Interval training: good for health as well as performance

Interval training has traditionally been regarded as the preserve of elite athletes seeking enhanced performance. In recent years, however, evidence has begun to accumulate that intervals can benefit the performance of sportsmen and women at all levels of sport. But is interval training a safe activity for the less than elite, and what are the implications for health? Andrew Hamilton looks at the evidence.

Most people associate high-intensity exercise with elite athletes rather than the recreational club sportsman or the weekend warrior, and that’s perhaps not surprising. After all, the oft-repeated mantra of doctors and other health professionals is that gentle-moderate exercise is the kind recommended for health. Intense or very vigorous exercise, on the other hand, tends to be regarded much more as the preserve of the younger, highly competitive sportsman or woman.

It’s easy to see how this consensus has arisen; a number of studies have shown that 30 minutes of gentle-moderate exercise performed on most or all days of the week may result in a reduction in the risk of cardiovascular disease. Meanwhile, well documented cases of sudden cardiac death as a result of unusually vigorous exercise abound – witness the case of the footballer Fabrice Muamba in 2012. Had he not had immediate expert medical attention, he would have almost certainly died as a result of his heart attack. Hardly surprising, then, that many health professionals see intense exercise such as intervals as not only unnecessary, but even as something to be avoided.

Intense is best

However, there’s a problem with this approach – there’s little evidence to suggest it’s correct! Several large studies have found that the rates of death from coronary heart disease (CHD) and all cause mortality (rates of death from any condition) are lower among vigorously active individuals than among moderately active individuals. For example, in a nine-year study of 18,000 British civil servants, the CHD rate among office workers who took part in swimming, running and other vigorous activities was less than half the CHD rate of office workers who reported no vigorous exercise(1,2). This same pattern was observed in smokers, those with high blood pressure and those with a family history of CHD, suggesting that vigorous exercise exerts protection independent of other risk factors.

In a famous study known as the Harvard Alumni Health Study, the relationship between physical activity and CHD was assessed at different time points in men who enrolled at Harvard University between 1916 and 1950. Baseline and follow-up questionnaires sought information about walking, stair climbing and participation in sport and exercise. The early results showed that in the 17,000 men followed for over 20 years, there was an inverse relationship between physical activity and all-cause mortality(3). In other words, the less physical activity, the greater the rate of all-cause mortality. Importantly, however, participation in non-vigorous (ie gentle-moderate) exercise was not associated with an increased lifespan. The researchers subsequently concluded that ‘a half hour of vigorous activity expends as much energy as moderate activity carried out for twice or three times as long – and it can provide greater heart health benefits(4).’

Another important study known as the ‘Health Professionals’ Follow-up Study’ is noteworthy because of its large sample size and its rigorous methodology (5). The subjects consisted of 44,500 health professionals, from dentists to veterinarians, aged 40–75 years, who were followed from 1986 to 1998. Their levels of physical activity were assessed at baseline and then at two-year intervals using a questionnaire.

The results showed that the risk of CHD was reduced by 18% in those who walked 30 minutes per day but more encouragingly, CHD risk was reduced by 42% in men who ran for one hour per week. Moreover, men who consistently engaged in any form of vigorous exercise enjoyed a 30% reduction in CHD risk compared to men who maintained a lower intensity of exercise. Interestingly, men who increased their exercise intensity from low to vigorous enjoyed an extra 12% reduction in CHD risk. This study of health professionals reached a similar conclusion to that of Harvard alumni: while gentle-moderate intensity exercise such as brisk walking or slow jogging may reduce CHD risk, greater protection can be gained by engaging in more intense exercise.

Figure 1.



Figure 1 shows the relationship between physical fitness and cardiovascular mortality and between physical fitness and all-cause mortality (6). The ‘fitness quintiles’ numbered 1-5 rank fitness with fitness quintile #1 being the lowest fitness bracket, fitness quintile #5 the highest, with the intermediate quintiles representing the 20% fitness brackets in between. You can see that individuals with fitness levels in quintile 4-5 have by far the lowest risk of CHD and early death from any cause.

But now think about this: to enter fitness quintile #2 and enjoy any protection from disease, someone aged 40–49 years would need to have an aerobic capacity of 11 METs31. However, one MET is equivalent to the energy expended at rest, and brisk walking requires only 4 METs. You can hopefully see therefore that while gentle/ moderate intensity exercise will gain you some health benefits, much greater benefit can be had with higher levels of aerobic fitness, for which much more intense exercise is required. And that’s where interval training comes in because, as we’ve said previously, interval training is an extremely efficient and manageable way of increasing exercise intensity and building aerobic fitness!

Why does interval training benefit health?

