In this investigative “Wellness Girl chat,” Dr. Kelsy Vick breaks down how fitness trackers estimate caloric expenditure—and why those numbers can be misleading for assessing workout effort, effectiveness, or weight loss. She defines calories (including the difference between food-label “calories” and kilocalories), explains total daily energy expenditure, and outlines what wearables typically model: basal metabolic rate (BMR) plus active energy expenditure, while often failing to capture thermic effect of food and non-exercise activity thermogenesis (NEAT). She reviews how BMR is estimated from static equations using age, sex, height, and weight (with limitations and common error ranges), then explains how active calories rely on proprietary algorithms driven largely by heart rate, supported by sensors like accelerometers, gyroscopes, barometers, GPS, and (for Oura) skin temperature—often switching sensor priorities based on the selected activity.

The episode also connects heart rate to oxygen consumption and energy expenditure, explains VO2 and the lab-based VO2 max testing process, and shows how wearables use heart-rate-based VO2 relationships to estimate calories. Dr. Vic compares common devices: Apple Watch (Move vs Total Calories, activity classification, VO2 feature accuracy cited within 4% when using VO2 integration), Whoop (step-free strain model, continuous heart rate monitoring, algorithm shifts based on heart rate reserve), Oura Ring (finger-based sensing, emphasis on resting metrics, temperature and sleep inputs for BMR, MET-based intensity categories with user-reported effort), Garmin (GPS- and pace-informed VO2 estimates, cycling power-based calculations, with studies showing wide accuracy ranges and underestimation at low and very high intensities due to heart-rate lag and anaerobic work), and Fitbit (older models leaning more on accelerometer data, newer models incorporating heart rate).

Dr. Vick summarizes research findings showing large real-world error in wearable calorie estimates, including a Stanford Benchmark study (most accurate device averaging 27% off; least accurate 93% off, with factors like skin color and BMI affecting results), a 2025 head-to-head comparison reporting varying under- and over-estimation across devices, and a systematic review where no brand consistently met acceptable accuracy limits. She notes that resting heart rate and step count are generally more accurate than calorie burn estimates, while sleep duration is moderately accurate and sleep staging is weak. The takeaway: calorie metrics may be useful for broad trends, but shouldn’t drive dietary or training decisions; she recommends emphasizing other wearable metrics such as heart rate, VO2/VO2 max, heart rate variability, and recovery instead.

Resources:

https://www.empirical.health/blog/apple-watch-calories-accuracy/

https://www.apple.com/health/pdf/Heart_Rate_Calorimetry_Activity_on_Apple_Watch_November_2024.pdf

https://pmc.ncbi.nlm.nih.gov/articles/PMC5738849/

https://pmc.ncbi.nlm.nih.gov/articles/PMC9950693/

https://blogs.sas.com/content/efs/2025/06/25/can-you-trust-your-smartwatch-a-deep-dive-into-calorie-burn-estimates/

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00:00 Calories Culture Backstory

02:49 What Trackers Claim

04:05 What Is A Calorie

04:51 TDEE Basics BMR Active

06:41 BMR Formula Limits

08:28 Sensors And Algorithms

11:55 Heart Rate And VO2

12:55 VO2 Lab Gold Standard

15:35 Apple Watch Breakdown

17:14 Whoop Strain Model

18:57 Oura Ring And METs

22:14 Garmin VO2 And GPS

26:03 Fitbit And Accuracy Studies

29:01 How To Use Calories

31:08 Wrap Up And Takeaways

Let's do a little investigative journalism and Wellness Girl chat

2

about calories and fitness trackers.

3

Welcome back to Wellness the Pod.

4

I'm your host, Dr.

5

Kelsey Vic, a board certified

orthopedic doctor, physical therapy,

6

and a pelvic floor physical therapist.

7

And if you're like me, you have

come from an era hyper-focused on

8

calories and caloric expenditure.

9

It was the only thing I used to track on

MyFitnessPal back in college to make sure

10

I didn't gain the freshman 15 or have

any weight gain from being out on my own.

11

I feel like so much of the

messaging back then was fear-based

12

messaging about weight gain.

13

When you go out on your own and are

having to cook and you're a little

14

more stressed in college and you're

having to exercise on your own, you're

15

no longer in high school sports.

