In the U.S., it has been estimated that one-third of adults spend at least 9 waking hours and over two-thirds spend more than 7.5 waking hours being sedentary (SED) every day. In addition, more than two-thirds of adults achieve zero minutes of moderate-to-vigorous physical activity (MVPA). Interestingly enough, one-third of adults are obese and two-thirds are overweight. Coincidence? Not according to research suggesting that both increasing MVPA and reducing SED are required to substantially reduce the risk of obesity.

More recently, a wave of publications has appeared in several journals that continue to support the what-should-be-obvious-by-now association between SED and/or MVPA and health. Although all these studies are epidemiological and thus prevent causal inferences, they still provide interesting information and identify associations for future direct testing.

First up is the work of Maddison et al from University of Auckland, who sought to characterize typical activity profiles of Americans and determine their association with cardiovascular disease risk (CVDR). Data was extracted from the NHANES 2003-2004 and 2005-2006 surveys, which together provided information on 6499 individuals aged 30-75 years. They were divided into one of four categories: busy exercisers (reference group; high MVPA, low SED), techno-actives (high MVPA, high SED), potterers (low MVPA, low SED), and couch potatoes (low MVPA, high SED).

Overall, participants spent about 17 minutes engaging in MVPA and 8.5 hours being SED. As could be expected, CVDR was significantly associated with increased SED and reduced MVPA. Compared with the busy-exercisers, couch potatoes and potterers had a more than 2-fold and 64% increased CVDR, respectively, whereas techno-actives were not significantly different. Notably, SED was similar between busy exercisers and potterers, as well as techno-actives and couch potatoes, but the busy exercisers and techno-actives had far more MVPA, suggesting that MVPA plays a more important role in CVDR reduction.

The categories of SED and MVPA are a great start, but they don’t adequately capture various levels or modes of physical activity. Thus, Loprinzi and Davis from the University of Mississippi analyzed data on 12,321 people from the NHANES 1999-2004 survey in an attempt to examine the individual and combined effects of 4 different movement-based behaviors (MBB) on all-causes and CVD-specific mortality. The 4 MBBs were (1) moderate-intensity aerobic activity (MPA), (2) vigorous-intensity aerobic activity (VPA), (3) muscular strength activities (MSA), and (4) active transport (AT).

Unlike the previous study that relied on objective accelerometer data, the current classifications were determined by self-reported answers to questions such as, “Over the past 30 days, did you do any physical activities specifically designed to strengthen your muscles such as lifting weights, push-ups or sit-ups?” If the respondent answered “yes” then they would receive 1 MBB point, and a MBB index variable was created by summing the number of MBBs that each individual engaged in (range: 0–4).

Over the 57.7 month follow-up period (4.8 years), 654 participants died of all-cause mortality, with 231 dying of CVD. Compared with having a MBB index of zero, having an index of 1, 2, or 3-4 (because the number of people with all 4 MBBs was too few to stand alone) reduced the risk of all-cause mortality by 39, 51, and 76 %, respectively, and reduced the risk of CVD-specific mortality by 51, 61, and 71 %, respectively. However, the only MBBs independently associated (e.g. VPA vs no VPA) with reduced mortality were VPA and MPA, and the only MBB that was associated with reduced all-cause mortality when it was the only MBB performed was VPA.

Another way to look at these findings is that more MBBs is better, but that VPA > MPA > MSA = AT. I do find it very odd that MSA had no associations with mortality and I speculate it is because of the self-reported nature of these activities, especially considering previous research has found muscular strength to be independently protective against death from all causes and cancer, even after adjusting for cardiorespiratory fitness, physical activity levels, and other potential confounders.

Regardless, the two studies thus far support current recommendations for MVPA participation, but they don’t tell us how including more MVPA time would impact health when it displaces SED time. Fortunately for us, the recent publication by Matthews et al from the National Cancer Institute analyzes data on 154,614 elderly individuals (50-71 years) to give us our answer. These people were provided a survey that asked how much time per week was spent in 16 activities during the past 12 months. The activities were classified as exercise or nonexercise activity, and the estimated energy cost of the activities was combined with participation time to determine each person’s MET hours per day.

