A new study suggests that under-exercising, rather than overeating, may be at the heart of America's obesity epidemic. 
 
 Researchers from Stanford University School of Medicine report a strong correlation between the rise in obesity and a striking drop in the amount of time Americans spend exercising when not at work over the last 22 years. 
 
 Their analysis uncovered no evidence that American's have increased their daily calorie count in the same time period. 
 
 "We wouldn't say that calories don't count, but the main takeaway is that we have to look very carefully at physical activity. The problem is not all in the intake of calories," said Dr. Uri Ladabaum, a professor of gastroenterology at Stanford Medical School. Ladabaum is also the lead author of the study that will be published in the August issue of the American Journal of Medicine. 
 
 The study relies on data collected by the National Health and Nutrition Examination Survey from 1988 to 2010, which Ladabaum and his team used to look for trends in obesity, abdominal obesity, physical activity and caloric intake in the last two decades. 
 
 The most startling finding in the study is the drop in the amount of exercise Americans do in our free time. The researchers found that from 1988 to 2010, the percentage of adults who reported doing no exercise in their free time grew dramatically from 19% to 52% in women, and from 11% to 43% in men. 
 
 "We suspected there was a trend in that direction, but not that magnitude," said Ladabaum. "People can get exercise in other ways, but most people don't walk or bike to work, and most people are not in jobs that require physical activity." 
 
 At the same time, the researchers found that the prevalence of obesity increased from 25% to 35% in women, and from 20% to 35% in men. In that time period the proportion of normal weight men and women dropped, while the proportion of overweight men and women remained the same. 
 
 Most people don't walk or bike to work, and most people are not in jobs that require physical activity. — Dr. Uri Ladabaum 
 
 But here comes the surprising part: The researchers did not find any evidence that people were ingesting more calories on a daily basis in 2010 than they were in 1988. 
 
 "The one caveat here is that the amount of calorie intake was based on self report, so it is possible people were not recalling correctly what they ate, or not reporting correctly," said Ladabaum. 
 
 Ladabaum notes that the study can tell us only that a major drop in time spent exercising occurred at the same time as a rise in obesity, not that one caused the other. 
 
 "The study looks at trends and certain associations, but does not prove any cause and effect between these," he said. 
 
 He also wants to make clear that the fact that the average caloric intake did not change substantially does not mean that caloric intake has been optimal at the population level or at the level of individuals. 
 
 "We simply did not detect a substantial increase over time," he said. 
 
 Even with all those caveats, however, this study could still be used to inform policy on managing America's obesity epidemic. 
 
 "Even though it is very difficult to prove directly that public health interventions promoting physical activity will make a difference, I think they will," he said. "This study should serve as a reinforcement of the message that we need to think of a multi-component solution where diet is a big part of it, and physical activity is a big part as well." 
 
 However, there is a big difference between telling people that they should be exercising and actually getting them to do it. 
 
 "The finger wagging Puritan in me wants sedentary folks to get off the couch and exercise, but my public health background cautions me to go beyond the data tables and look at the lives of Americans today," writes Pamela Powers Hannely, managing editor of the American Journal of Medicine in an editorial. 
 
 She notes that single mothers in particular, may have difficulty figuring out how to work exercise into their already hectic lives. She also wonders if this may explain why the rise in obesity has been most prevalent in women between the ages of 18 to 39. 
 
 For more news from the world of science, follow me @DeborahNetburn ||||| Image caption Children are advised to do at least an hour of vigorous activity every day 
 
 Many children cannot run as fast as their parents could when they were young, a study of global fitness says. 
 
 Experts say the work - being presented at the American Heart Association's annual meeting - suggests children's fitness levels may be declining. 
 
 Researchers analysed data spanning 46 years and involving more than 25 million children in 28 countries. 
 
 On average, children today run a mile 90 seconds slower than did their counterparts 30 years ago, they said. 
 
 If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life Lead researcher Dr Grant Tomkinson , University of South Australia BBC - Health BMI calculator 
 
 Obesity 
 
 Across nations, cardiovascular endurance - gauged by how far children can run in a set time - has dwindled consistently by about 5% every decade, according to the findings. 
 
 The decline is seen in boys and girls and across all ages from nine to 17 years, and is linked to obesity, with some countries faring worse than others. 
 
 Lead researcher Dr Grant Tomkinson of the University of South Australia's School of Health Sciences said: "In fact, about 30% to 60% of the declines in endurance running performance can be explained by increases in fat mass." 
 
 The problem is largely one of Western countries, but some parts of Asia like South Korea, mainland China and Hong Kong are also seeing this phenomenon. 
 
 Dr Tomkinson said children needed to be inspired and encouraged to do more vigorous exercise. 
 
 If not, the public health consequences could be dire. 
 
 Huff and puff 
 
 "If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life," said Dr Tomkinson. 
 
 To stay healthy, children and young people need to do at least an hour of physical activity - such as walking or cycling to school and running in the playground - every day. It can be done in small chunks rather than one session. 
 
 Prof Michael Gwitz of the American Heart Association said: "The type of exercise is really important." 
 
 He says exercise must be something that "makes you sweat" and is "sustained and dynamic" to promote cardiovascular fitness. 
 
