Plenty of research has shown a strong association between income inequality and obesity. In developed countries in particular, individuals at the bottom of the socio-economic scale are much more likely to be obese than people nearer the top.
Well, a new study that used smartphones to track the physical activity — measured in number of steps taken daily — of more than 700,000 men and women in 111 countries has identified another kind of obesity-related disparity: activity inequality. This is the difference in each country between people who walk a lot and those who walk very little. The study found this type of inequality to be a powerful predictor of obesity — more powerful than comparing the average numbers of steps recorded for each country.
“If you think about some people in a country as ‘activity rich’ and others as ‘activity poor,’ the size of the gap between them is a strong indicator of obesity levels in that society,” said Scott Delp, one of the study’s authors and a bioengineer at Stanford University, in a released statement.
Depp and his colleagues also found a strong correlation between a country’s gender-activity gap — the difference between how much men and women walk, on average — and its obesity levels. Previous research involving mostly people in the U.S. have shown that men walk much more than women, and this study demonstrated that was true globally as well.
Comparing the U.S. with Mexico
People in the United States and Mexico, for example, take similar average daily steps (around 4,700, according to the study). In Mexico, those steps tend to be spread out across the population, while in the U.S., they are concentrated among a small percentage of people. The disparity between the number of steps taken by men and women is also greater in the U.S. than in Mexico.
Those two activity inequalities, the study’s authors say, predict the wide difference in obesity rates between the two countries — 27.7 percent in the U.S. versus 18.1 percent in Mexico.
Other countries, like Sweden, are more equal: more people there — including more women — tend to take the average number of steps (around 5,900) recorded for that country in the study.
“Sweden had one of the smallest gaps between activity rich and activity poor, and the smallest disparity between male and female steps,” said Tim Althoff, another author of the study and a Stanford University doctoral candidate in computer science, in the released statement. “It also had one of the lowest rates of obesity [12.6 percent].”
More study details
For the study, the Stanford University researchers examined data collected from 68 million days of minute-by-minute step recordings from 717,527 anonymous users of Argus, a free Argus smartphone app. Although the data came from people in 111 countries, the researchers focused their analysis on the 46 countries that had at least 1,000 users of the app.
The app also records each person’s age, gender, weight and height. The researchers used the latter two data points to determine each person’s body mass index, or BMI, and thus whether they were obese or not.
The average user in the study recorded 4,961 steps per day over an average span of 14 hours. Among the 46 countries, the average number of steps ranged from 3, 513 in Indonesia to 6,189 in China.
But it’s the countries with the greatest activity inequality — such as the U.S., Canada, Australia, New Zealand and Egypt — rather than those with the least average number of steps taken that have the highest obesity rates in the world (all over 20 percent). In fact, the study found that people in those five countries are almost 200 percent more likely to be obese than people from the countries with the lowest activity inequality scores (Hong Kong, China, Sweden, South Korea, Japan, the Czech Republic).
The data revealed that the gender-activity gap explains 43 percent of the observed difference in activity inequality in the countries.
“This suggests that activity inequality could be greatly reduced through increases in female activity alone,” the researchers write.
The importance of ‘walkability’
The study also looked at whether increasing a city’s walkability — creating a structured environment within a city that makes walking easy, safe and enjoyable — could reduce activity inequality and the gender-activity gap.
“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,” said Delp. “In cities like New York and San Francisco, where you can get across town on foot safely, the city has high walkability.”
Delp and his colleagues analyzed data from 69 U.S. cities — including Minneapolis and St. Paul — and found that higher walkability scores (as determined by prior research) are associated with lower activity inequality.
“Looking at three California cities in close geographic proximity — San Francisco, San Jose and Fremont — we determined that San Francisco had both the highest walkability score and lowest level of activity inequality,” said Jennifer Hicks, yet another of the study’s author and a data scientist at Stanford University, in a second released statement. “In cities that are more walkable everyone tends to take more daily steps, whether male or female, young or old, healthy weight or obese.”
The five cities with the top walkability scores were New York (87.6) Jersey City (84.4), San Francisco (83.9), Boston (79.5 and Philadelphia (76.5). Those with the lowest scores were Charlotte (24.4), Jacksonville (25.5), Nashville (26.5) Indianapolis (28.7) and Raleigh (28.8).
A societal, not an individual problem
“Our findings highlight activity inequality as an important indicator of activity disparities in the population and identify ‘activity poor’ subpopulations, such as women, who could benefit most from interventions to promote physical activity,” the study’s authors conclude.
The findings also suggest that public health interventions to reduce obesity need to focus less on lecturing individuals about their behaviors and more on making the “built environment” of communities conducive to physical activity — particularly walking.