Monday June 24 2013
Exercise lowers the risk of developing metabolic syndrome
"Hurrah! One workout a week is enough," the Mail Online reports. The bad news? The session has to last two and a half hours.
There is a general consensus that to stay fit and reduce the risk of chronic diseases, adults should perform at least 150 minutes (two and a half hours) of moderate to vigorous physical activity (MVPA) a week. But does it have to be 30 minutes for five days or, for those with busy lifestyles, can you get away with 150 minutes over the weekend?
This was the question posed by a recent Canadian study, which used data from more than 2,000 Canadian adults to look at how frequency of physical activity related to risk of metabolic syndrome.
Unsurprisingly, the study found that people who followed the recommended MVPA were less likely to have metabolic syndrome than those who didn't. Crucially, how this was split over the week did not seem to make a difference to the risk of metabolic syndrome.
However, this study has not proved a cause and effect relationship and is limited by its design. For example, activity was only measured over one week and therefore may not be a true reflection of people's typical activity levels.
A final important note is that a single 150-minute bout of MVPA may not be appropriate for everybody. Check with your GP before adopting this type of exercise regime.
Where did the story come from?
The study was carried out by two researchers from the School of Kinesiology and Health Studies at Queen's University in Kingston, Canada, and was funded by the Heart and Stroke Foundation of Ontario.
It was published in the peer-reviewed journal Applied Physiology, Nutrition and Metabolism.
While the reporting of the study was reasonably accurate, no mention was made of its limitations. These include the fact that the study design cannot prove a cause and effect relationship, and that only a week's worth of data was looked at.
Perhaps more importantly, there was no mention that suddenly engaging in 150 minutes of vigorous exercise once a week may not be appropriate or safe for everyone.
What kind of research was this?
This was a cross-sectional study looking at whether frequency of exercise throughout the week in physically active people has an impact on markers of health – in this case, metabolic syndrome.
Metabolic syndrome is a collection of risk factors – including obesity, high blood pressure, high cholesterol and diabetes – that increase a person's risk of cardiovascular disease, such as heart disease and stroke.
The researchers report that current World Health Organization physical activity guidelines recommend that adults accumulate at least 150 minutes of MVPA a week, but there is no stipulation on how many days a week the person has to be active.
Other guidelines, including from the UK Department of Health, recommend that a person perform at least 30 minutes of MVPA on most or all days of the week.
The study included data from physically active men and women who participated in the Canadian national survey. The researchers looked at clinical measures of metabolic syndrome and asked participants to wear an accelerometer to look at their physical activity over the course of a week. An accelerometer is a device that measures acceleration, which can provide a reasonably accurate assessment of total energy expenditure.
The main limitations of such a study design is that it is cross-sectional and so does not prove cause and effect. It is also possible that a person's normal activity levels may not be reliably represented when they are only asked to wear an accelerometer for one week. For example, being asked to wear an accelerometer may consciously or unconsciously cause a person to be more active than they normally would.
What did the research involve?
The study looked at physical activity in adults and assessed whether the frequency of any MVPA they did was related to the likelihood that they had metabolic syndrome, rather than just the total amount of exercise they did.
It included data from two rounds of the Canadian Health Measures Survey (CHMS), which collected data from Canadians aged 6-79 years on the first round, and from Canadians aged 3-79 years in the second round three years later. The majority of the Canadian population is said to be captured by this survey.
Information on sociodemographic data and general health was collected through interviews, and participants then attended a mobile clinic for physical measurements and blood tests.
The current study included 2,324 adults (aged 18-64 years) who had complete data available on their physical activity and on risk factors for metabolic syndrome. This is said to be a response rate of just over one half (53.5%) of those asked to participate in the surveys.
All participants who could walk who also attended the mobile clinic were provided with an accelerometer so that acceleration could be measured. They were asked to wear this on a belt around their waist for one week and then return it to the researchers.
The accelerometer recordings were used to measure the intensity of a person's activity throughout the day. A bout of MVPA was defined as a period of at least 10 consecutive minutes above the moderate-intensity level of activity.
Energy expenditure was calculated using a type of standard unit of activity called metabolic equivalent tasks (METs). Total weekly MET minutes was calculated by adding the MET energy expenditure values for all bouts of MVPA (MVPA accumulated in periods of activity lasting for 10 minutes or more) and sporadic MVPA (MVPA accumulated in periods of nine minutes or less). Current guidelines were reported to recommend 500 MET minutes per week of total MVPA.