 Simply going to the gym or belonging to a school sports team might not be enough, unless you are moving around a lot. 
 
 Christopher Allen of the British Heart Foundation, said: "It's well established that being physically inactive in childhood can have serious health implications later in life. 
 
 "Keeping active can reduce the risk of cardiovascular disease and the sooner kids start, the better. 
 
 "By encouraging children to get active, we can help protect their hearts as they grow up. Parents, schools and community groups can all help kids on their way to 60 minutes exercise a day." ||||| Today's kids can't keep up with their parents. An analysis of studies on millions of children around the world finds they don't run as fast or as far as their parents did when they were young. 
 
 FILE - In this May 13, 2007 file photo, boys participate in 100 meter race during two-day World Athletics Day meet in Bangalore, India. An analysis of studies on 250 million children around the world... (Associated Press) 
 
 On average, it takes children 90 seconds longer to run a mile than their counterparts did 30 years ago. Heart-related fitness has declined 5 percent per decade since 1975 for children ages 9 to 17. 
 
 The American Heart Association, whose conference featured the research on Tuesday, says it's the first to show that children's fitness has declined worldwide over the last three decades. 
 
 "It makes sense. We have kids that are less active than before," said Dr. Stephen Daniels, a University of Colorado pediatrician and spokesman for the heart association. 
 
 Health experts recommend that children 6 and older get 60 minutes of moderately vigorous activity accumulated over a day. Only one-third of American kids do now. 
 
 "Kids aren't getting enough opportunities to build up that activity over the course of the day," Daniels said. "Many schools, for economic reasons, don't have any physical education at all. Some rely on recess" to provide exercise. 
 
 Sam Kass, a White House chef and head of first lady Michelle Obama's Let's Move program, stressed the role of schools in a speech to the conference on Monday. 
 
 "We are currently facing the most sedentary generation of children in our history," Kass said. 
 
 The new study was led by Grant Tomkinson, an exercise physiologist at the University of South Australia. Researchers analyzed 50 studies on running fitness _ a key measure of cardiovascular health and endurance _ involving 25 million children ages 9 to 17 in 28 countries from 1964 to 2010. 
 
 The studies measured how far children could run in 5 to 15 minutes and how quickly they ran a certain distance, ranging from half a mile to two miles. Today's kids are about 15 percent less fit than their parents were, researchers concluded. 
 
 "The changes are very similar for boys and girls and also for various ages," but differed by geographic region, Tomkinson said. 
 
 The decline in fitness seems to be leveling off in Europe, Australia and New Zealand, and perhaps in the last few years in North America. However, it continues to fall in China, and Japan never had much falloff _ fitness has remained fairly consistent there. About 20 million of the 25 million children in the studies were from Asia. 
 
 In China, annual fitness test data show the country's students are getting slower and fatter over the past couple of decades. 
 
 Experts and educators blame an obsession with academic testing scores for China's competitive college admissions as well as a proliferation of indoor entertainment options like gaming and web surfing for the decline. 
 
 China's Education Ministry data show that in 2010 male college students ran 1,000 meters 14 to 15 seconds slower on average than male students who ran a decade earlier. Female students slowed by about 12 seconds in running 800 meters. 
 
 Tomkinson and Daniels said obesity likely plays a role, since it makes it harder to run or do any aerobic exercise. Too much time watching television and playing video games and unsafe neighborhoods with not enough options for outdoor play also may play a role, they said. 
 
 Other research discussed global declines in activity. 
 
 Fitness is "pretty poor in adults and even worse in young people," especially in the United States and eastern Europe, said Dr. Ulf Ekelund of the Norwegian School of Sport Sciences in Oslo, Norway. 
 
 World Health Organization numbers suggest that 80 percent of young people globally may not be getting enough exercise. 
 
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 Healthy lifestyle guidelines: http://bit.ly/16ZnV7e 
 
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 Marilynn Marchione can be followed at http://twitter.com/MMarchioneAP
– The Physical Activity Council has a list of 104 physical activities—which do not include lifting 12 ounces of Bud Lite but do include table tennis, golf, and walking—in its annual survey of our activity habits, released yesterday. And the number of Americans who engaged in not a single one of those 104 activities in all of 2014 is more than one in four of us: That's 83 million people, or 28% of the US population over the age of 6, reports the Wall Street Journal. "We feel confident, in a sad way, that this is the largest number we’ve ever seen," says PAC member and Sports and Fitness Industry Association exec Tom Cove, who adds it's the most dismal result he's seen in 24 years. The number of completely sedentary Americans increased by 18% since 2007. That's not to say we don't like the appearance of fitness, just the actuality of it: The Journal notes the rise of "athleisure" apparel; such footwear saw sales jump 8% in the year ending April 11, while performance shoes took an 18% dive. The PAC traces the decline to our schools, which help form our lifelong fitness habits and where kids are spending less time in gym class and recess. Another problem? The increasingly cutthroat nature of sports is forcing more casual athletes to the sidelines. Says Cove: "There are way too many kids who leave sports at age 9, 10, 11, because they simply have to make a decision: Am I going to be a travel soccer kid and devote my life to this, or are there other things that I want to do?" (An editorial argues that it's not lack of exercise causing an obesity epidemic, it's the food industry's unhealthy products.)