Keeping exercise short and hard: what’s involved and is it worth it?
Giving lifestyle advice often involves encouraging patients to do exercise. Is there a less time-consuming way to exercise but achieve the same health benefits?
Physical activity is recommended as just one part of the overall strategy to remain healthy. Current UK guidelines suggest that adults between the age of 16 and 64 should aim to achieve at least two-and-a-half hours each week of moderately intense aerobic activity and strength training on two days per week or (in addition to strength training twice a week) an equal mix of moderate and vigorous activity.1
There is no doubt that physical activity is important for health. The Cooper Centre Longitudinal Study in Texas recently reported on a 40-year follow-up study of over 80,000 people, assessing the relationship between cardiorespiratory fitness (CRF) and metabolic risk. The conclusion was clear: higher levels of CRF decreased the development of cardiovascular disease risk factors.2
Furthermore, in a study of over 3,000 women followed for 16 years, lower CRF was found to be significantly associated with a higher risk of all-cause mortality3 and similar associations have been demonstrated in men.4 Additionally, a recent analysis has suggested that exercise may be a viable alternative to medication for the secondary prevention of heart disease, type 2 diabetes and the treatment of heart failure5 and that it might even reduce the risk of dementia.6 In short, there is now a good deal of evidence from epidemiological studies that physical activity is associated with a reduced risk of coronary heart disease, obesity, type 2 diabetes and many other chronic conditions.7
Despite these acknowledged benefits many people cite lack of time, money and tiredness as barriers to physical activity.8,9 Recently, there has been great interest in high-intensity interval training (HIT) as a means of achieving health and fitness but with a reduced time commitment. The technique was the subject of a BBC Horizon programme in 2012 and there have been several new books (eg, ‘Fast exercise’ by Michael Mosley) on the topic. But what exactly is HIT training and does the technique offer equivalent or even superior health benefits compared with continuous (eg, cycling, running) aerobic exercise?
This article provides an overview of the evidence for HIT training and whether this new approach is a suitable alternative to traditional methods.
In order to understand the effect of HIT better it is worth first considering how the body derives the necessary energy to exercise and the adaptations that occur as a result of any form of exercise.
Fuel for exercise
Muscle mass accounts for up to 40 per cent of the total body mass and contributes roughly 30 per cent of the resting metabolic rate in adults.10 Muscles require energy to contract and this is provided by adenosine triphosphate (ATP). Muscles possess a small amount of ATP which is soon exhausted and therefore metabolic pathways are activated in response to exercise to generate further ATP.
Three separate systems exist within muscles that are able to generate ATP. When starting exercise, ATP is produced via the phosphagen system in which creatine phosphate donates a phosphate residue to adenosine diphosphate to create ATP. The second system is the anaerobic breakdown of stored glycogen within muscles (anaerobic glycolysis). The third and final system is aerobic metabolism (the citric acid or Kreb’s cycle) which uses oxygen and is much more efficient at generating ATP and can use both stored carbohydrate and fats.11
The phosphagen system can only generate sufficient ATP to sustain maximal effort exercise for about 10 seconds12 but this is sufficient for activities such as the shot put, jumping events, etc. As the duration of exercise increases, more ATP is required and is generated by anaerobic metabolism (anaerobic glycolysis). Anaerobic metabolism, again, yields only a small amount of ATP that is adequate for short bursts of activity lasting up to two minutes (eg, sprinting or lifting heavy weights). Longer periods of exercise such as jogging, cycling, etc. require more ATP which is generated from aerobic metabolism.
However, even during a short burst of exercise, all three systems contribute to ATP production. For example, during a 30-second sprint, 23 per cent of the ATP is generated from the phosphagen system, 49 per cent from glycolysis and the remaining 28 per cent from aerobic metabolism12 although, over time, the anaerobic system contributes significantly less energy to muscles. For exhaustive exercise lasting up to four minutes, the contribution from anaerobic metabolism reduces to 21 per cent with the remainder supplied through aerobic metabolism.11
Adaptations in response to exercise
With repeated bouts of aerobic exercise, the body’s endurance increases, thus allowing someone to complete the exercise, for instance, to run a certain distance in less time. One commonly used measure of endurance is VO2 max. As exercise intensity increases, a point is reached at which the heart can no longer increase the rate at which oxygen is delivered to exercising muscles (ie, the VO2 max). In essence, VO2 max is a measure of fitness and will increase, to some extent, in response to aerobic exercise training and is considered the best measure of CRF. An alternative method of assessing endurance is a time test trial (eg, asking someone to cycle or run as fast as possible over a predefined distance or for a set period of time).
