The disparity in exercise levels between the most and least active individuals within a country is closely related to its prevalence of obesity, with much of the difference due to reduced exercise levels among women, the results of an innovative health-tracking study indicate.

Using anonymized smartphone data on over 700,000 individuals from more than 100 countries, the researchers found that activity "inequality" — the gap between people who walked a lot and those who walked very little — is a stronger predictor of obesity levels than simply the average number of steps walked per day, with over 40% of the variance due to gender differences.

The findings, which were published online in Nature on July 10, also show that a city's walkability is a key factor in the amount of steps walked on average per day, with the impact of this being greater among women than men.

"In regions with high activity inequality, there are many people who are activity poor, and activity inequality is a strong predictor of health outcomes," said study author Scott L Delp, PhD, James H Clark Professor of Bioengineering and director of the Mobilize Center at Stanford University, Stanford, California, in an National Institutes of Health (NIH) statement.

And "when activity inequality is greatest, women's activity is reduced much more dramatically than men's activity, and thus the negative connections to obesity can affect women more greatly," coauthor Jure Leskovec, PhD, associate professor of computer science at Stanford University, added in a Stanford University press release.

Thus, activity inequality is an important target for obesity prevention, the researchers stress.

Largest-Ever Study on Human Movement

Using a groundbreaking methodology, the researchers gathered anonymized, retrospective data on 717,527 users from 111 countries from the Argus app (Azumio) on Apple iPhone smartphones, yielding 68 million days of physical activity for the period between July 2013 and December 2014. They note that almost 75% of individuals in developed countries carry a smartphone, and in developing nations, about half of the population has such a phone.

Specifically, they collected steps walked per day, as determined via the iPhone's internal accelerometers, which measure movement, and Apple's proprietary algorithms for step counting, as well as — for users in the United States — each individual's city, based on the most commonly occurring weather update.

The users also enter their gender, age, height, and weight in the app settings, which can be changed at any time, and the most recently entered information was used to calculate body mass index (BMI).

"There have been wonderful health surveys done, but our new study provides data from more countries, many more subjects, and tracks people's activity on an ongoing basis in their free-living environments, vs a survey in which you rely on people to self-report their activity," Dr Delp said.

"The study is 1000 times larger than any previous study on human movement," he stressed.

Focusing primarily on 46 countries with at least 1000 users of the app, the team found that the average user recorded 4961 steps per day over an average of 14 hours.

And while there were between-country differences in the average number of steps walked per day, activity patterns across a country's population revealed more about its health dynamics than simply the number of steps walked.

So, examining activity "inequality" showed that countries with the biggest difference between the most and least active individuals had higher obesity rates, independent of gender and age biases and a country's income level.

For instance, the researchers found that, although the United States and Mexico have similar average daily steps, at 4774 vs 4692, the United States had a larger activity inequality, at 0.303 vs 0.279, and higher obesity prevalence rates, at 27.7% vs 18.1%.

Powerful Role of Gender in Country-to-Country Activity Differences

Moreover, it was shown that, in countries with high activity inequality, activity in females was disproportionately reduced compared with males, such that 43% of the activity inequality was explained by gender differences.

In Japan, for example, activity appears to be quite uniform across the population and similar among women and men, as is the case in Sweden, which also has one of the smallest gaps between activity-rich and activity-poor individuals, leading to the smallest disparity between male and female steps, resulting in one of the lowest rates of obesity.

But in Saudi Arabia and the United States, there is greater activity disparity, and activity is disproportionately reduced among females.

The United States ranked fourth from the bottom of the 46 countries in overall activity inequality and was fifth from the bottom in the gender step gap, resulting in high levels of obesity.

The team says: "We find that the prevalence of obesity increases more rapidly for females than males as activity decreases.…So, given two countries with identical average activity levels, the country with higher activity inequality will have a greater fraction of low-activity individuals, many of them female, leading to higher obesity than predicted from average activity levels alone."

Walkability Should Be a Key Aim

Data from 69 cities in the United States indicate that walkability scores were associated with lower activity equality. Taking three Californian cities as an example, the researchers showed that San Francisco had a higher walkability than San Jose and Fremont and the lowest activity inequality.

"If you must cross major highways to get from point A to point B in a city, the walkability is low; people rely on cars," explained Dr Delp. "In cities like New York and San Francisco, where you can get across town on foot safely, the city has high walkability."

And demonstrating that the effect of walkability on activity is greater in females than males, the researchers explain that for a 40-year-old woman, a 25-point increase in walkability equated to 868 more steps walked per day, while the equivalent increase in daily steps among men of the same age was only 622.

Discussing their results, the team says: "Our findings can help us to understand the prevalence, spread, and effects of inactivity and obesity within and across countries and subpopulations and to design communities, policies, and interventions that promote greater physical activity."

The researchers are sharing their findings on an activity inequality website.

Dr Delp was supported by grants from the NIH National Center for Simulation in Rehabilitation Research. Dr Leskovec and other contributors were supported by an NIH grant. Dr Leskovec was supported by an National Science Foundation grant and the Stanford Data Science Initiative and is a Chan Zuckerberg Biohub investigator. Disclosures for the coauthors are listed in the paper.

Nature. Published online July 10, 2017. Abstract

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