16

Like it was all fear-based messaging

around keeping your calories and

17

your energy expenditure relatively

level so that you did not become

18

a victim of the freshman 15.

19

As a side note, I had a TA in college.

20

I loved her, she was from Canada,

and she said when she came to

21

Texas she was unaware of how much.

22

Chips and salsa and queso was served at

every meal at Tex-Mex restaurants, and so

23

she called it the Texas 10 for her because

it was so much chips and salsa and queso

24

that she was not used to and would just

25

devour at every meal.

26

So she called it the Texas 10,

but whatever you called it, I feel

27

like calories were such a focus.

28

And back when I was in college,

Fitbit was like the big thing.

29

So I would always see what my

caloric expenditure was on Fitbit and

30

compare that with my MyFitnessPal.

31

And this was not a healthy

way to do it at all, but.

32

It was what was common at the time.

33

A lot of girls were using MyFitnessPal

and Fitbit and trying to make those

34

calories equal or even less and go

more into a caloric deficit in order to

35

maintain what they thought was a healthy

weight or a healthy body composition

36

for this new stage in our lives.

37

So I thought it would be fun to actually

look at how different fitness trackers,

38

common fitness trackers today actually

look at and calculate caloric expenditure.

39

I know Apple has moved calories.

40

Whoop has strain.

41

So today we'll dive into some of

the context into how different

42

fitness trackers actually

estimate caloric expenditure.

43

And it's not the same across the board.

44

They use different algorithms

and different formulas to

45

get caloric expenditure.

46

We're gonna talk about why it differs so

drastically compared to what's actually

47

happening within our bodies and why we

shouldn't use calories as measured on a

48

fitness tracker as the primary objective

measure that we're looking at, especially

49

as it pertains to workout effectiveness,

effort of the workout or weight loss.

50

So I know there are so many

fitness wearables out there.

51

The ones we'll talk about today are the

common ones, I would say today, or a ring.

52

Whoop Apple, watch Garmin and Fitbit.

53

Which I don't see as many Fitbits

nowadays, but I still figured since

54

it was kind of the OG one that I

feel like a lot of people knew.

55

I'd still put it in there just in case.

56

Someone was curious on how their old

Fitbit and how some of the newer Fitbit

57

models track caloric expenditure compared

to more popular fitness wearables today.

58

So let's go over a few like basics.

59

starting with what even is a calorie.

60

So a calorie with a big C is the

energy to heat one gram of water

61

by one degree Celsius at standard

pressure, which equals about 4.184

62

joules.

63

Such a complicated definition

for what a calorie is.

64

in food labeling and nutrition

labeling, when we talk about a calorie

65

with a little C, what we're really

referring to are kilo calories.

66

And kilo calories represent the energy

your body gets from oxidizing nutrients

67

like carbs, proteins, and fats.

68

For example, a 250 calorie item

on a label means 250 kilo calories

69

or 250,000 small calories Now what

are the basics behind how fitness

70

trackers actually measure calories?

71

So every fitness tracker

estimates the total daily energy

72

expenditure using two variables.

73

And remember, energy expenditure is a

fancy term for calories expended because

74

calories is a unit used to measure energy.

75

So those two variables that go

into total daily energy expenditure

76

are your basal metabolic rate.

77

The calories required to sustain

basic life functions at rest.

78

Basically breathing, digestion,

circulation, cellular repair,

79

which typically accounts for about

60 to 75% of daily calorie burn.

80

And the second variable is

active energy expenditure.

81

So the additional calories burned

through physical activity and

82

movement from walking around

the house to intense exercise.

83

What fitness trackers

don't actually track.

84

That also goes into our physiologic

total daily energy expenditure are

85

what we call thermic effect of food and

non-exercise activity thermogenesis.

86

So the thermic effect of food

accounts for about 10% of daily

87

caloric energy expenditure, and it's.

88

The amount of calories used to just

digest the food that we're consuming.

89

And then neat or non-exercise activity.

90

Thermogenesis includes fidgeting,

gesturing while talking.

91

This is actually really cool.

92

There's studies done on

even, , like a soleus raise.

93

So like when you're sitting and

just lifting your heel up and how

94

many extra calories that burns.