Overall, participants were grouped into three activity categories:
- inactive – less than 250 MET minutes per week of total MVPA, less than half of current guideline recommendations
- somewhat active – 250-499 MET minutes per week of total MVPA, over half but not meeting current guideline recommendations
- active – ≥500 MET minutes per week of total MVPA, meeting current guideline recommendations
Metabolic syndrome was considered to be present if the person had three of more of the following:
- high blood pressure (≥130/85mmHg) or taking blood pressure medication
- high triglycerides (≥1.7 mmol/l) or taking cholesterol-lowering medication
- low high density ("good") cholesterol (<1.0 mmol/l for men, <1.3 mmol/l for women) or taking cholesterol-lowering medication
- high fasting blood glucose (≥5.6 mmol/l),
- self-reported diabetes or taking anti-diabetic medication
- high waist circumference (≥102 cm for men, ≥88 cm for women)
The researchers then looked at the association between activity levels and metabolic syndrome, taking into account age, gender and education. Somewhat oddly, they did not take smoking into account, which could also influence results.
What were the basic results?
The average age of participants in this study was 41 years, and just under half (48%) were men. Just under a fifth of the sample (16%) met criteria for metabolic syndrome.
When only bouts of MPVA of 10 minutes or more were assessed, around a quarter of the sample (24%) were defined as "active", meeting current recommendations of performing 500 or more MET minutes per week in bouts of activity. However, when the total MET was summed – including that achieved in bouts and in sporadic MPVA activity – half of participants (51%) were defined as "active".
When the researchers calculated the odds of metabolic syndrome according to total MVPA (performed in bouts or sporadically) or performed in recommended bouts only, they found that:
- looking at total MVPA, compared with people who met the definition for being "active", people who met the definition for only being "somewhat active" were more than three times as likely to have metabolic syndrome (odds ratio [OR] 3.34, 95% confidence interval [CI] 1.91 to 5.87)
- people who met the definition for being "inactive" were more than four times likely to have metabolic syndrome (OR 4.43, 95% CI 2.26 to 8.69)
- counting only MVPA taken in bouts of 10 minutes or more, people who were "somewhat active" had no difference in risk of metabolic syndrome than those who were active, while people who were "inactive" were at three times the risk (OR 3.10, 95% CI 1.56 to 6.15)
All additional analyses were based on total MVPA.
The result the media has latched onto came from participants who met enough total MVPA time to meet recommendations of 500 MET minutes per week or more. In people in this group, there was no difference in risk of metabolic syndrome between people who performed MVPA frequently (five or more days a week) or infrequently (one to four days a week).
How did the researchers interpret the results?
The researchers say that while the total weekly amount of MVPA was strongly associated with metabolic syndrome, the frequency of this activity was not.
This study used a large quantity of data from more than 2,000 participants of the Canadian Health Measures Survey to try to gauge how often adults should perform 150 minutes of exercise a week, as recommended by most guidelines.
The main finding was that those who met this total physical activity requirement were less likely to have metabolic syndrome than those who were less active, which is not particularly surprising. Lack of exercise is associated with many of the risk factors for metabolic syndrome, such as obesity.
Importantly, however, the frequency of time that MVPA was conducted over – whether this was made up of bouts of activity of 10 minutes or longer, or sporadic episodes of less than this – did not seem to make a difference to the risk of metabolic syndrome.
However, this study has three very important limitations:
- This was a cross-sectional survey. As such, it cannot prove cause and effect: it cannot prove that the person's activity level has caused them to have metabolic syndrome, and it could equally be that the person's metabolic syndrome (such as being overweight or having high blood pressure) causes them to be less active. However, it is already known that being physically inactive does contribute to the risk of metabolic syndrome, along with other factors such as diet.
- The fact that the participants were asked to wear the accelerometer for just one week means that the activity they did during this week may not be representative of what they would normally do. Being aware of the fact that they were wearing the accelerometer may have encouraged participants to engage in more activity than they would normally have done. This could influence the results.
- Although metabolic syndrome is an important measure of health, it is not the only measure of health. Greater physical activity is believed to have benefits across many areas of wellbeing, physical and mental health. This study cannot tell us about the association between frequency of exercise and these other measures.
Current UK recommendations suggest that adults should aim to be active daily. Weekly activity adding up to at least 150 minutes (two and a half hours) of moderate-intensity activity in bouts of 10 minutes or more, such as 30 minutes on at least five days a week, is recommended. An alternative is 75 minutes of vigorous activity spread across the week.
A case could be made for some people adopting the more practical "weekend warrior" model, as many people find it difficult to spare the time for exercise during the working week.
But there are other ways you can build an exercise routine into your week, such as walking to work rather than using public transport. Read more about how you can get active your way.
A single extended moderate to vigorous exercise session may not be appropriate for some people, depending on their current fitness levels and health. Check with your GP.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.