Heavy and "obese" people read headlines recently urging them to "Walk Slowly For Weight Loss". The *news*, which made headlines throughout the mainstream media including CNN, Yahoo, Science Daily, and MNBC, originated from a study and subsequent press release from the University of Colorado, Boulder June 14, 2005. The study's authors urged the exact opposite advice of what people have tried to adhere to for years. Reliable sources including the American Medical Association (AMA), the American Obesity Association (AOA), The American Heart Association (AHA), among others, have long encouraged everyone to "walk briskly!" We have been told to "pick up the pace from leisurely to brisk", not only to improve overall health , but to decrease the risks of cardiovascular disease, stroke, and diabetes, as well as to lose or maintain healthy weight.
The headlines referred to research published in the May issue of Obesity Research titled: "Energetic Cost and Preferred Speed of Walking in Obese vs. Normal Weight Women", by Raymond Browning, Roger Kram and others. Here's how the CU's press release summarized the researchers results, quoting Ray Browning, the lead author (emphasis ours):
"The message is that by walking more slowly, obese individuals can burn more calories per mile and may reduce the risk of arthritis or joint injury."
We found the study in order to look at the original results. According to the CU press release, the authors started with the hypothesis that:
"Obese adults would have a greater energy cost when walking [based] on previous studies by Kram's lab team. In one study, energy expenditure increased by about 25 percent when normal-weight people walked with a deliberately wider stance"
Based on these previous results, authors expected that further research would show that heavier people walked more slowly then lighter weight people, with higher energy cost and commensurately greater cardiovascular effort. Instead, Browning and Kram found that the two groups walked at similar speeds. Surprisingly, the heavier group only burned only 11% more calories per pound. Browning and Kram had theorized that the heavier group would use at least 100% more calories per pound.
Said the Browning:
"This was a surprise...The subjects probably are unwittingly altering their posture and walking with straighter legs, conserving calories in the process."
Here's an approximate graph of the results. It differs slightly from the graph published by the study because the authors used "2.5" as the starting Y axis value, instead of "0". The results (our lines are based on theirs, which are second-order least squares regression calculations), show that as the walkers increase their speed they decrease the energy they use until the optimal "preferred speed" for walking is achieved. This "preferred speed" was reached at 1.47m/s for the normal group and 1.4m/s for the obese group. These speeds represent approximately the points where the walkers cover the greatest distance using the least amount of energy.
Interestingly, both the thinner and the heavier subjects used more energy at slower speeds. So why then, did the researchers only recommend that obese subjects walk more slowly? According to the authors it was because the heavier cohort were the only ones at risk for osteoarthritis. The authors indicate in their press release that their research shows this risk. However nowhere in the original study was this question addressed except for a brief reference to a paper from different authors.
However the research group whose work Browning and Kram referenced, had done work only with a normal weight cohort, not with the obese who walked more efficiently. So in essence the basis for their press release is not their own research, at least not published research, but their approximations based on another lab's study. This is all very vague in their paper and completely obscured in the press release. The study that they reference carefully noted that their results were relevant to "normal" healthy people. This is important because according to co-author Kram:
"As people become gradually obese, they also seem to become particularly graceful...there appears to be some sort of a physiological drive for them to minimize the amount of energy they expend."
In other words, not only is the obese cohort that Browning and Kram study not "normal" but according to the authors, they walk differently. So considering their very own observations, should they be generalizing the results of the other study of normal weighted people without at least doing the research and submitting it for peer review? The fact that obese might conserve energy by adjusting their gait is not a revelation, since previous studies, including ones we talked about with Nepalese porters here, show that humans often economize energy expenditure when walking with heavy loads. Like porters or African women who carry weight on their head, overweight people appear to alter their gait to conserve energy expenditure. That's the interesting result.
It seems like Browning and Kram jump the gun on the research, especially since other studies have found that the obese do not increase the stress on their knees when walking. This is because, again, as the authors concluded, there is a change in gait that seems to favor preservation in energetics. So why would this energy optimization, these self-preservation optimizations, extend to joint preservation and biomechanics as well?
There are two things that are curious about the news of this study. One is the conclusion of the authors. The second is how the research was portrayed in the media. It's hard to know where one ends and the other begins. There's potentially a point where researchers lose control of their words to the machinations of media. The authors admit the confusion about the results on the part of the media. However since the original press release is from the authors own university it's difficult to believe that the confusion between the undone biomechanics research and the kinetics data that was the subject of their published study couldn't have easily been corrected at the source.
Instead the misconceptions were seemingly *allowed* to propagate throughout the media, which risks misleading and misinforming people. Since the researchers are telling people to 'walk more slowly', which flies in the face of a bulk of public health research, it is important to know just how slow obese people are being advised to walk.
Notably, the authors advise that researchers have found disparate "preferred walking speeds" for men and women of various weights. The range varies from study to study, 1.18 m/s, 1.19 m/s, 1.09 m/s, or 0.75 m/s. So the "preferred walking speeds" are widely variable, but the media never modulates their conclusions to accommodate the potential variation. It's possible then, that obese people are already walking slowly. However since the authors conclude that only "obese" people, should "walk slowly" they isolate this population for their recommendation, which could encourage already slow walkers to slow down more and could impede the healthful benefits of walking. Should the public health community be concerned about recommendations that are so wide reaching yet whose benefits may be inconclusive?
Further clouding the result, the researchers tested the walkers for only 5 minutes. They then extrapolated this result to a 45 minutes walk, which was in turn extended to an hour or hour and a half in the media reports. It is quite possible that over longer periods of time the energetics of walking would change. Five minutes of walking is vastly different (especially for an obese person) than 1.5 hours. Shouldn't more work be done at different time periods before publicizing health recommendations?
Finally, is osteoporosis the greatest health risk? What about all the studies that concluded that "brisk exercise" (not slow exercise) lessens health risks such as diabetes, cardiovascular disease, weight loss, cancer risk, and even osteoporosis? We are all concerned about public health and as citizens we deserve to hear more robust science based recommendations that will actually inform us, rather than just sell stories about obesity.
