Norman swan
Sitting has been accused of being as toxic as obesity and smoking. Those concerns have spawned a whole new line of office furniture: the standing desk. At my organisation, the Australian Broadcasting Corporation, we even have standing studio consoles and headsets that allow twitchy broadcasters such as me to walk around gesticulating while on air.
Just focusing on the amount of time you spend sitting each day, however, ignores the amount of exercise you get when you’re not sitting. It also doesn’t take into account what you’re doing while sitting. Watching television for prolonged periods is considered a risk factor in its own right for obesity and premature mortality.
So just how bad is sitting for you? A recent series of studies in the Lancet aimed to find out. The researchers analysed sitting time, TV time and physical activity in relation to premature mortality.
They looked at data from previous studies that had followed groups of people for several years to document lifestyle and physical activity. The studies also took into account obesity, age and prior illness – factors that could confound any direct link between sitting and disease. After eliminating poor quality or inappropriate studies, they ended up with information on more than a million people who had been followed for up to 18 years.
Physical activity was measured in metabolic equivalent of task (METs), which reflect the intensity of exercise over time. Crudely speaking, the more METs in your life, the better. For instance, regular walking for an hour is around three METs, moderate exercise around four METs and vigorous exercise around seven METs.
They confirmed what everyone already knows: physical inactivity is linked to early death from all causes. People in the lowest activity groups had up to a 59% increased risk of premature mortality when compared to people who didn’t sit much and who had over 35 METs a week.
But when it came to sitting, the results were actually quite comforting. You could abolish the deadly effects of sitting by taking additional exercise. People who sat for eight hours a day needed to take 60-75 minutes of physical activity daily, amounting to more than 35 METs per week to abolish the risk.
Now, that didn’t need to be an hour and a quarter bashing away at the weights at the gym. Those gains could be accumulated as semi-structured exercise like walking the dog. While 75 minutes a day sounds a lot, data from Australia suggest 25% of adults over 45 years old already take that amount. Then again, there’s another 25% who take fewer than five minutes a day.
When it came to television sitting time, the story was somewhat different. Watching television for three hours a day was associated with higher chances of dying prematurely regardless of your level of activity, except for the most active people (more than 35 METs a week) who could get away with five hours in front of the box. So the ill effects of prolonged sitting in front of the TV, as opposed to other types of sitting, are far harder to counteract with exercise.
How do you explain all this biologically?
In specific studies where researchers intervened with exercise rather than just watching people’s behaviour over time, they found that one hour of moderate intensity exercise helps you to metabolise your blood sugars more effectively after a meal. Looking at people with type 2 diabetes, they found that cycling at moderate intensity for 45 minutes after sitting for 10 hours improved their sugar metabolism. There is also evidence that interrupting sitting time with physical activity helps reduce the blood levels of glucose and fats.
TV viewing is more complicated. Adults who watch most of their television after dinner are also more likely to snack while doing so. We already know that our body handles food intake differently at night – eating in the evening is more likely to lead to unhealthy high blood sugar levels. There’s also some evidence that TV advertisements may encourage people to consume more junk food.
Bottom line: don’t become too paranoid about your sitting time. Just make sure not too much of it is in front of the television, and get as much moderate exercise as you can at other times.