There are a number of reasons why vigorous exercise is more effective in fighting conditions such as obesity, type-2 diabetes and coronary heart disease, all of which can lead to early death. We know that weight loss occurs when energy expenditure exceeds energy intake. Because vigorous exercise burns more calories per minute than moderate-intensity exercise, it offers an advantage in controlling weight. In a 2003 report, the International Association for the Study of Obesity concluded that 30 minutes of moderate activity per day was insufficient to prevent unhealthy weight gain(7). Instead, the expert panel concluded that 60–90 minutes of moderate activity per day or ‘lesser amounts of vigorous activity’ was needed to prevent weight regain in formerly obese individuals.

Fat burning, insulin and type-2 diabetes

We also know that aerobic exercise increases the ability of the hormone insulin to work in the body, which is needed to lower blood sugar and prevent type-2 diabetes. However, recent research has shown that the ‘insulin-sensitising’ effect of exercise is far greater during vigorous exercise than during moderate exercise (8). In fact, it’s likely that any exercise actually needs to be pretty intense in order to deplete muscle glycogen (stored muscle carbohydrate) and therefore increase insulin sensitivity (9). This is probably because when muscle stores of glycogen are depleted during intense exercise, signalling molecules are released, increasing the muscle cells’ sensitivity to insulin and thus helping to restore muscle glycogen more rapidly when carbohydrate is eaten.

A good example of these benefits in practice can be seen in a study by researchers at McMasters University in Canada published in 2011(10). Gibala and his team investigated a low-volume HIT protocol on skeletal muscle oxidative capacity and insulin sensitivity in middle-aged adults, who are often at a higher risk for inactivity-related disorders. Seven sedentary but otherwise healthy individuals performed six training sessions during a two-week period. Each session involved 10 × 1 minutes of cycling at around 60% of peak power equating to an intensity that produced 80%-95% of heart rate reserve (ie relatively intense but not flat out), with 1 minute of recovery between intervals. The results showed that the ability of the subjects’ muscles to burn fat for energy increased by around 35% after training. This was perhaps unsurprising as levels of the key signalling molecule called PGC-1α (see section 2) increased by 56%. What was even more impressive was that the levels of ‘glucose transporter protein’ (a protein that sits in muscle cell walls and allows muscle cells to soak up glucose from the bloodstream) increased by 260%. This helped increased overall insulin sensitivity in the subjects by 35%!

These results support findings from earlier studies. For example, a 2008 study on young women compared the metabolic benefits of high intensity intervals performed three times per week for 15 weeks to the same frequency of steady state exercise (11). What the researchers found was that, compared to the steady-state exercise, the intense intervals resulted in significantly larger reductions in total body fat, subcutaneous leg and trunk fat, and greater gains in insulin sensitivity. The researchers concluded therefore that high intensity intervals may help to prevent type-2 diabetes.

The most recent research

Over the last year or two, a solid body of evidence has accumulated that interval training (particularly high-intensity intervals) brings about performance benefits to not just elite athletes, but purely recreational sportsmen and women too, even those with little training history. This in turn has spurred researchers on to investigate more thoroughly the health impacts of this kind of training, and the results are encouraging to say the least.

For example, a large study published 11 months ago looked at the effect of an interval training programme on the management of hypertension (high blood pressure) in 245 male subjects (12). C-reactive protein is a protein found in the blood that is associated with inflammation. High C-reactive protein levels are associated with future development of hypertension and cardiovascular problems, which suggests that hypertension is in part an inflammatory disorder.

The subjects (who had mild to moderate hypertension - systolic BP between 140-179 mmHg and diastolic between 90-109 mmHg) were age matched and randomly grouped to interval training (140 men) or control groups (105 men). The interval groups performed eight weeks of interval training with sessions lasting between 45-60 minutes and at intensities of 60-79% of HR reserve. The control group, meanwhile, remained sedentary during this period. The results showed that in those men performing intervals, there was a significant drop in C-reactive protein and that this was correlated with a drop in both systolic and diastolic blood pressure reading. The researchers concluded: “Interval training is effective in the non-pharmacological management of hypertension and may prevent cardiovascular event through the down-regulation of C-reactive protein hypertension.”

Just six months ago, a British study examined the effects of interval training in men at risk for insulin resistance (a precursor to type-2 diabetes (13). What was interesting about this study was that it compared three months of steady state aerobic training to interval training in a work matched manner; regardless of which group the subjects trained in, the total workload was the same – 12Kcals per kilo of bodyweight per week. In particular, the researchers wanted to see how each mode of training affected oral glucose tolerance and HOMA-IR (a measure of insulin resistance based on glucose response and insulin levels) 24 and 72 hours after each participant’s last exercise session.

Both training modes improved oral glucose tolerance but only the interval training significantly improved HOMA-IR scores. Moreover, the interval training seemed to be particularly beneficial in reducing the scores for ‘metabolic syndrome’ (the medical term for a combination of diabetes, high blood pressure and obesity – a combination that increases the risk of heart disease, stroke and other conditions affecting blood vessels), especially in those participants who entered the study with poor HOMA-IR scores (see figure 2).

