Commentary on “Association of daily sitting time and leisure-time physical activity with body fat among U.S. adults”

In their study, association of daily sitting time and leisure-time physical activity (LTPA) with body fat among U.S. Adults, Liao et al.¹ analyzed data from the 2011–2018 National Health and Examination Survey to examine associations between h/day sitting, meeting (or not meeting) the U.S. Physical Activity Guidelines of ≥150 min/week of moderate- and vigorous-intensity LTPA, and combinations of sitting time and LTPA on total and trunk percent body fat (BF%). LTPA and total sitting time were measured with the Global Physical Activity Questionnaire.² For LTPA, respondents recalled the minutes spent in moderate- and vigorous-intensity LTPA in the past week. For total sitting time, respondents recalled the h/day sitting at school, at home, getting to and from places, and with friends. BF% was measured using dual-energy X-ray absorptiometry scans to identify total body fat (head, limbs, and trunk area) and trunk body fat, reflecting abdominal body fat (herein referred to as BF%).

Results showed a positive association between total sitting time and a negative association between LTPA and BF% in 10,808 males and females aged 20 years to 59 years. LTPA had a moderating effect on BF% for combinations of sitting time (<4 h/day, 4–<6 h/day, 6–8 h/day, and >8 h/day) and LTPA categories (0 min/week (inactive), <150 min/week (insufficiently active), and ≥150 min/week (sufficiently active)). Inactive and insufficiently active respondents had a graded increase in BF% at all levels of sitting time. Sufficiently active respondents had increases in BF% only when sitting ≥6 h/week. These results show the deleterious effects of prolonged sitting time on BF%, especially in persons who are physically inactive or insufficiently active. They also show the moderating effects of sufficient LTPA on sitting time in protecting against increased BF%.

Liao et al.’s¹ results are consistent with studies showing independent3, 4, 5, 6, 7, 8 and joint effects9, 10, 11, 12, 13 of prolonged sitting and physical inactivity on mortality, morbidity, and health-related conditions. For example, in an analysis of National Health and Examination Survey data from 2007 to 2015, Cao et al.⁹ showed that inactive and insufficiently active cancer survivors who sat for ≥6 h/day were 3.92 times (odds ratio (OR) = 3.92, 95% confidence interval (95%CI): 2.15–7.16) more likely to die from all-causes and 4.71 times (OR = 4.71, 95%CI: 1.60–13.9) more likely to die from cancer as compared to sufficiently active cancer survivors who sat for ≥6 h/day. Park et al.¹⁰ analyzed data from the 2014 and 2015 Korea National Health and Nutrition Examination Survey showing joint effects of sitting and LTPA on chronic low back pain. Respondents with low LTPA (inactive and insufficiently active) and sitting more than 7 h/day had a 46% increased odds (OR = 1.46, 95%CI: 1.13–1.88) of chronic low back pain compared to sufficiently active respondents who sat more than 7 h/day (OR = 1.18; 95%CI: 0.87–1.60). In a subsequent analysis of the Korea National Health and Nutrition Examination Survey, Choi et al.¹² examined the associations between sitting time, LTPA, and health-related quality of life measured with the EuroQol 5 Dimension 5 Level scale¹⁴ in older adults ages ≥65. Compared with sufficiently active respondents who sat for <8 h/day, ORs (OR and 95%CI) for adverse health-related quality of life among inactive and insufficiently active (<150 min/week) respondents sitting ≥8 h/day were: poor mobility (OR = 1.90, 95%CI: 1.47–2.46), low self-care ability (OR = 2.14, 95%CI: 1.41–3.27), low usual activity (OR = 3.38, 95%CI: 2.37–4.82), pain and discomfort (OR = 1.79, 95%CI: 1.38–2.31), and high anxiety and depression (OR = 1.69, 95%CI: 1.19–2.41). Alternatively, in sufficiently active respondents sitting ≥8 h/day, 3 (of 5) items on the health-related quality of life questionnaire (mobility, self-care, anxiety, and depression) were not statistically different from sufficiently active respondents sitting <8 h/day. Results from these studies and others show a consistent pattern that sitting for prolonged periods and engaging in <150 m/week in moderate- and vigorous-intensity LTPA have adverse effects on mortality, morbidity, and health-related conditions.

Sedentary behaviors are pervasive in societies.¹⁵ Prolonged sitting occurs at worksites, transportation, leisure, home, and garden activities, in recreation, and in spectator sports settings.¹⁶ According to the 2022 American Time Use Survey,¹⁷ men spent nearly 4.5 h/day on weekdays and 6.7 h/day on weekends in sedentary activities. Women spent less time than men (4 h/day weekdays; 6 h/day weekends). Most time was spent watching television (2.8 h/day), accounting for over half of all leisure time. Reading, playing video games, and computer use for leisure ranged from 29 min/day to 1 h/day and 38 min/day for each activity, depending on one's age. Socializing with others ranged from 25 min/day on weekdays to 53 min/day on weekends. These patterns of physical inactivity are pervasive in countries worldwide, with the highest prevalence found in urban areas, middle-to-upper, and upper-income countries.⁸ Sedentary behaviors have increased over the years,¹⁸ whereas the prevalence of insufficient LTPA remains constant.¹⁹ In 2009, Steven Blair²⁰ proclaimed that physical inactivity was the greatest public health problem in the 21st century. Arocha-Rodulfo²¹ also proclaimed that sedentarism was a vital public health problem of the 21st century. The World Health Organization published its 2020 PA Guidelines on Physical Activity and Sedentary Behavior.⁷ They recommend individuals of all ages engage regularly for varying durations in moderate- and/or vigorous-intensity PA, strength train ≥2 days/week, and limit their amount of sedentary time.

How can inactive people become motivated to be more physically active? Research shows that people engage in LTPA or purposeful walking when the environment is pleasurable; there are safe places to be active, easy access to parks, playgrounds, sports fields, and places to walk or bicycle for shopping close to home.22, 23, 24 Additionally, many people engage in LTPA for health, social interaction, and personal and spiritual reasons; they like how they feel physically and mentally.²⁵ Adults seeking to avoid increased body fat should aim to meet the PA guidelines of ≥150 min/week moderate- and vigorous-intensity LTPA and intentionally try to sit <6 h/day. These behaviors require a thoughtful change in completing daily activities. An excellent start to adopting an LTPA program is to try public health approaches to being more active: take the stairs instead of the escalator or elevator, park the car further from entrances to worksites, stores, and restaurants, walk or ride a bike to places instead of driving, if possible. Finding ways to sit less can apply to using a standing computer desk, engaging in less screen time, which often involves sitting, or taking movement breaks during pauses in a program or video game. Intentionally sitting less and engaging in regular LTPA may require breaking habits, but the payoff results in better management of BF% and overall health,¹² reducing the risks of morbidity¹⁰ and mortality.^9,11,13

This study by Liao et al.¹ observed that inactive and insufficiently active adults had a graded increase in total- and trunk BF% at all categories of sitting time (from <4 h/day, 4–<6 h/day, 6–8 h/day, and >8 h/day). Alternatively, sufficiently active adults had no increases in BF% until sitting 6 h/day; no additional increases in BF% were observed when sitting longer than 6 h/day. Even though data were from a self-report questionnaire, the results are consistent with studies showing the deleterious effects of prolonged sitting and physical inactivity on health status, morbidity, and mortality. Personal, social, and environmental efforts are needed to encourage sedentary persons to engage in sufficient LTPA for better health and maintenance of a lower BF%.

Competing interests

The author declares that she has no competing interests.