Wednesday July 3 2013
Many of us now have desk jobs
"Getting up from your desk for two minutes every half an hour could slash the risk of diabetes" is the advice in the Daily Mail.
The Mail reports on a small study looking at people who were asked to either:
- sit still for nine hours
- walk for 30 minutes before sitting still for nine hours
- interrupt the nine hour sitting period with short bursts of regular activity (walking for 1 minute 40 seconds) every 30 minutes
As you would expect, the regular activity group had healthier blood sugar levels than the ‘sitting still’ group. Interestingly however, they also had healthier blood sugar levels than the longer exercise group.
The findings, while intriguing, are far from conclusive. The study was small and short term so it would be a mistake to draw firm conclusions from it. Also, it only recruited people of healthy weight – if the group of 70 had included people who were overweight or obese, the results may have been different.
While blood sugar control is a marker for diabetes risk, diabetes was not tested directly in the study.
Despite these limitations, the results reinforce the established fact that regular exercise is beneficial for health and wellbeing and that prolonged inactivity is unhealthy.
But whether exercising little and often may be just as good, or better than more intense blocks of exercise, is still unproven.
Where did the story come from?
The study was carried out by researchers from University of Otago in New Zealand. The funding source was not reported but the authors declared no personal financial conflict of interest.
The study was published in the American Journal of Clinical Nutrition, a peer-reviewed medical journal.
The media reporting of the story was broadly accurate although it failed to highlight any of the limitations of the study. Many of the headlines incorrectly used the term “two minute walks”, but this can be forgiven, as it is more straightforward than “one minute and forty second walks”.
What kind of research was this?
This was a randomised crossover trial looking at the effects of regularly interrupting long spells of sitting down on blood sugar (glucose) regulation.
The researchers state that quick and effective regulation of blood glucose is important because past research has associated this with lower risk of developing diabetes.
The researchers highlight that being inactive, for example sitting for long periods of time at a desk, is a known risk factor for diabetes and cardiovascular diseases. Based on this, the researchers wanted to find out if interrupting prolonged sitting periods with exercise might reduce the risks, and what type and duration of exercise might be best.
A randomised control trial is one of the best ways of assessing whether a specific intervention (getting up from your desk and going for a short walk) is effective at reducing the risk of a certain disease (for example diabetes), or risk factors for a disease (such as insulin and glucose regulation).
A crossover design means that all participants in the trial receive all of the different interventions, but are allocated to them in a random order. This design tends to be used when smaller numbers of people are recruited to a trial to increase the apparent sample size and power of the study.
What did the research involve?
The researchers compared the effects of various activity levels on measures of glucose and insulin regulation after eating.
Seventy adults participated in the study. There were three intervention groups:
- “prolonged sitting intervention” involved sitting for nine hours (control)
- “physical activity intervention” involved walking for 30 min and then sitting for nine hours
- “regular-activity-break intervention” involved interrupting the nine hour sitting period with walking for 1 minute 40 seconds every 30 minutes
Participants consumed a “meal-replacement beverage” (presumably some type of soup, smoothie or fortified shake) at 1 hour, 2 hours, and 7 hours into the 9 hour sitting period, after which they had blood tests to monitor how effectively they metabolised their food. Sixteen blood tests were performed for each participant to monitor the effect over time and how it differed based on their intervention.
The participants were randomly assigned to complete all three interventions, but in one of six different orders. For example, some did the nine hours sit first, then the physical activity before the sit intervention, and then completed the regular breaks intervention. Others had the reverse order and others variants in between.
All participants were asked not to exercise for three days before the interventions began. There was also a six day washout period between each consecutive intervention to try to limit carry over effects from other activities.
Eligible participants were:
- 18–40 years of age
- had a predominantly sedentary occupation
- did not participate regularly in more than 2.5 hours of physical activity per week
- did not have a history of diabetes, cardiovascular disease, or other medical conditions that prevented them from participating in physical activity or that affected fat or carbohydrate metabolism
Patients were excluded if they had abnormal blood fats, glucose or cholesterol levels or high blood pressure.
The analysis used a statistical method called incremental area under the curve (iAUC) to detect differences in insulin, glucose, and fat levels in the blood depending on the activity intervention. iAUC involves plotting how the levels varied over time and allows them to be compared statistically to detect differences and is a widely used method in diabetes research.
What were the basic results?
The key results were that the regular-activity-break intervention lowered both glucose and insulin levels compared to both the prolonged sitting intervention the physical activity intervention.
Specifically, the regular-activity-break intervention lowered insulin levels by an average of 866.7IU per litre per nine hours when compared with prolonged sitting intervention and by 542.0IU per litre per nine hours when compared with physical activity intervention.
For glucose, regular-activity-break intervention lowered levels by 18.9mmol per litre per nine hours when compared with prolonged sitting intervention and by 17.4mmol per litre per nine hours when compared with physical activity intervention.
All were statistically significant differences.
How did the researchers interpret the results?
These results led the researchers to conclude that “regular activity breaks were more effective than continuous physical activity at decreasing postprandial [after eating] glycemia and insulinemia [blood glucose and insulin levels] in healthy, normal-weight adults”.
The results of this study suggest that regularly taking a break from prolonged sitting with a short (1 minute 40 second) bout of activity may be more effective than a single continuous (30 minute) bout of physical activity at lowering blood glucose and insulin concentrations after eating in healthy, normal-weight adults.
The following limitations should be borne in mind when considering the reliability of the evidence from this study.
The study was relatively small, recruiting just 70 participants. This group of 70 may not represent the wider population of the UK and had it been a different set of 70 people, the results may have been slightly different. This highlights the problems with reading too much into small studies and why many researchers call for larger studies to be carried out before any solid conclusions can be made.
The study measured glucose and insulin levels over a nine hour period. It was not designed to assess whether the exercise interventions led to any long term benefits in health or well-being, particularly related to risk of diabetes. This would require a different study design and longer study duration to investigate.
The results were obtained in normal-weight individuals. The effect of short bursts of exercise on those who are overweight or obese, or who already have diabetes may well be different and were not directly assessed here.
Finally, it is currently unclear if there is an optimal pattern of regular activity that best regulates blood glucose levels. The researchers chose a “30 minutes rest – 100 seconds walking” pattern of exercise, but this was an entirely arbitrary choice. Alternative patterns such as “15 minutes rest – 50 seconds walking” may be more effective. Similarly, the intensity of the exercise is likely to be as important as the duration to optimise health.
The findings of the study are consistent with the idea that regular exercise is beneficial for health and wellbeing and that prolonged inactivity is not healthy. However, this study provided little extra clarification of the precise intensity and frequency required to maximise health benefits. It did highlight the possibility that exercising little and often may be just as good, or better than more intense blocks of exercise. But this possibility, attractive as it may be to desk-jockies, is far from conclusive.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.