95

For people who are just

fidgeters, naturally fidgeting.

96

So the fidgeting, the hand gesturing,

all of that goes into this neat or

97

non-exercise activity thermogenesis,

which fitness trackers do not track.

98

It's a lot harder to track hand

gestures and movements and little

99

fidgets throughout the day with

something you wear on your wrist or

100

your fingers, so that makes sense.

101

But those are the two variables that

fitness trackers don't accurately

102

estimate or include into their

calculations for total energy expenditure.

103

So BMR or basal metabolic rate.

104

most fitness trackers estimate this

from a few well-known equations based

105

on your age, sex, height, and weight.

106

So a lot of the time when you're setting

up an account, this is what you will

107

enter, and this helps to estimate your

BMR And there's a few standard equations.

108

There's differences amongst all of them,

but there's a few, I guess, well-known

109

equations that estimate BMR based on the

variables that you enter when you create

110

your account on these fitness trackers.

111

However, even these equations have their

own drawbacks, oftentimes citing high

112

accuracy for the given value of your BMR

within about 200 calories, plus or minus

113

So there's a high amount of accuracy

within a 200 plus or minus calories of

114

what you're actually expending each day.

115

So the formulas that help estimate BMR

that are a part of these fitness trackers

116

already have some limitations to them.

117

So a few drawbacks for some of these

equations that these fitness trackers use

118

is number one, it's a static equation.

119

our BMR is affected by a

lot of different factors.

120

Things like our hormones, our sleep, our.

121

Stress are environmental temperatures.

122

So it's not this static measurement

that we can just copy paste every day.

123

It fluctuates day to day, week by week.

124

It's not this static measurement.

125

So that's one of the

drawbacks of these equations.

126

And another drawback is that the

studies that are been used to come up

127

with these equations aren't inclusive

of all variations in our genetics.

128

So these formulas basically already

have their limitations and these fitness

129

trackers are using these formulas, which

are researched and are the best ones when

130

it comes to what has been researched.

131

But since they're already using

these to get your BMR you're already

132

starting from this limited viewpoint

of your actual caloric expenditure.

133

So the second variable that fitness

trackers track when it comes to

134

total daily energy expenditure is

your active energy expenditure.

135

So this is that movement on top of

what is required to sustain basic life.

136

So active calorie estimates are really

the bread and butter of these fitness

137

trackers, and they are very secretive

on what their algorithms are because

138

that's usually the r and d that has

gone into creating some of the best

139

algorithms for their fitness trackers

based on the sensors involved and based

140

on the data that they have from all

of us as fitness wearable consumers.

141

So it's very secretive, but there's a few.

142

Tools that a lot of these fitness

trackers use to help estimate

143

active energy expenditure.

144

most of them use a combination

of an accelerometer, which is

145

movement and velocity changes.

146

So this is something like step count

they oftentimes use an optical heart

147

rate sensor, so this is that like

red flashing light that picks up

148

heart rate via light absorption.

149

Some of them have a gyroscope or

a tracker sensor for rotational

150

movements, especially used in activity

classifications like swimming or

151

different rotational movements of the arm.

152

Especially, a lot of the time the

trackers are primarily wrist based, or

153

if it's the R ring, it is finger based.

154

So different rotational movements

sensed via the arm typically.

155

A barometer, which helps detect

incline decline, A-G-P-S-G-N-S-S,

156

geolocation, outdoor pace and distance

estimator, and then skin temperature.

157

So general metabolic state, and

this one's primarily used by Aura.

158

So these are all of the different

sensors that fitness trackers can

159

utilize to estimate total daily energy

expenditure or caloric expenditure.

160

And these aren't all included in all

sensors, but these are just sort of

161

the basic categories of sensors that

are integrated into fitness wearables.

162

what I found really cool based on my

research was that the reason they have you

163

choose different activity levels whenever

you're recording a workout is because

164

their algorithms will switch to different

sensors and prioritize those sensors

165

or the data coming from those sensors

more than data coming from other sensors

166

based on the activity that you're doing.

167

So.

168

If you're doing something like swimming,

they're going to use the data coming

169

from the gyroscope more, or weight

that heavier than the data coming from

170

the barometer or the incline decline.

171

So if you think about it.

