Effects of Substituting Sedentary Time with Physical Activity on Metabolic Risk

Purpose: The detrimental effects of sedentary time on health may act by replacing time spent in physical activities. The aim of this study was to examine cross-sectional associations between objectively assessed sedentary and physical activity domains and cardiometabolic risk factors using a novel isotemporal substitution paradigm.

Methods: Participants were 445 healthy men and women (mean age, 66 ± 6 yr), without history or objective signs of cardiovascular disease, drawn from the Whitehall II epidemiological cohort. Physical activity was objectively measured using accelerometers (ActiGraph GT3X) worn around the waist during waking hours for 4–7 consecutive days. We examined the effects of replacing sedentary time with light activity or moderate-to-vigorous physical activity (MVPA) on a range of risk factors (HDL cholesterol, triglycerides, HbA1c, and body mass index) using an isotemporal substitution paradigm.

Results: In partition models, where the time in each of the intensity categories was held constant, only MVPA remained associated with risk factors. In isotemporal substitution models that held total (wear) time constant, replacing 10-min sedentary time with an equal amount of MVPA was associated with favorable effects in all risk factors, including HbA1c (B= −0.023; 95% confidence interval (CI), −0.043 to −0.002), BMI (B = −0.39; 95% CI, −0.54 to −0.24), HDL cholesterol (B = 0.037; 95% CI, 0.021–0.054), and triglycerides (B = −0.035; 95% CI, −0.061 to −0.009).

Conclusions: The associations between sedentary behavior and cardiometabolic risk may be dependent on the types of activities that are displaced by sedentary time.

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Alex’s Notes: There is a research method called the isotemporal substitution model which is designed to simultaneously model the specific activity being performed and the specific activity being displaced in an equal time exchange fashion. It was originally designed for nutritional epidemiology and would be used along the lines of holding calories constant and seeing what would happen when one food replaced another. But the researchers of our study at hand wanted to use it for something else. They wanted to,

“Use the isotemporal substitution method to explore associations with a wider range of cardiometabolic risk markers when replacing sedentary time with light activity or MVPA in a sample of older adults.”

The subjects recruited for the study had no history or signs of cardiovascular disease (CVD) and no previous diagnosis of hypertension, inflammatory diseases, or allergies. All the subjects (n=445) were white Europeans 57-79 years of age (average 66-years) and in decent health with an average BMI of 26, HbA1c of 5.7%, triglycerides of 122 mg/dL, and HDL-C of 67 mg/dL. All subjects wore an accelerometer on the hip for seven days, which allowed classification of activity levels as sedentary (<1.5 MET), light (1.5–3 METs), or moderate-to-vigorous physical activity (MVPA; >3 METs) based on the frequency, intensity, and duration of movement. However, they did not wear it when sleeping or when doing water-based activities (like showering). All the results were adjusted for age, sex, smoking, statin use, and employment state.

On average, 72% of the day was spent being sedentary, with 24% and 4% being light and MVPA, respectively. So assuming eight hours of sleep per night and ignoring the shower time, this translates roughly into 11.5 waking hours of sedentary behavior, 3.8 hours of light activity, and almost 40 minutes of MVPA. Since the cut-off for MVPA was anything greater than 3 METS, we don’t know exactly how intense this activity was. In fact, walking at a 3mph pace would be 3.3 METS but that is hardly moderate-to-vigorous. Even worse, “general house cleaning” is 3 METS. Light activity would be walking at a 2mph pace (the speed you would use if you had a treadmill desk) or light gardening, and sedentary behavior is basically anything that involves sitting or lying down. Even standing comes in at 2.0 METS.

Without considering other forms of activity performed throughout the day, sedentary activity was adversely associated with BMI and HDL-C; light activity was favorably associated with HDL-C; and MVPA was favorably associated with all risk factors. In the isotemporal substitution model that held time constant, replacing sedentary time or light activity with MVPA was associated with favorable effects in all metabolic risk factors, with the magnitude of change not differing between the two. There was also no benefit when sedentary time was replaced with light activity.

The above really struck me as surprising upon first glance because we all hear about how beneficial short breaks in sitting time and other sedentariness can be for health. Indeed, walking for 2 minutes at a 2 mph pace every 20 minutes improves glucose responses to a test meal in overweight & obese adults compared to sitting the entire time. However, the effect was more pronounced in those who had to walk at a 3.6 mph pace, and these people had the added benefit of lower insulin levels as well, which was not seen in the lower walking speed.

A separate study conducted in healthy normal weight college students (85% women) had subjects sit 14 hrs/day, walk 1 hr/day, stand 1 hr/day, and spend 8 hr/day sleeping, with a second group replacing 1 hour of sitting with 1 hour of vigorous supervised bicycling, and a third group replacing 6 hours of sitting with 4 hours of walking at a leisure pace and with 2 hours of standing. The bicycling and walk/stand replacements were designed to both expend roughly 450 kcal. Only the third group showed improvements in insulin sensitivity and plasma lipids relative to the sedentary group.

The above discrepancies could be explained by the subjects and study methods. In the first study, the subjects were obese, whereas they were barely overweight in the current study, suggesting that light activity may be more beneficial among those with greater health complications. In the second study, the subjects were young and it is likely that their “leisurely walking” was greater than 3.0 METs. This appears to be the case from personal experience, but we cannot say for certain. Either way, it is clear that MVPA is the most beneficial.

Remember, MVPA was defined as activity above 3 METs. If you are curious about your activity levels, record what you are doing and when, and then go look up that activity from this chart.

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