The idea of using a fitness tracker is that you should pay more attention to the amount of exercise you get and, therefore, be more likely to get enough. This should support weight loss in people who are carrying too much weight.

A recent study appears to question that basic assumption with evidence. The Times reported the story (on the front page) with this headline:

Fitness trackers can make you fatter.

Here is how Gizmodo reported it:

In a study published on Tuesday, researchers found that fitness trackers weren’t the boon to longterm weight loss they expected them to be...

The study put overweight people on a standard diet for 6 months and then split them into two groups: one with fitness trackers and one with just an internet fitness diary. At the end of another year and a half:

...all of the volunteers returned to the lab to repeat their measurements from the start.

Most were thinner now than at the start of the study (although many had regained some of the weight that they had lost during the first six months).

Those who had not worn activity monitors were, on average, about 13 pounds lighter now than two years ago.

Those who had worn the monitors, however, weighed only about 8 pounds less than at the start.

So having a tracker seems to make you fatter (compared to the study control group).

This is a pretty unexpected result. Is it right?

  • 2
    Please be aware that unreferenced answers, theoretical answers and original research are not allowed on this site, and are deleted even if highly upvoted.
    – Sklivvz
    Commented Sep 22, 2016 at 18:56
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    "The tracker does not make you fatter, rather it was observed that those who used the tracker lost less weight. With a 99.8% certainty, those who used a tracker lost less weight." Please edit your question accordingly.
    – Mazura
    Commented Sep 22, 2016 at 21:21
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    @Mazura The headline and the question accurately report the way the story was reported in the media (which may count as clickbait but so do newpaper headlines). There is no point in editing the question to correct the claim: that is a job for answers which can point out that the headlines are a poor summary of the science.
    – matt_black
    Commented Sep 23, 2016 at 7:21
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    I don't get it. The title asks if they make you fatter, but nowhere in the question this is stated. Actually the opposite gets mentioned even: They lost weight. So click-bait has once again become a thing on StackExchange? Sigh ...
    – Num Lock
    Commented Sep 23, 2016 at 9:28
  • 5
    "Do fitness trackers make you fatter?" It depends on how many you eat. Commented Sep 24, 2016 at 2:54

2 Answers 2


No. Using a tracker does not make you fatter, and that is not what this study claims.

Lets start with the participants:

Among young adults with a BMI between 25 and less than 40

(so no already fit people here).

And this important little note:

If weight loss exceeded 6% during each 4-week period or if BMI was 22 or less, prescribed individual calorie intake was increased.

Accourding to the U.S Department of Health 22 is just under overweight (25 being overweight) and most people trying to lose weight wouldn't suddenly increase their caloric intake if their bmi hit 22. So participants were actively monitered and restricted in the type of weight loss they could obtain. This applied to both groups equally, but is important to note.


During months 7 to 24, participants in the standard intervention group self-reported their daily MVPA using a website designed for this study, and this information was available to the staff during the intervention telephone contacts. Participants in the enhanced intervention group self-monitored their MVPA using the technology described below.

Combined with:

Of the 237 participants randomized to enhanced intervention, 191 participants received the wearable device that was a component of the intervention starting after month 6 and wore the device for 1 day or longer (median days worn, 170.0...On days that the device was worn, the median wear time was 241.1 min/d ...).

And part of the graph of results: (I couldn't actually save the whole thing as anything but powerpoint, go figure)

enter image description here

Makes it clear that those that were asked to self-report were significantly more diligent in their reporting then the group with the watches were in their self-monitoring. This shows up in the amount of total exercise over the last 12 months, the caloric intake and in the final results.

Then there is the time period, it's funny because the data collection was finished in 2014, but the study initially started in 2010. Now neither of those years seem very long ago, but in terms of this technoligy, those years describe generations for both hardware and software .See the 2nd gen Applewatch or latergen Fitbit for just how far and fast this tech has come in reliablity to monitor fitness in 2 years.

Finally this was a general study and the author himself stated it best when he said there were limitations to the study, namely:

Additional investigation is also needed to examine for whom wearable devices and other technologies may be effective within the context of weight loss efforts and how these technologies influence other components of weight loss, namely, eating behavior and dietary intake.

As with all tech, it's not a silverbullet one size fits all, it will motivate and push some, while others will still benefit more from the more traditional method of reporting their progress.

Study can be found here.

  • What is 22%? BMI is the ratio between weight and height (in meters) squared. There is no "times 100" anywhere...
    – Bakuriu
    Commented Sep 23, 2016 at 9:47
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    A BMI of 22 is not "just under overweight". It is lies in the middle of the normal weight range [19-25]. Going below that when starting above 25 (and doing it fast) is more of a malnutrion symptom and unhealthy (weight loss of 12+% in a rather short time).
    – Chieron
    Commented Sep 23, 2016 at 15:53
  • @MrLister, Bakuriu, my mistake not the studies, sorry. Have a 1 year old am used to being told where her weight/height ratio put her percentile wise. I removed the % sign the numbers are the same though.
    – Ryan
    Commented Sep 23, 2016 at 16:19

"The idea of using a fitness tracker is that you should pay more attention to the amount of exercise you get and, therefore, be more likely to get enough. This should support weight loss in people who are carrying too much weight." ...The above statement is erroneous and assumes that exercise alone will cause weight loss. If a person exercises but then eats the calories he just burned, then he won't lose weight.

In the study, people that kept track of their food intake lost more weight than people with a fitness tracker. People that utilized a journal lost 13 pounds. People that used a tracker lost 8 pounds. Both methods were effective at weight loss. In this study, keeping track of food intake appears to be a more effective way to lose weight. The group that kept track of food intake had a larger calorie deficit than the group that used the trackers.

  • It assumes, that people will change their energy input less or negligible in compare to the change of the output. My eating behaviour is not affected much when doing nothing (go to work by bus) or biking there (2x20 minutes). I am skinny and stil have some reserves so I feel the need of diet change only if the output is exceeding the average for longer time (days). I suppose that the tested people changed their outputs slowly without stressing the income, so the body didn't treat is as shock and started to burning reserves (fat).
    – Crowley
    Commented Sep 22, 2016 at 18:46
  • To fullfill your contradiction, the testee must have estimated his extra output and reward himself with the extra cake of simmilar energy. Otherwise, they are losing weight.
    – Crowley
    Commented Sep 22, 2016 at 18:48

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