172

It makes sense to weight certain

data a little bit higher based on

173

the activity that you're doing.

174

So this might be why some of those

Caloric estimates change once you

175

change your activity after the fact.

176

So if you just start exercising without

actually going in the app change or going

177

on your watch, changing the activity

that you're doing and then performing the

178

workout and then stopping the workout,

if you go in after the fact, this is why

179

some of those estimates might change,

is because their algorithms pull or.

180

weight certain data heavier based on

the activity that you're doing, which

181

I thought was actually pretty cool.

182

It makes sense.

183

But once I realized like, oh yeah,

of course they're getting this data

184

from so many different sensors and

it makes sense to weight data coming

185

from the gyroscope for swimming higher

than step count data, of course.

186

So I thought that was pretty cool.

187

But overall, heart rate is typically

the primary driver of caloric

188

expenditure across all models.

189

they will weight other data a little

more depending on the activity.

190

But overall heart rate is one of the

primary factors that goes into how these

191

fitness wearables and how this fitness

tech estimates caloric expenditure.

192

so, why is that?

193

Heart rate actually correlates

with oxygen consumption, which

194

correlates with energy expenditure.

195

And there's a formula for this, although

the formula is said to be more accurate at

196

greater than 50% of your max heart rate.

197

So again, another estimation formula

that has error built in that will affect

198

the accuracy of the caloric data that

these fitness wearables are giving you.

199

So oxygen consumption or VO O2 is a

measure of how much oxygen your muscles

200

can actually utilize, not necessarily how

much oxygen they're receiving, but how

201

efficient are your muscles at actually

utilizing the oxygen that is sent to them.

202

it goes a step further and takes into

account things like mitochondrial

203

density and how much your muscles

can actually utilize the oxygen

204

at more of a cellular level.

205

There's a lot of formulas that can look

at heart rate and estimate your VO O2.

206

And many of the fitness trackers use

a variation of this relationship in

207

order to estimate caloric expenditure.

208

But before we dive into that, let's

talk about the gold standard way of

209

measuring your VO two max so that we

know how these fitness trackers are

210

getting the estimate compared to that

gold standard testing in the lab.

211

So to measure VO O2, you typically are

using a treadmill or a cycle like a,

212

a stationary bike, depending on the

activity that you perform most often.

213

A lot of the time they're doing these

in elite athletes, and so specificity

214

is very important, but we'll use the

treadmill as an example, you'll put a

215

mask on and it's a very uncomfortable

mask that is tied up to a lot of different

216

sensors to measure and estimate different

gas concentrations of O2 and CO2.

217

So what you'll do is you'll start

running on a treadmill and over time the.

218

Treadmill will increase

in incline and speed.

219

So you're basically ramping

up intensity really quickly.

220

It's usually a pretty quick test because

your goal is to reach this relative

221

fatigue point or a certain ratio of.

222

Oh two to CO2 before the

test is actually terminated.

223

So you put the mask on, you run

on the treadmill, the mask starts

224

measuring different gas concentrations,

primarily CO2 as it's expelled.

225

As intensity increases

your heart rate rise.

226

You consume more O2 and expel more CO2,

at some point, the intensity of the

227

workout will exceed the amount of oxygen

that your heart and lungs can supply.

228

So CO2 will continue to increase

as you're expelling, air, as

229

you're exhaling and O2 plateaus.

230

So that test is terminated

once that O2 plateaus, but.

231

Ideal.

232

It also might terminate if the intensity

gets too high for the testing subject or

233

if that O2 CO2 ratio reaches a certain

number and then the test is terminated.

234

So it's a very challenging test.

235

It requires a good amount of lab

gear, a good amount of sensors.

236

So a lot of the time we just

try and estimate VO two based

237

on some of these formulas.

238

And just to link it back to this

entire episode, V two and heart rate

239

are linked together, and both of those

variables, along with certain other

240

variables and data coming from other

sensors are utilized to estimate caloric

241

expenditure in these fitness wearables.

242

Okay, let's dive in to each of the

different fitness wearables and if you

243

guys have one that I didn't mention.

244

I know Microsoft has one.

245

I know Samsung has one.

246

I'm sorry I haven't gotten to these, but

I feel like a lot of them have similar, I.

