues that an experienced personal trainer should be able to manage with their clients.
High-Intensity Intermittent Exercise-Mediated Fat Loss
So how are the fat loss effects of HIIE mediated as opposed to aerobic exercise. One of the mechanisms is thought to be due to the significantly higher catecholamine release in response to HIIE. One study showed plasma epinephrine increased 6.3-fold, while norepinephrine increased 14.5-fold at the end of sprinting5. Catecholamines, especially epinephrine, have been shown to drive lipolysis and are largely responsible for fat release from both subcutaneous and intramuscular fat stores6. Significantly, more β-adrenergic receptors have been found in abdominal compared to subcutaneous fat7 suggesting that HIIE may have the potential to lower abdominal fat stores. Another mechanism by which HIIE stimulates fat loss may be via HIIE effects on growth hormone release. One study examined the response of growth hormone to treadmill sprinting in female and male athletes and showed that there was a marked growth hormone response to only 30 s of maximal exercise8. The response was similar for men and women but greater for sprint compared to endurance trained athletes. Moreover, growth hormone concentration was still ten times higher than baseline levels after 1 hour of recovery8.
HIIE for Obese vs Overweight People
The majority of studies that have conducted HIIE interventions for weight loss have involved young adults with normal BMI and body mass and have been relatively short (i.e. 2 to 6 weeks). Accordingly, many of these studies have only resulted in negligible weight loss9. Studies that used longer duration HIIE protocols with individuals possessing moderate elevations in fat mass2 have resulted in greater weight/fat reduction. Interestingly, the greatest HIIE-induced fat loss was found in two studies that used overweight type 2 diabetic adults (BMI > 29 kg/m2) as subjects10, 11. Given that greater fat loss to exercise interventions has been found for those individuals possessing larger initial fat mass12, it is feasible that HIIE will have a greater fat reduction effect on the overweight or obese.