Meanwhile, a study published in 2013 investigated the changes in oxidative stress biomarkers and antioxidant status indices caused by a three-week high-intensity interval training (HIT) regimen (14). Eight physically active males performed three HIT sessions/week over three weeks. Each session included four to six 30-second bouts of high-intensity cycling separated by four minutes of recovery.

As a result of the three weeks of HIT, the levels of oxidative stress markers produced by an exercise session dropped significantly (protein carbonyls by 13.3% and thiobarbituric acid reactive substances by 7.2%). And this was despite the fact that total work done was 10.9% greater in the post-HIT training test compared with the pretraining exercise test. The HIT also induced a marked elevation of antioxidant status indices – ie had fortified the subjects’ own antioxidant defence systems. Even more remarkable was that these positive changes occurred after only nine training sessions totalling 22 minutes of high-intensity exercise, further supporting its positive effect not only on physical conditioning, but also on health promotion!

Despite the same number of total calories expended during both modes of training, interval training resulted in a greater total mass reduction and greater drop in % body fat than did steadystate aerobic training. These findings suggest that interval training was therefore more effective at reducing the risk of metabolic syndrome.

Intervals are for everyone

However, it’s not just unfit and/or overweight people that have been shown to gain health benefits by a period of interval training; studies have also been carried out on individuals with coronary artery disease and patients who have suffered a heart attack, patients with severe chronic obstructive pulmonary disease, individuals with metabolic syndrome, spinal-cord-injured individuals and those suffering with intermittent claudication (pain in the lower limb associated with atherosclerosis of the lower extremity arteries)(15).

In a comment article, McMasters University experts in the field of interval training for health concluded the following: “Although interval-type exercise training has been used by athletes for many years, the perception that it is unsafe and or unfeasible for less ‘fit’ populations is now being challenged by emerging evidence. Indeed, the body of accumulated evidence demonstrates that high-intensity interval training is a safe and valuable tool which can be used to combat many inactivity related disorders in a wide range of populations, and is no longer simply a training tool for the elite athlete.”

Compliance

A final word on the health benefits of intervals. Previous studies have consistently demonstrated the beneficial physiological adaptations associated with exercise. However, for training to produce a substantial and significant impact on health, functional capacity and quality of life, any exercise programme has to be both adopted and then consistently adhered to. This is perhaps one of the biggest reasons why interval training is so effective for health.

In a 2009 study, a research team lead by Tjønna found that overweight adolescents, who performed three months of twice-weekly highintensity exercise sessions, experienced reductions in several known cardiovascular risk factors to a much greater degree than those who received a more general multi-treatment strategy (diet, lifestyle counselling etc)(16). In fact, the superior effect of interval training was observed not just at the end of the three-month study period but up to eight months after the formal exercise training had been completed! Moreover, the authors also pointed out that ‘informal’ comments from the adolescents in the interval training group indicated that they were enthusiastic about the variation provided by the interval programme and were so encouraged by their own perceptions of increased fitness that they were motivated to continue interval exercise on their own after the 13-week experimental period!

One of the most appealing aspects of intervaltype exercise is that it mimics the activity patterns that
people typically experience in their activities of daily living – for example, climbing a flight of stairs. It may be, therefore, that this type of training translates into increased confidence and enthusiasm for similar challenges in daily life, thereby permitting greater gains in fitness and further improvements in health.

Summary

If you’re new to intervals and are pondering the health implications of this mode of training, the evidence is overwhelmingly positive. Research shows that incorporating relatively intense exercise into a training programme can indeed provide valuable health benefits over and above steady-state training. Common sense is still needed, of course; you still need to introduce intervals into your programme gradually – for example, by starting with a few moderate intensity intervals and then slowly increasing the intensity and number. That said, however, interval training is an efficient and safe way of incorporating relatively intense bouts of exercise into a training programme, and it is therefore recommended for anybody seeking to maximise the health benefits of exercise.

References
1. Lancet. 1980;2:1207-10.
2. Lancet. 1973;1:333-9.
3. JAMA. 1995;273:1179-84
4. Prev Med. 2001;29:37-52
5. JAMA. 2002;288:1994-2000
6. JAMA. 1989;262:2395-2401
7. Obes Rev. 2003;4:101-14
8. Diabetes Care. 1996;19:341-9.
9. N Engl J Med. 1996;335:1357-62
10. Med Sci Sports Exerc. 2011; Oct;43(10):1849-56
11. International Journal of Obesity 2008; 32 (4): 684–91
12. Afr J Med Med Sci. 2012 Dec;41(4):379-86.
13. Int J Sports Med. 2013 Apr;34(4):355-63
14. Food Chem Toxicol. 2013 Jun 6. pii: S0278- 6915(13)00352-9
15. Clinical Science 2009; 116, 315–316
16. Clin. Sci 2009; 116, 317–326