247

Technologies and ways of calculating,

but I'd be happy to do another episode,

248

bonus episode of just those devices.

249

If you guys tell me

which ones you utilize.

250

I haven't seen too many of my own

friends using like the Samsung or

251

the Microsoft or, I know I keep

getting ads for the new earrings,

252

so I'm interested to see that one.

253

I don't think it's out yet,

but if you guys have any other

254

ones that you'd like me to dive

into, I'd be more than happy to.

255

But today we'll just cover Apple

Watch Garmin Ora Ring, whoop, and the

256

Fitbit, starting with the Apple Watch.

257

So some of the key features move

calories on the Apple Watch.

258

Looks just at active energy expenditures.

259

So remember there's the BMR basal

metabolic rate, and then there's

260

active energy expenditure, and these

move calories on your Apple Watch.

261

Constitute the active energy expenditure.

262

Total Calories combines BMR and

Active Energy expenditure, and the

263

Apple Watch has a VO O2 integration.

264

So it's different from calories,

but it gives a good picture

265

of overall heart health.

266

so the VO O2 feature specifically has

been found to be accurate within 4% of

267

the actual value of your actual VO O2

if you're using that VO O2 integration.

268

So I think you actually have to go in

and do the VO O2 testing in order to get

269

that more accurate within 4% estimate.

270

But that's super cool.

271

So the Apple Watch basics, it uses the

heart rate, the optical heart rate sensor.

272

It uses the accelerometer, the

gyroscope, the barometer, and GPS data.

273

It also has activity based

individualization, so it uses machine

274

learning based activity classification

to better estimate caloric expenditure

275

based on activity specific formulas.

276

So it weights data coming from the

accelerometer, the gyroscope, GPS data,

277

et cetera, based on that activity.

278

So super important.

279

If you want the most accurate estimate to

actually start your workout for most of

280

these fitness wearables with the workout

that you're actually doing that weight, it

281

can weight some of the data a little bit

282

more appropriately for the

activity that you're doing.

283

So if you're swimming, it will weight data

coming from gyroscope, a little higher.

284

GPS and accelerometer will be weighted

higher for walking and running, and

285

then yoga, Pilates, et cetera, with

heart rate weighted a little bit more.

286

For the whoop the key features, it

notably rejects step counts entirely in

287

favor of a strain model, which integrates

continuous heart rate monitoring.

288

That's a big thing for whoop,

continuous heart rate monitoring.

289

BMR calculations based on

height, age, weight, and sex.

290

And this is how all of them

get their BMR calculations.

291

Whoop claims their product doesn't

count steps because step count

292

ignores intensity and other movements.

293

With their preferred method being

strain, which accounts for heart

294

rate alongside physical activity.

295

So to calculate the active energy

expenditure for Whoop, they utilize your

296

heart rate to estimate strain data and

therefore active caloric expenditure.

297

They utilize heart rate reserve,

which is max heart rate, minus your

298

resting heart rate to gauge intensity.

299

Inactivity and when to switch algorithms

to measure caloric expenditure.

300

So once it goes higher than about 30% of

your heart rate reserve, which is your max

301

heart rate, minus your resting heart rate,

then it will switch algorithms to be more

302

of an active exercise based algorithm to

calculate that active energy expenditure.

303

And just as a reminder, we've

done an episode on Max heart rate

304

and rest and heart rate, but max

heart rate is typically estimated.

305

Again, there's a few formulas for

this, but it's estimated to be two 20

306

minus your age is your max heart rate.

307

So intensity is based on a

percentage at that max heart rate.

308

It's definitely affected by heart rate

medications, blood pressure medications,

309

any cardiovascular medications.

310

So that's a little disclaimer.

311

If you're a normal, healthy adult,

you can use two 20 minus age to

312

estimate your max heart rate.

313

And then your resting heart rate

is typically the lowest heart rate

314

that your body reaches at rest

315

and heart rate reserve is the max heart

rate minus your resting heart rate.

316

so whoop will start calculating

active energy expenditure at 30%

317

of that heart rate reserve number.

318

For the R ring key features, it utilizes

the gyroscope, accelerometer heart

319

rate, sensing body temp, and then user

entered body metrics again, specifically

320

for that basal metabolic rate.

