Fitness trackers put to the test inside a metabolic chamber

ABC News' Mara Schiavocampo shares more from her experience inside a metabolic chamber.
4:42 | 03/29/17

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Transcript for Fitness trackers put to the test inside a metabolic chamber
metabolic chamber this morning. We're seeing how fitness track and calculators stack up. Mara spent 23 hours in that room and you have more information. Good morning, guys. There are tons of app, products and websites that claim to be able to tell you your metabolism, bmi and how many calories you're burning but just how much can you count on those calculations and could they actually be derailing your weight loss efforts? They're the tech tools for weight loss. Online calculators and wearable trackers measuring your fitness and calorie burn. But just how accurate are they? Everything online equations and, you know, devices, they only predict an estimate so it's a rough guesstimation. Reporter: We compared their findings with the most precise measuring device available. This room, the metabolic chamber at mt. Sinai St. Luke's hospital. I spent 23 hours inside. I'm stuck in here for the next day. To determine exactly how many calories I burned. By measuring my oxygen consumption and carbon dioxide production. First up my resting metabolic rate. Basically my metabolism. The calories my body burns just to function. Handling things like breathing and pumping blood. I use three popular online calculators that look at things like your height and weight. One finding my metabolism is 1 W. 346 challengeries a day. Another finding it's 13,45. One calculating it's 1670. But the chamber says for me those are all off. My true resting metabolic rate almost 1800 calories a day. That shocks me. Much higher than I expected it to be. If this is what you do when you're not eating enough over time your body will slow down its metabolism. Reporter: Next calculating my bmi considered one of the best indicators of weight-related health. According to online calculators mine is 25.6 in the overweight category. But after determining my exact body fat percentage using this body pod, doctors atst sinai St. Luke's found my real bmi is actually lower, 23.4, well within the healthy range. Why? Because online calculators don't account for muscle mass. Just take a look. These two people have the same bmi, but clearly very different body compositions. It's very, very important but it has major limitation. Reporter: Last we compare our results with three popular fitness trackers. According to the chamber I burned a total of 3,240 calories in 23 hours. But all of the trackers significantly overestimated that number. One off by 200 calories, another off by 300. The last saying I burned 624 calories more than I actually had. People need to realize that they may be burning less than when they actually think they are. Now, since we first started airing these pieces three weeks ago a lot of people have been reaching out to me asking if they can do this. The bad news is it is very expensive and it's only in new York. The good news is there are places around the country that offer similar services, you have to search for metabolic testing and you can find places locally that will help you. Explain again how the chamber work. Basically it's measuring how you're breathing so it's your oxygen consumption and carbon dioxide production. Since it's vacuum sealed it is measuring temperature and humidity and giving you a completely accurate analysis without any predicting. Really interesting on the bmi. Put those two images next to each other. If you're muscular you might be better off than you think. Bmi is fascinating. Only measuring height and weight and doesn't take muscle mass into account. Muscle is heavier than fat so a body builder may have the bmi of one who is obese when clearly they're the picture of health. It is just one indicator of health. You can't put too much stock in those numbers. Doesn't paint the whole picture. That's going to be my excuse. Sticking to it. Sticking to it but I'm our Kus for you. You got the results. Are you doing anything differently. I am. You know, the biggest thing for me was how many calories I'm burning. They told me I wasn't eating enough and slowing down my metabolism. That's great news, right? That's great news. If you find out you can eat more. I like that. Mara was saying literally you doubled your portions of healthy food. If I might have a four-ounce serving of salmon now I'll have nine ounces and full day day. It's fantastic. Doesn't mean double the cookies.

This transcript has been automatically generated and may not be 100% accurate.

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