By Ellen Feldman, MD

Altru Health System, Grand Forks, ND

Dr. Feldman reports no financial relationships relevant to this field of study.

SYNOPSIS: Investigators found an association between higher number of steps taken daily and lower all-cause mortality, lower mortality from cardiovascular disease, and lower mortality from cancer, but no association between intensity of steps and mortality in any of those areas.

SOURCE: Saint-Maurice PF, Troiano RP, Bassett DR, et al. Association of daily step count and step intensity with mortality among U.S. adults. JAMA 2020;323:1151-1160.

In a 1786 letter to his son-in-law, Thomas Jefferson wrote, “I have known some great walkers and had particular accounts of many more; and I never knew or heard of one who was not healthy and long lived.”1 More than 200 years later, Saint-Maurice et al have used research methods from 2020 to understand and delineate the relationship among walking, health, and longevity long ago noted by one of the United States’ “founding fathers.”

Saint-Maurice et al noted the lack of evidence-based studies supporting the popular goal of achieving 10,000 steps each day. Prior investigations involving walking and health have affirmed the health benefits of walking, but typically involved limited and specific populations.2,3 Therefore, findings are not necessarily applicable to other groups or to a more general population. In addition, there have been conflicting results in studies of step intensity and association with health benefits.4,5 Citing these reasons, this team set out to look at any association between step count, step intensity, and mortality in a representative sample of the U.S. population older than age 40 years.

The National Health and Nutrition Examination Survey (NHANES), a Centers for Disease Control and Prevention-sponsored program, annually collects data from a nationally representative sample of the U.S. population.6 From 2003 to 2006, 4,840 respondents aged 40 years or older from NHANES wore standardized hip accelerometers during a seven-day period. Saint-Maurice et al used data from the accelerometers to determine participant step count and step intensity. Considering the breadth of information collected by NHANES, the team adjusted results for a variety of factors, including age, education, substance use, specific medical diagnosis, and self-reported general health. There were 6,355 respondents initially eligible for inclusion in the study, but there were sufficient data for only 4,840 people; 1,515 individuals either elected not to wear the accelerometer or did not submit a record for at least one 12-hour period. The authors accessed the National Death Registry periodically until Dec. 31, 2015, to determine death of any participant from any cause. Indications that a participant had died by either cancer or cardiovascular disorder were noted, too.

The mean number of steps/day for the entire group of 4,840 participants was 9,124. The mean length of time wearing the accelerometer was 5.7 days, while the mean time each day was 14.4 hours. Respondent data were stratified according to number of daily steps:

  • < 4,000 steps/day (n = 655)
  • 4,000-7,999 steps/day (n = 1,727)
  • 8,000-11,999 steps/day (n = 1,539)
  • > 12,000 steps/day (n = 919)

Considering the differences between the groups, and an analysis showing significant attenuation of results with adjustment for some variables, Saint-Maurice et al presented several interpretive models for review and discussion. In addition to all-cause mortality, the authors examined death as the result of cardiovascular disease and cancer. Each revealed a similar pattern of a significant decrease in mortality rate with increasing step count. In addition, results indicated consistent findings: decelerated mortality rate with more steps taken when reported separately for men, women, age groups, and ethnicity.

The second major part of this study concerned step intensity and mortality rate (MR). Step intensity was calculated in several manners (based on time and number of steps). Higher step intensity was associated with significantly lower MR — until adjusting these figures for number of steps daily. For example, the unadjusted MR was 5.2 (95% confidence interval [CI], 3.2-7.3) per 1,000 adults/year for those in the highest quadrant of step intensity and 10.0 (95% CI, 7.1-12.9) for the respondents on the other end of the scale. However, when adjusting for the number of steps in both quadrants, this difference virtually vanishes, with an adjusted MR of 8.4 (95% CI, 4.0-12.9) per 1,000 adults/year and 9.2 (95% CI, 6.9-11.8) per 1,000 adults/year. The P value for the trend is 0.34.

COMMENTARY

This study gives backbone to the principle of viewing physical activity as essential “medicine” for health. The association between step count and longevity found by Saint-Maurice et al is consistent with studies of step count and health both in investigations centered on older adults and in studies of persons with chronic illnesses.2,3 This work adds to the literature by generalizing the findings from these narrower studies and pointing toward a dose relationship between step count and longevity.

It is important to note this was an observational study; there is no evidence for causation. There are some other significant limitations, including the fact the participant group was self-selected, and that wearing the accelerometer itself may have produced an effect (such as motivating more walking or motivating a more active lifestyle). Future investigations with randomization and controls will be helpful in further understanding and quantifying the relationship between step count and longevity.

It is notable that even when adjusted for a substantial number of variables, results still indicate a dose-response relationship between step count and longevity, with hazard ratio indicating a 65% reduced chance of death among the group taking 12,000 or more steps daily when compared to the referent group (taking 4,000 steps or fewer daily). The authors of future investigations may want to look more closely at the lifestyle of the most active group to see if there are other variables influencing longevity, such as participation in activities that may not register on an accelerometer (e.g., biking or swimming). Another point to consider is that the only accelerometer data came from a seven-day period between 2003 and 2006. There is little evidence to suggest this period is representative of a respondent’s lifestyle in general and/or over time. More information and data from subsequent years will be helpful in evaluating whether accelerometer patterns stay stable over time and ultimately may help in attempts to address the question of causation. The investigators expected to see an association of step intensity with mortality, and were surprised at not finding this relationship. They noted there has been only a smattering of studies of the effect of step intensity on mortality and suggested further studies to understand this in full. In this paper, it appears that step intensity is associated with greater number of steps in general. Even with the limitations noted, the Saint-Maurice et al study represents a true “step” forward in the field. While it is prudent to wait for evidence of causation, there is no need to wait for recommending an active lifestyle to patients. Today, many patients wear devices that measure not only step count, but also many other health parameters associated with activity, including heart rate and recovery time. Using these devices and active monitoring may become an integral component of a modern wellness plan. We know activity is helpful for many health measures and conditions, including reducing the risk of type 2 diabetes, obesity, and heart disease. This study strengthens the argument for continuing to keep physical activity as the foundation of a healthy lifestyle and adds evidence to a role for physical activity in longevity.

REFERENCES

  1. Thomas Jefferson’s Monticello. Exercise. Nov. 6, 1990.
  2. O’Donnell J, Smith-Byrne K, Velardo C, et al. Self-reported and objectively measured physical activity in people with and without chronic heart failure: U.K. Biobank analysis. Open Heart 2020;7:e001099.
  3. Dohrn IM, Welmer AK, Hagströmer M. Accelerometry-assessed physical activity and sedentary time and associations with chronic disease and hospital visits – a prospective cohort study with 15 years follow-up. Int J Behav Nutr Phys Act 2019;16:125.
  4. Lee IM, Shiroma EJ, Kamada M, et al. Association of step volume and intensity with all-cause mortality in older women. JAMA Intern Med 2019;179:1105-1112.
  5. Ekelund U, Steene-Johannessen J, Brown WJ, et al. Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonised meta-analysis of data from more than 1 million men and women. Lancet 2016;388:1302-1310.
  6. Centers for Disease Control and Prevention. About the National Health and Nutrition Examination Survey. Last reviewed Sept. 15, 2017.