Stepping Off the Scale

People who are obese walk with surprising economies of scale. Both the stress they place on their knees and the energy they expend per kilogram are less than proportional to how much excess weight they carry, according to recent studies.

The finding suggests a way to help heavyset men and women lose weight, though with a possible drawback. The authors of the new work say that obese people might force themselves to walk in ways that take more energy, which would help them lose weight but might also elevate their risk of developing osteoarthritis in their knees.

Being obese obviously adds to the mass that someone must move while walking. But the condition burdens locomotion in other ways. For example, obese people have relatively heavy thighs, so swinging a leg forward with each stride requires considerable effort. Their thighs also tend to be thick, which forces them to swing their legs wide to keep them apart, and their footsteps tend to fall farther apart from side-to-side, which means that a good deal of their motion does not translate into forward momentum.

Those characteristics make walking more energetically expensive for the obese than for people of normal weight—not only overall but also on a per-kilo basis—according to locomotion researcher Rodger Kram of the University of Colorado in Boulder.

Studies that simulate obesity in nonobese people suggest that putting on pounds substantially increases the metabolic cost of walking. When lean women walk with heavy, bulky gear strapped to their legs and bodies, Kram says, “their energy costs skyrocket.”

But in recent experiments, Kram and his University of Colorado colleague Raymond C. Browning found that obese women somehow avoid paying most of the expected penalty. The heavy women seem to have learned to carry their bodies efficiently.

That’s unfortunate in one sense, Browning says, because high metabolic efficiency means that some calories get conserved when they’d otherwise be burned. By understanding how obese people conserve energy and by encouraging them to alter their walking motion to neutralize energy-saving adaptations, the researchers say they might be able to promote weight loss.

In their study, Kram and Browning asked 10 obese women and 10 women of normal weight to walk at various speeds on a treadmill for 5 minutes at a time. During the trials, the researchers monitored oxygen and carbon dioxide in each volunteers’ breath and computed how many calories she burned at her most energy-efficient speed. That speed corresponded closely to the rate that each woman chose to move on a level sidewalk.

In the May Obesity Research, the researchers report that, compared with the other volunteers, the obese ones spent only 11 percent more calories per kilogram when moving at optimal speed over a given distance. Given the difference in size between the groups, the metabolic cost could have been more than 100 percent greater for the obese women, the researchers say.

“That suggests [the obese volunteers] are changing their gait to conserve energy,” Browning says. He’s now using video cameras and other equipment to test that idea by comparing the gaits of the obese and nonobese volunteers.

Paying the price

Paul DeVita, a biomechanist at East Carolina University in Greenville, N.C., says he’s surprised by how small the metabolic cost increase is, but he doesn’t question the findings. He says he suspects that people change their gait subconsciously as they become obese. Energy efficiency may not be the only motivator, he adds.

In an earlier study, DeVita and Timor Hortobágyi found that a person who weighs twice as much as another experiences no greater torque in the knees when both walk at the same, reasonable speed. When the obese volunteers were allowed to walk at a slower speed, which they found more comfortable, they exerted even less force on their knees than people in the other group did at the faster pace. Greater force on the knee had previously been presumed to explain why obese people are susceptible to osteoarthritis in that joint.

Furthermore, DeVita and Hortobágyi reported in 2003, obese volunteers walked with shorter strides and straighter knees than did other people. Those behaviors reduce certain stresses on the joint, DeVita says.

DeVita and Hortobágyi’s findings suggest that teaching obese people to use a less energy-efficient gait might not be a good idea, assuming it’s even possible, Browning says.

For example, walking quickly burns more calories than does covering the same ground at a comfortable speed. But since it also increases the skeletal impact of each footfall, it might raise the risk of osteoarthritis, he says. Walking slower than normal reduces joint stress, but it also cuts down cardiovascular benefits.

However, certain motions might enable obese people to increase their metabolic output without putting undue strain on their joints. “We may be able to get around their energy-conserving mechanism by making them walk uphill,” Browning says.

That could offer a second benefit, as well. Compared with walking downhill or on a level plane, DeVita says, “walking uphill has a gentler impact.”

Other research suggests that uphill and downhill exercise have different effects on health and that uphill workouts improve the body’s processing of cholesterol (SN: 12/11/04, p. 380: Available to subscribers at Up and down make different workouts). Further studies could show that climbing slopes brings people closer to the peak of metabolic health.