Sedentary behavior and physical activity were associated with age, education, obesity, smoking, comorbid health conditions, and general health status, and therefore all of these variables were controlled for in the analyses. As would be expected, more SED was associated with a graded increase in the risk for all-cause mortality and CVD-specific mortality, and greater amounts of both exercise and nonexercise activities were independently associated with lower all-cause mortality. In fact, mortality was 20-30 % lower for men and women reporting 1-2 hours per day of exercise, and 1-2 hours per day of nonexercise activity was associated with 30% reduction in mortality in men and 50-60 % reduction in women. Interestingly, the exercise mortality curve was “U” shaped, suggesting that more exercise is actually hazardous and the optimal daily amount was about 1 hour.

The researchers then went on to fit isotemporal models to estimate the mortality benefits associated with replacing SED with an equal amount of different types of physical activity in less active (<2 hours per day overall activity) and more active participants (≥2 hours per day). For LESS active people, replacing 1 hour of SED with 1 hours of exercise and nonexercise activity was associated with a 42 and 30 % reduction in risk of all-cause mortality, respectively, with stronger associations observed for higher intensity activities. For MORE active people, replacement of SED with exercise reduced risk by 9% but there was no association when replaced by nonexercise activity. Similar results were observed for CVD-specific mortality.

It seems pretty clear-cut that more SED is associated with a greater risk of death and that more physical activity, especially more vigorous types of exercise, is protective against all causes of death. Moreover, it appears that the less active one is, the more they benefit from increased activity. Could this be because they have more opportunity for improving cardiorespiratory fitness (CRF) through increased physical activity?

Whether there is a risk gradient for all-cause mortality among individuals with the lowest CRF was explored by Farrell et al from the Cooper Institute in Texas. These researchers analyzed data on 6251 men aged 40-69 years that attended their clinic from 1971-2006 and followed them from baseline testing to death or the end of 2008, which ever came first. At baseline testing, CRF for each person was determined by a maximal treadmill exercise test using a modified Balke protocol, and the cohort was divided into quintiles. The lowest quintile was further stratified into tertiles for analysis.

When using CRF as a continuous rather than a categorical variable, we found that for every 1-min increment in treadmill time, there were corresponding 9%, 11%, and 15% decreased risk of mortality for 40- to 49-yr-old, 50- to 59-yr-old, and 60- to 69-yr-old groups, respectively. Because clinicians may be more familiar with METs than the modified-Balke treadmill protocol, we also report that for every 1-MET increment in treadmill performance, there were corresponding 16%, 20%, and 23% decreased risk of mortality for these same age groups.

Okay, that settles it! The evidence thus far strongly supports the notion exercise is healthful, especially among those who are most sedentary, perhaps because it increases their CRF. So the question remains, how do we get these couch-potatoes to start moving? The final study published this last week address this exact question. Mutikainen et al from the University of Jyväskylä, Finland used data from the Body and Future Health randomized controlled trial to evaluate what baseline characteristics predicted changes in physical activity over the 6-month follow-up period. All 51 of the analyzed participants were aged 25-40 years and overweight or obese but otherwise healthy.

The results clearly showed that a strong sense of meaningfulness (ability to find meaning in the situation) and having better recovery and adaptability (assessed via heart-rate variability) on non-work days significantly predicted increased physical activity, but that there were no associations between measures of body composition, stress and recovery on work-days, or psychological flexibility.

So what have we learned? If there is a single take-away from this potpourri of research investigating the associations between physical activity and death, it is that exercise counts. Depending on where you start, a reduced risk of death from any cause can be achieved by simply performing household chores, gardening, or walking more often so that it displaces time spent being sedentary. However, the most bang-for-the-buck comes from engaging in structured moderate-to-vigorous physical activity, ideally for about 1 hour per day.

So what is moderate-to-vigorous activity look like? Well, according to published intensity thresholds, the answer would be about 2,000 accelerometer counts per minute, which is equivalent to walking about 3 miles per hour. In other words, walking 3 miles per day would net you the low-end benefits. That is a great place to start for many people, but since health improves with cardiorespiratory fitness, it is prudent to up the intensity as soon as one feels able.