Runners cast shadows as they make their way east on Lafayette Avenue during the 2016 TCS New York City Marathon. (Drew Angerer/Getty Images)

A new report, which calls itself the largest-yet analysis of U.S. road race results, has concluded that “American runners have never been slower.”

The report, based on more than 34 million individual race results from 1996 to 2016 and published on the Danish website RunRepeat.com, focuses on marathon results. It finds that the average American marathon time has slowed from about 4:15 to about 4:40 over the 20-year period.

A principal author of the report, Jens Jakob Andersen, says he was watching the Copenhagen Marathon several years ago, and was surprised by the number of five- and six-hour finishers. He wondered whether U.S. marathoners were similar, and whether they had always been as slow. Andersen and co-author Ivanka Nikolova, who has a PhD in mathematics, spent four months gathering U.S. race results, and analyzing trends.

A road race trade organization named Running USA has been compiling similar data since 1980, when the typical male American marathoner finished the 26.2-mile distance in 3:32:17. According to Running USA’s most recent analysis, the equivalent time in 2016 was 4:22:07.

But why the slow-down?

American marathoning has changed dramatically over the past three or four decades, thanks in large part to the women’s running boom. In 1980, Running USA estimates that only 10 percent of marathon finishers were women. Last year, that figure reached 44 percent. Because women are, on average, 10 percent slower than men, more women participants will necessarily slow the average times. (Most marathon experts have tied slower marathon times to the increase in women and in less-serious racers.)

The RunRepeat report, however, says that slowing men have contributed more to the decline (54 percent) than increased participation by women (46 percent).

It also asserts that casual, back-of-the-pack runners have slowed only slightly more than those in the front of the pack.

Ken Young, dean of worldwide road race statisticians, says those casual racers share more of the blame than the RunRepeat report shows. Young has been compiling race data since the mid-1970s, and serves as unofficial director of the global Association of Road Race Statisticians. “Average times are slowing because more and more races are emphasizing their social aspects,” Young says. “They seek to attract recreational runners and walkers. Look at the race websites. You can find all sorts of social media links, party details and merchandise for sale, but it’s hard to find the race results.”

Young’s view is shared by Rich Harshbarger, chief executive of Running USA. “Events have become so much more social,” he notes. “Look at the races produced by Walt Disney World. You can meet a Disney character and get your photo snapped almost every half-mile. In today’s market, event producers position their races more as festivals than traditional races.”

While average times are getting slower, American elites have continued to improve. In 1996, the American marathon records stood at 2:10:04 (men) and 2:21:21 (women). Today they are 2:05:38 and 2:19:36.

Many of the best Americans enter the Boston Marathon each April. Due to Boston’s tough qualifying standards, rare among big marathons, only about 10 percent of all U.S. marathoners run times fast enough to enter Boston. Yet it has also gotten slower, on average, over the past two decades. In 2000, according to MarathonGuide.com, the average finish time at Boston was 3:41:39. Three months ago, in the 2017 Boston Marathon, the average finisher crossed the line in 3:58:03.

“In recent years, the general emphasis among marathoners has shifted from performance to participation,” acknowledges Tom Grilk, chief executive of the Boston Athletic Association, which organizes the Boston Marathon. “If that means more people are running for their long-term health and fitness, it’s a trend I applaud. I think it’s healthy that fitness ‘sustainability’ is more important than speed.”

The RunRepeat report notes that the slower American marathon times coincide with increases in obesity, diabetes and medical expenses, as reported by the Centers for Disease Control and Prevention but stops short of claiming causation. “I would love to say that obesity is the cause for the slow-down, but I cannot,” Anderson admits. “We can only point out the correlations.”

Most runners understand that the more they weigh, the slower they will run. The imprecise formula is often given as two seconds per pound per mile. That means that a marathoner who gains 10 pounds over 20 years will run eight to nine minutes slower for 26 miles. Although data gathered by the National Runners Health Study indicates runners gain weight at only about half the rate per year of non-runners, the pounds can still add up over time.

Writer-nutritionist Matt Fitzgerald, author of “Racing Weight: How to Get Lean for Peak Performance,” has observed that the runners he counsels generally have the same nutrition problems as non-runners. “The biggest one is that they eat too many processed foods, including pastries, chips, fried foods and sweetened beverages,” he notes. Fitzgerald has created a RacingWeight.com calculator to help runners determine their optimal performance weight.

Mayo Clinic health and endurance expert Michael Joyner has been following marathon trends for more than 40 years. He notes several reasons for slowing marathon times. First, most runners no longer aspire to performance goals, as they did in the 1970s and 1980s. Second, the marathon has become a “suburban Everest,” where the goal is to reach the summit rather than to test one’s limits. And third, a kinder-gentler zeitgeist encourages a broader range of body types to enter marathons. “There aren’t a lot of people who want to run more miles, add interval training and lose weight,” says Joyner. “But that’s what it takes to run faster marathons.”

This story has been updated to correct the country of the website RunRepeat.com. It is Danish, not Dutch.