How to use an activity tracker to help with weight loss

Today's study out of JAMA (2016;316(11):1161-1171) has already been all over the news: "Weight Loss On Your Wrist? Fitness Trackers May Not Help" (NPR.org) and "Wearable Activity Trackers May Not Help You Lose More Weight" (Fortune.com) are two of probably dozens of stories on the article.

In case you've missed it, briefly: 470 overweight and obese women between the ages of 18 and 30 were assigned reduced-calorie diets and increased exercise for a two-year period. After the first six months, half the participants were given fitness trackers to wear on their upper arms. Both groups used a diet and fitness tracker website, but those who wore the fitness trackers used the device manufacturer's website while the control group used the researchers' website. At the end of two years, both groups lost weight, but the control group lost more - 5.9 kilograms (about 13 pounds) on average, as opposed to the device wearer's 3.5 kilograms (about 8 pounds).

While some news articles have pounced on this study to assert that fitness trackers may actually hamper people's attempts to lose weight, authors conclude, appropriately, that the use of fitness trackers may not be as effective in "changing diet or physical activity behaviors compared with what was achieved [in the first six months of the study]." It's this difference between the first six months of the study and the second six months of the study that interests me: the authors had access to the participants' diet and exercise records as well as the data from the fitness trackers.

In the first six months of the study the control group lost an average of 9.4 kilograms, while the fitness tracker group (who, mind you, did not have fitness trackers yet), lost an average of 8.4 kilograms. Both groups reported being slightly less sedentary in their daily lives in those first six months, but strangely enough, while the control group increased their amount of light physical activity (LPA) significantly (by about 3 hours per week), the fitness tracker group only increased their light physical activity by about an hour. Worse yet, the control group increased their moderate-to-vigorous physical activity (MVPA) by almost 70 minutes per week while the tracker group actually decreased their amount of MVPA by 30 minutes! Once again, this was before anyone was given a fitness tracker.

The second six months of the trial are even worse: the control group could not sustain that greatly increased level of physical activity, with their weekly minutes of LPA being only 14 minutes greater than they were at the start of the study. Those wearing fitness trackers actually decreased their LPA even more significantly: by 76 minutes over their amount of physical activity at the start of the study. In terms of moderate-to-vigorous physical activity, while the control group was still doing more than they had at the start of the study, their increase was now only about 9 minutes more - not 3 hours as it had been for the first six months. Those wearing the fitness trackers further cut their MVPA - now by almost 80 minutes.

In the third six months of the study the numbers for the control group go back up almost to that of the first six months for their levels of light physical activity, but their levels of MVPA never get back to more than about half of that first six-month push. Those wearing fitness trackers managed to improve their levels of LPA but their MVPA remains lower than it was at the start of the study.

The upshot of all this is that yes, the control group lost more weight even though the group wearing the fitness trackers consistently ate fewer calories than those who did not wear a fitness tracker. So does wearing a fitness tracker actually hamper weight loss? I think the jury is still out: the difference in activity levels between the two groups in the first six months of the study are significant enough that I wonder how effective the initial behavioral (group diet and exercise counseling) weight loss sessions really were. Certainly it's possible that those wearing the trackers may have succumbed to a bit of magical thinking: I've seen that some of my patients proudly display their fitness devices as if simply wearing the device meant they were exercising more.

What this means for you

If you're going to wear a fitness tracker, first, take its accuracy with a grain of salt. Instead, challenge yourself to meet a target, whether it's more calories burned than yesterday or a certain number of steps. Use it as a tool to keep you focused on your physical activity goals and to stay motivated. You'll still be healthier whether the tracker helps you lose more weight or not.

First posted: September 28, 2016