The aerobic metabolic capacity of muscle cells depends on the number of mitochondria present and aerobic training increases the number and size of mitochondria within cells. Muscles also adapt to training by increasing their glycogen storage capacity,13 shift towards a greater use of fatty acids as fuel during prolonged exercise14 and improve insulin sensitivity.15
Although exercise protocols vary considerably, HIT generally involves vigorous but brief and intermittent bursts of exercise interspersed by periods of rest or low intensity exercise. HIT has been used to train athletes for many years and became widespread among European runners during the 1950s16 although, more recently, interest has grown in the role of HIT as a means of improving the health and fitness of the non-athletic population.
For HIT to be perceived as a credible alternative to aerobic training, it was necessary to demonstrate comparable improvements to those induced by endurance training. Several studies have addressed this specific point and have established either greater or comparable improvements in outcomes. For example, one study compared between four and six 30 second “all out” sprints on a stationary bike with four-and-a-half minutes rest between sprints, with 40 to 60 minutes cycling each day for five days per week for a total of six weeks. Measured outcomes included VO2 max, changes in fuel utilisation, increases in glycogen storage and levels of peroxisome-proliferator-activated receptor gamma-coactivator (PGC)-1-alpha, which is measure of mitochondrial biosynthesis. The results showed comparable improvements in each of these outcomes yet the time commitment for the sprint group was approximately one-and-a-half hours per week compared to four-and-a-half hours per week.17
In contrast, others have found greater improvements in VO2 max compared with endurance training.18
HIT and fat loss
Although it seems intuitive that a combination of diet and exercise leads to weight loss, results to date have been disappointing. A systematic review in 2009 of randomised trials of diet and exercise found that the mean weight loss after two years was 1.64kg19 and a recent review of exercise interventions in obese and overweight patients for 12 months concluded that “isolated aerobic exercise is not an effective weight loss therapy”.20
Since obesity is defined in terms of an excess of fat, studies have explored whether HIT leads to fat loss. The results from several trials reveal that HIT programs do cause fat loss although this effect is only observed in programmes continued for longer than six weeks. The amount of fat loss observed varies and has been reviewed in detail elsewhere.21
One of the greatest losses recorded occurred in a 15-week study with 45 young women. The group undertaking HIT experienced a mean fat loss of 2.5kg compared with a mean increase in fat mass of 0.44kg in the continuous exercise group. Although it was reported that there was a significant decrease in abdominal fat for the HIT group, the actual amount was only 0.15kg.22
The term “insulin sensitivity” is an assessment of the body’s response to the effects of insulin. The higher the insulin sensitivity, the lower the amount of insulin required to dispose (into peripheral and nervous tissue) of a given amount of glucose. In contrast, low insulin sensitivity or insulin resistance, which occurs in type 2 diabetes, effectively means that larger amounts of insulin are required to dispose of a specific glucose load. Skeletal muscle is the peripheral tissue responsible for absorption of as much as 90 per cent of the glucose from an oral or intravenous load.23 Exercise programmes, therefore, have a potentially important role in patients with type 2 diabetic patients and evidence from several studies demonstrate that HIT significantly improves insulin sensitivity in these patients.24
HIT in patients with CV disease
The use of HIT has also been explored in patients with cardio-metabolic disease and appears to lead to greater improvements in VO2 max than moderate intensity exercise.25 The authors suggest that the suitability of HIT should be assessed on an individual basis. For example, HIT is not appropriate for patients with unstable angina, recent myocardial infarction and those with high blood pressure.
Is it worth it?
There is a large body of evidence which demonstrates that HIT leads to improvements in aerobic fitness. Additionally, the benefits are at least equivalent and, in some cases, superior to those obtained through continuous aerobic training though with a substantially reduced time commitment. There is some evidence from epidemiological studies which suggests that vigorous intensity exercise confers a greater cardio-protective benefit than moderate activity when energy expenditure is held constant.26 It seems that, with HIT, the health benefits are achieved with a considerably reduced energy expenditure. Unfortunately, the wide range of HIT protocols used in studies means that it is not possible to make specific recommendations on a suitable programme. Currently, HIT studies have been undertaken in the laboratory and whether these same benefits would accrue in the real world remains to be seen. One such study has found that the improvements in VO2 max were less than expected which the authors assumed was due to reduced adherence to the program.27
Nonetheless, given that many people do not exercise because they lack the necessary time, HIT might prove to be a suitable alternative method of improving CRF which, ultimately, should lead to improvements in health and mortality.
• Improved physical fitness is associated with a reduced risk of all-cause mortality
• Lack of time is often cited as a reason for not undertaking exercise
• High-intensity interval training (HIT) improves fitness to the same extent as traditional aerobic exercise with a reduced time commitment
• HIT appears to be safe for patients with cardio metabolic disease
• Currently there is no standardised HIT programme
Citation: Electronicjuice DOI: 10.1211/PJ.2014.11135528
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