Reference
Lee IM, Shiroma EJ, Kamada M, Bassett DR, Matthews CE, Buring JE. Association of step volume and intensity with all-cause mortality in older women. JAMA Intern Med. 2019;179(8):1105-1112.
Design
A prospective cohort study testing the association between exercise, measured as steps per day, and all-cause mortality
Participants
The study cohort included 18,289 women in the Women’s Health Study who wore an accelerometer during waking hours for 7 days during 2011 and 2015. Only women who wore the device for 10 hours per day or more on 4 or more days without malfunction were included in the final data analysis (n=16,741). Their mean age was 72 years.
Outcome Measures
Deaths were ascertained through either family members or public records.
Key Findings
During an average follow-up of 4.3 years, 504 women died. The risk of dying was inversely associated with number of steps taken per day. Risk of death significantly decreased with increasing number of daily steps. The women were divided into 4 quartiles by number of steps and intensity of exertion interpreted from frequency of steps. Those in the first quartile took an average of 2,718 steps per day; in the second quartile, 4,363 steps per day; in the third, 5,905 steps per day; and in the fourth quartile, 8,442 steps per day.
If the relative risk of dying for those women in the lowest quartile of steps per day is set to 1.00, the risk decreased significantly for each quartile. Risk decreased to 0.51 for those in the second quartile, 0.44 in the third, and 0.33 in the fourth (P<0.01; adjusted for age and wear time). Further adjustment for potential confounders weakened the associations only slightly, to 1.00, 0.59, 0.54, and 0.42, respectively (P<0.01).
Mortality rates continued to decrease with increasing steps up to approximately 7,500 steps per day, at which point benefit leveled off.
The absolute rate reduction for women in the fourth quartile compared to those in the first quartile was 9.3 deaths per 1,000 person-years. For every additional 1,000 steps per day, the hazard ratio (HR) declined by 15%. Additional adjustment for body mass index (BMI), hypertension, high cholesterol, and diabetes shifted quartile HRs to become more similar to those adjusted for age and wear time only (1.00, 0.54, 0.47, 0.34; P<0.01). When adjusting for stepping intensity in addition to confounders, associations were attenuated, but all remained significant (HRs per quartile: 1.00, 0.68, 0.68, 0.58; P<0.01).
The largest impact was the 41% reduction in mortality rates between women in the first and second quartiles—that is, women who walked only 2,700 steps per day compared to 4,400 steps per day. Mortality rates continued to decrease with increasing steps up to approximately 7,500 steps per day, at which point benefit leveled off. Stepping intensity (basically how fast one walked) had relatively little impact on mortality rates. Number of steps mattered more than intensity.
Clinical Implications
Many of our patients are obsessed with step counting. It’s beyond fashionable. Fitbit sold 14 million devices between 2010 and 2018.1 Most people who own a Fitbit or any other step-tracker are certain that their goal is to walk 10,000 steps per day, even though this number is not based on any evidence.
Rather the 10,000-step target apparently originated with Yamasa, a Japanese company that designed a pedometer in 1965 called the Manpo-kei, a name that translates as “10,000 steps meter.”2 At the time there was some thought that taking 10,000 steps per day was necessary to decrease risk of coronary heart disease. This was more a marketing idea than a medical finding.
There is nothing new about step counting. People have used it as a method to measure distances since ancient times. The word mile is Latin in origin, from milia passuum, or thousands of paces.3 If one could count, one could create accurate maps and know where one was. The idea of using mechanical aids to count steps goes back to da Vinci. Thomas Jefferson had a pedometer custom-made for him while he lived in Paris and noted that he took fewer steps to walk a mile in cold weather than warm.4 He sent a pedometer to James Madison as a gift in 1788.5
There is little evidence that informs us how many steps people actually need to take to improve health. A 2018 meta-analysis by Oja et al that examined walking intervention trials reported improvements in cardiovascular risk factors with increasing steps.6 Yet the authors were not convinced that the results they examined were really significant. Kelly et al’s 2014 meta-analysis of observational studies reported amounts of walking and bicycling were inversely related to mortality rates but recorded their conclusions in metabolic equivalent of task (MET) hours per week.7 In 2017 Stamatakis et al reported that faster-paced walking was also inversely related to mortality rates.8
There have been a number of specific mortality studies worth noting. In 2015 Dwyer et al reported number of steps and all-cause mortality on a cohort of Tasmanians and found a linear relationship between increasing daily steps and reduced mortality risk (n=2,576). Compared to those described as sedentary, those taking 10,000 steps per day had a 46% lower risk of mortality in the decade of follow-up. Those who increased their daily steps over the course of the study also experienced substantial reduction in mortality risk. A 1,000-step-per-day increment was associated with a 6% risk reduction.9 In a 2018 British study by Jefferis et al on older men (n=1,655), each 1,000-step-per-day increment was associated with a 14% reduction in mortality risk.10
Other step-counting studies have reported lower risk of dysglycemia, decreases in adiposity, and improved insulin sensitivity with increasing steps.11 However, in patients’ thinking, shifts in mortality risk generally outweigh these sorts of improvements.
This current study by Lee et al was substantially larger than prior research trials, and pedometer technology has only become more sophisticated over time. Based on these findings, the evidence suggests health benefits, at least in terms of mortality, level off at about 7,500 steps per day. This may pose a problem as the 10,000-step goal is now so cemented in public belief that saying anything less may lead only to your patients' questioning your expertise.
Perhaps it is best to let patients continue to think their goal is 10,000 steps, and if they confess that they reached only 7,500 steps, then explain the evidence. Where the real payoff will be is in getting the couch potatoes, the lowest quartile (<2,700 steps per day), to move more. While we should encourage all our patients to walk, we should put our greatest effort where it will make the most difference.
