Positive Health Online

After a whole day in front of a PC, many of us rush to the gym or go for a run, believing that a bout of vigorous exercise will compensate for a sedentary lifestyle. Up till now it was indeed that simple: if you exercised for at least 60 minutes a day for five days a week, you were considered physically active. But recent research suggests that what you do the rest of the day matters as well.

In 2010 Dr Patel and colleagues noticed that people who sit down for longer have a higher mortality rate, irrespective of the amount of exercise they do. By 'sit down' they meant any activity during which you use less energy than 1.5 metabolic equivalents, such as watching TV, reading or driving. Usually those very sedentary activities are indeed done while sitting down. In this study, many people who used to sit down for a long time each day are physically active but perform less often very light exercise, like standing or slow walking. Sitting down therefore seems to replace very easy work, but not the moderate to vigorous activities.

Those of us who sit the most are about 50% more likely to die from cardiovascular diseases than those who sit the least, independently of the classical risk factors such as obesity, smoking, high blood pressure, cholesterol, diet and vigorous exercise. This means that all these factors being equal, the person who sits the most is at the highest risk. Even if we are working out vigorously for an hour or so a day, we have about 14 to 16 waking hours that we are not exercising, and Dr Patel concluded that staying immobile during that time was a risk factor on its own.

His data confirmed studies from Canada, Australia and Japan.

This is worrying, as modern life involves a large amount of sitting such as driving, studying, attending meetings or working at a computer.

Why?
The researchers used sitting time as a measure of inactivity. It might be that in general it is a better measure of the amount of energy spent by exercising, but this does not fully explain the data since even in the most strenuous training group, more sitting time was associated with a higher mortality rate.

In 2007 Dr Naomi Hamburg studied the effect of bed rest on insulin resistance, cholesterol and triglycerides in 20 healthy subjects, and showed that physical inactivity leads rapidly to an increase of these biomarkers. Higher levels have since long been associated with an increased risk of cardiovascular and metabolic diseases. Animal studies obtained similar results.

Sitting time affects our health by reducing the activity of lipoprotein lipase (LPL). LPL is an enzyme that allows our muscles and adipose tissues to take up free fatty acids from the bloodstream. Hamilton and colleagues showed that decreased levels of LPL are associated with increased triglycerides, lowered HDL cholesterol ('good cholesterol') and a higher risk of cardiovascular disease. They also demonstrated that the reduction in LPL due to inactivity is found in oxidative muscle fibres, while the increase due to exercise happens in glycolytic fibres.

Moreover, the decrease in LPL in response to inactivity is at least four times more important than its increase due to exercise. The mechanism linking LPL to exercise is thus different from the one linking it to inactivity, and Hamilton concluded that sitting less is different from exercising more.
Several researchers have shown that the carbohydrate metabolism is also affected by inactivity, due to a reduced uptake of glucose.

Healthy bones require a balance between bone resorption and deposition. Studies have shown that inactivity increases resorption without affecting bone formation, leading to osteoporosis. Vigorous exercise can prevent some of the losses, but in 2007 Sara Zwart and colleagues showed that this is not always enough to avoid osteoporosis. Less sitting time might also be necessary.

Evolution Versus Modern Life
In 2004, Dennis Bramble and Daniel Lieberman showed from anatomical studies that humans have evolved to become good endurance runners. Early humans were hunter-gatherers, and long distance running was the only way to get close enough to prey to throw projectiles. They might even have exhausted mammals by pursuing them relentlessly, because humans are much better at running long distances in the heat than most animals. This was vigorous exercise. After the hunt, they had to perform light physical activities such as picking fruit, making tools, maintaining a fire...Our bodies have thus evolved to perform vigorous as well as light exercise, and we cannot swap one for the other without risking health problems.

What Can We Do?
Dr Genevieve Healy has published evidence that short breaks can help to lower the health risk. She demonstrated that a higher total number of breaks was associated with a smaller waist size, a lower BMI and lower blood levels of glucose and triglycerides, irrespective of the total sitting time. The average duration of the breaks was less than five minutes, and the intensity was very light. Short breaks will also increase the number of calories you are burning, and even though it is each time only a minimal amount, it adds up and can make a difference in the long run.

Dr Healey concluded that we should take as many breaks as possible, for example by standing up while answering the phone or while the PC is downloading stuff. But the problem is that this can slow our work down.

Sophisticated Help
Dr James Levine from the Mayo Clinic designed the treadmill desk, a desk mounted on a classical treadmill, which allows us to walk slowly while working. But treadmill desks are expensive, noisy and take up too much space to be practical.  It is easier and much cheaper to have a pedal exerciser, which is a stationary bike without wheels that can be placed under a desk. A study conducted by Dr Lucas Carr and colleagues showed that this was feasible, and participants used it on average 12 out of 20 days.

Dr McAlpine investigated the use of a small stepper placed under the desk. His study showed that this device can reduce inactivity time and increase energy expenditure at a low cost. Pedal exercisers and small steppers are available in specialised shops or online.

What We do not Know Yet
More than 60 years of scientific research has shown a link between physical activity and health. It is only now that there is some evidence for an independent effect of sitting time, and there are still several important questions. We do not know yet if there is a safe amount of sitting, how many breaks of how long we have to take, or how much walking, pedalling or stepping we should do. But the evidence is so overwhelming that we do not have time to wait for further studies, and we have to act now. It is up to each of us to find a method that works, and just do it.
Remember that we are talking here about light exercise by whole-body movements, which does not replace moderate to vigorous physical activity. We still have to follow the advice of the American College of Sports Medicine and exercise moderately or vigorously for at least 30 minutes five times a week, and do some muscle-strengthening workouts as well.

References
1. L Carr, KA Walaska, BH Marcus. Feasibility of a portable pedal exercise machine for reducing sedentary time in the workplace.  Br J Sports Med: published online first 14 Feb 2011.
2. DM Bramble, DE Lieberman. Endurance running and the evolution of Homo. Nature 432 (7015):345-52. Nov 18 2004.
3. DW Dunstan, ELM Barr, GN Healy et al, Television viewing time and mortality. Circulation 121: 384-391. 2010.
4. HT Hamilton, DG Hamilton, TW Zderic.  Role of low energy expenditure and sitting in obesity, metabolic syndrome, type 2 diabetes and cardiovascular disease. Diabetes 56 (11): 2566-67. 2007.
5. Genevieve N Healy, David W. Dunstan et al. Breaks in sedentary time: Beneficial associations with metabolic risk.  Diabetes Care 31 (4): 661-666. 2008.
6. Peter Katzmarzyck. Timothy Church et al. Sitting time and mortality from all causes, cardiovascular disease, and cancer. Med Sci in Sports Exerc 419(5): 998-1005. 2009.
7. James A Levine and Jennifer Miller. The energy expenditure of using a "walk-and-work" desk for office workers with obesity. Br J Sports Med 41(9): 558-561. Sep 2007.
8. DA McAlpine, CU Manohar, SK McCrady. An office-place stepping device to promote workplace physical activity Br J Sports Med 41(12): 903-7. Dec 2007.
9. AV Patel, Lesley Bernstein, Anisula Deka et al. Leisure time spent sitting in relation to total mortality in  a prospective cohort of US adults. Am J Epidemiol 172(4): 419-429. 2010.
10. E Stamatakis, M Hamer, DW Dunstan. Screen-based entertainment time, all cause mortality, and cardiovascular events: population-based study with ongoing mortality and hospital events follow-up. J Am Coll Cardiol 57(3):292-9. Jan. 18 2011.