Tuesday, 19 January 2016
A place for high-intensity interval training in disease management
Since writing my previous piece on HIT in which I wrote about the benefits to physical fitness(http://www.ll.dlpa.bru.nihr.ac.uk/brublogs-5072.html), I have focussed my attention on metabolic responses and the effects it might have for those with or at risk of type 2 diabetes. As a side, I've also been interested in its practical application; however great HIT might turn out to be in the lab, will people do it on a day to day basis?
What I have found over the past 18 months of research is that, ultimately, HIT works. It's good for fitness and it's beneficial for glucose regulation. It's not necessarily better than traditional continuous exercise, but it's certainly better than nothing. I also believe (and I have no money to make from saying this) that, if sold properly (unlike in this article I was asked to contribute towards - I did not say it had to hurt; https://www.sciencenews.org/blog/scicurious/high-intensity-interval-training-has-great-gains-%E2%80%94-and-pain), HIT, or to be more accurate "interval training", is the best way to get a non-exerciser to exercise.
What is HIT?
First I should explain what HIT is. The academic definition puts it simply; "short bursts of vigorous intensity exercise interspersed with periods of recovery or absolute rest". If you try to explain this in more detail however, you realise how nuanced this form of exercise training is and the range of work it encompasses.
Firstly, "vigorous intensity" is categorised as any physical activity that elicits a heart rate of 77% or greater of an individual's maximum. This sounds high, but in fact can range from work that feels relatively easy to all-out maximal efforts. How "hard" the intensity feels depends on factors such as age and fitness. Things are complicated further when you consider that even when exercising at their maximum HR, individuals can keep on working at higher speeds, gradients or watts. In the HIT research, effort intensities range from the lower end of the vigorous spectrum to all-out maximal efforts.
Next you have to work out how long to sustain the effort for. Intuitively, the higher the intensity, the shorter the effort. In the literature, intervals range from 6 seconds to 4 minutes.
The duration and intensity of the recovery periods also need to be determined. The extent to which the exerciser recovers between intervals affects the type of adaptation that the activity will stimulate.
Finally, the number of intervals completed can be decided upon. This is generally dependent on the desired energy expenditure or overall time commitment the work out requires. Deciding upon a prescription for a certain audience requires consideration of a number of factors including ability and desired outcome.
HIT and type 2 diabetes
Ideally, evaluating the effect of HIT on markers related to type 2 diabetes would pool results from studies employing the same HIT protocol investigating the same outcomes. However, HIT has only recently been applied in a health setting and there is not enough information available to come to a conclusion regarding the specific effects of specific protocols. I therefore did my best to combine the results from the available studies, which used different HIT protocols, and run some analyses that would help to tease out which might be the most influential components.
While I found that overall, the various HIT programmes that had been tested improved insulin sensitivity1, reduced HbA1c2 and, consistent with other reviews, physical fitness, I was unable to determine what it was about HIT specifically that stimulates these improvements . There is clearly something about this mode of exercise that enhances the body's adaptations to physical activity because equivalent benefits to moderate-intensity activity are observed following a substantially lower volume of exercise. The mean "active" duration (i.e. not including rest periods) of the HIT work-outs I analysed was 21 minutes compared to 44 minutes of moderate-intensity exercise and yet greater or equal benefits were produced.
Taking the HIT programmes alone, I looked at whether the magnitude of the improvements observed were related to the interval intensity, total exercise volume or duration of training programme. In other words, do you get better effects with higher intensities, longer exercise sessions or more weeks of training? I found that none of these factors predicted outcomes. There are a number of possible reasons for this. First of all it is quite likely that the variation in the studies I included diluted the effects each factor was having; its not to say that there is no relationship, but I was unable to see one. Secondly, it could be something else; the change from high to low intensity for example, which predicts how well HIT works.
The article can be accessed here: http://www.ncbi.nlm.nih.gov/pubmed/26481101
What this means is that currently we know that HIT can work, but we don't know why it works.
Practically speaking though, that's what is important. Other research groups are investigating the mechanism of action of HIT and this will help to optimise HIT protocols for different populations. For now though, we can tentatively recommend HIT in the prevention and management of type 2 diabetes.
Working alongside healthcare professionals for the past 2 years has led me to be sceptical about the uptake of HIT by a clinical population. Indeed, participation in any physical activity by the population as a whole is remarkably low and now I want those who are suffering most from the consequences of inactivity to exercise hard?!
The media attention HIT has received, I feel, has appealed largely to Lycra-clad gym-goers; a minority population. It has focussed on all-out, maximal effort HIT the intervals of which cause the participant to grunt and groan and leave them gasping for breath. Yes, HIT needs to feel hard, but as alluded to earlier, 'hard' can start at 'not comfortable'. Then of course you've got the rest periods; you get to stop! Perhaps placing more emphasis on the recovery periods will attract a wider range of people to HIT.
Admittedly, to date, my patient contact has been limited to a few willing study participants (not entirely typical of the general population), but what I have consistently observed is that once someone has tried HIT, they realise that it is not as unpleasant as they thought and, in many cases, easier overall than continuous exercise of a lower intensity. In my opinion, the biggest challenge with HIT is selling it right. One of my ideas is to drop the 'H' and call it 'interval training'. This way you remove the focus from the high-intensity part and emphasise the change in intensity between intervals.
As part of my follow-up project to the meta-analysis, I will be asking participants to rate how hard they find HIT compared to continuous exercise. By the end I should have some idea as to how people rate each type of exercise, as well as how effective they are in controlling blood glucose.
In conclusion, there are still many unanswered questions with HIT, but what we know is that as long as heart and breathing rates have risen, and/or 'hard' effort has been exerted during the workout, HIT is going to have some kind of beneficial effect. The difficulty is breaking the barriers associated with HIT and convincing people it is safe and achievable.
1 Insulin sensitivity reflects the bodies ability to utilise the insulin it produces. It is a term used widely in research as it is the physiological state which precedes type 2 diabetes. Insulin resistance has not been employed as a clinical term due to the difficulty of its measurement and lack of normal range values.
2 HbA1c is glycated haemoglobin, which is a measure of an individuals' average blood glucose over a period of weeks.