321

So it's worn on the finger rather than

the wrist giving it better heart rate.

322

Sensing from what Aura says.

323

I still question this because

when we take pulse metrics,

324

we're taking it at the wrist.

325

So I question this a little bit, but I

do agree with them saying the fingers

326

might move a little bit less so the data

that they're getting from the finger

327

might be a little bit more accurate

because you're not necessarily having

328

some of these hand gestures at the wrist.

329

Your fingers are typically

not moving as much as.

330

Your wrist, so I can see that a

little bit, but I still, we definitely

331

take pulse measurements and heart

rate measurements at the wrist, so I

332

question those claims a little bit.

333

And if anyone has some of the science to

back that up, I would love to read it.

334

But I still question the claim

that the finger has better

335

blood flow than the wrists.

336

The R ring emphasizes passive resting

metrics and uses a combination

337

of finger heart rate, sensing,

temperature, and accelerometer

338

to estimate calorie expenditure.

339

So passive calories or that BMR

is derived from user entered body

340

metrics, body temp, and sleep pattern.

341

So this takes a more holistic

approach to estimate BMR.

342

As we said earlier, some of those

BMR formulas do not take into

343

account stress and sleep in some

of the dynamics of your BMR.

344

They use it as more of

a static measurement.

345

So Aura tries to account for that

with some of its sleep technology

346

sensing and body temp sensing, although

we'll talk about it later, that

347

technology isn't quite accurate either.

348

So take all of that with a grain of

salt, but at least they're trying

349

to integrate some of those more

holistic components into your BMR

350

into some of their measurements.

351

For active calories, it uses

mets to classify activities as

352

low, moderate, or high intensity.

353

So a MET is kind of a confusing thing.

354

I think it's meant to be simple,

but to me it's confusing, but

355

it's a metabolic equivalent.

356

So mets are used as a

multiplier of your BMR.

357

So one met is the energy

required to sustain you at rest.

358

So if something is three mets, it's

three times as intense as at rest.

359

If it's five mets, it's

five times as intense.

360

So it's a multiplier.

361

I think it's a little bit more confusing.

362

I have never really used it, but.

363

I also understand that in order to

gauge intensity, they can use it as a

364

multiplier based on low effort, moderate

effort, or high intensity effort.

365

And I have used an ora ring.

366

And after the workout, I'll say,

what is your intensity effort?

367

And this is why, is because they will use

a multiplier for low intensity, moderate

368

intensity, and high intensity to calculate

your caloric expenditure for that workout.

369

However, if it's a highly intense

upper body day, I have found that

370

if I put high intensity, I don't

necessarily think that I burned as

371

many calories that the AA ring says.

372

So I feel like there is some.

373

Definitely subjectivity to

that measurement based on

374

your own gauge of intensity.

375

So the fact that you are the one

deciding whether that workout was

376

low, moderate, or high intensity

to then calculate the active energy

377

expenditure, there's a little bit of

a limitation in that calculation Then.

378

So basically your intensity ranking

then helps them to calculate the

379

estimated caloric expenditure

380

for Garmin, some of the key features.

381

It uses GPS when available to supplement

accelerometer data for outdoor

382

activities and incorporates VO two

max estimates, which it calculates

383

from heart rate versus that pace

data to improve calorie estimates

384

for trained athletes specifically.

385

So it gets those VO O2 max

estimates from heart rate and

386

running pace or cycling power.

387

If your heart rate is lower at a given

pace, they assume higher aerobic fitness.

388

So they're taking your pace.

389

If you've got a relatively quick

pace and your heart rate is a little

390

bit lower of your max heart rate,

they assume your VO O2 max based on.

391

That data that they're collecting and then

can use that to help estimate total active

392

caloric expenditure or energy expenditure.

393

So because of this integrated GPS data,

it can actually estimate your VO O2

394

that you're working at during a workout.

395

So once Garmin actually has that VO

O2 max estimate, based on your heart

396

rate at a certain pace, it can then

calculate how much oxygen you are

397

consuming at a given heart rate.

398

so this relationship is

pretty linear, above 50%.

399

They also reference PACE data to

help layer that into the equation to

400

make it a little bit more, or try and

make it a little bit more accurate.

401

In general, one liter of oxygen

consumed is estimated to take

402

about five kilo calories of energy.

403

It's a little different depending

on the fuel source, so whether fats

404

or carbs, but they estimate it to be

about Five kilo calories of energy.

405

So once VO O2 max is established as

a ceiling, the current VO O2 that

406

you're working at is estimated by

seeing where your real time heart

407

rate falls relative to your heart

rate reserve, which is again, is that

408

heart rate max minus heart rate rest.

409

So the VO two max is your ceiling

heart rate during the workout

410

tells you at what fraction of

that ceiling you are working at.

411

and since we have that formula of one

liter of oxygen consumed is estimated to

412

be about five kilo calories of energy,

we can convert some of that VO O2 data,

413

some of that heart rate data to then

get total active energy expenditure.

414

So for cycling, it's a

little bit different.

415

It switches the algorithm to estimate

direct power output rather than

416

heart rate, so it's more accurate,

especially if it's actually

417

connected to a direct power meter.

418

So there's a wide range

in accuracy for Garmin.

419

One study says they found accuracy within

7% of lab measurements for calories,

420

specifically with some as low as 6.7%

421

during medium to high effort activities.

422

So that's the bottom part of the range.

423

Another study reported up to 49.3%

424

error with the use of what they call

this first beat technology, which

425

is a company that Garmin acquired

426

to put this energy expenditure

data into their products.

427

Overall, Garmin seems to underestimate

at lower and higher intensities

428

for your caloric expenditure.

429

At lower intensities of effort, say

yoga or an upper body day, your heart

430

rate, which is a key measurement in

the Garmin active energy expenditure

431

formula, won't elevate despite

the high amount of muscular effort

432

that your body's putting in, which

would then underestimate calories

433

and then at the higher intensities.

434

Like during high intensity interval

training or sprints, we are reaching

435

about 80 to a hundred percent of our max

heart rate, depending on the workout.

436

But the energy system within our

bodies that we utilize during all out

437

sprints is our anaerobic system or a

system that doesn't utilize oxygen.

438

Heart rate lags about 15 to 30 seconds

behind a sudden high intensity effort

439

because the cardiovascular system

takes time to ramp up and during

440

that window, the muscles are already

working anaerobically, so without

441

oxygen at near maximal output.

442

So since the algorithm utilizes

oxygen consumption as a

443

factor for deciding caloric.

444

Expenditure and takes

into account heart rate.

445

It would underestimate caloric

expenditure at these higher intensities

446

due to the use of our anaerobic

systems and that heart rate lag.

447

Overall.

448

Garmin is more accurate for people using

GPS because it gives the pace data that

449

helps in the VO two estimate equation.

450

For the Fitbit, some of the key

features are it uses accelerometer

451

and heart rate sensing if the

device has a heart rate sensor.

452

Older models typically rely a little

bit more on accelerometer data over

453

the heart rate and newer models.

454

With heart rate sensors now utilize

that heart rate data a little bit more.

455

So some of the studies that looked at

a wide variety of fitness trackers,

456

what do they say when it comes to

caloric expenditure and how accurate

457

these fitness trackers are at measuring

total daily energy expenditure with

458

that BMR and those active energy

expenditure part of the equation.

459

So overall devices over or

underestimate energy expenditure

460

by greater than 30% on average.

461

And the error exceeds the 10%

acceptable threshold for all

462

devices in real world conditions.

463

So there was a Stanford Benchmark

464

they looked at, I think six or seven

devices, but not all of them were.

465

Either around or talked about

like the R ring wasn't included,

466

the Garmin wasn't included.

467

They did include the Apple Watch and

the Fitbit surge, which were part of our

468

investigative journalism, but the most

accurate device was off by an average

469

of 27% when comparing how it estimated

total daily energy expenditure, and

470

then also what was found in the lab.

471

The least accurate device was off

by 93% factors including skin color,

472

and BMI affected those measurements.

473

There's a 2025 head-to-head

comparison done that looked at

474

some of the more modern fitness

trackers compared to lap testing.

475

So they looked at some of the

results as a percentage of

476

actual lab measured calorie burn.

477

So whoop was 66% of the actual calorie

burn, so it underestimated by about 34%.

478

Fitbit charge was 72%, so it

underestimated it by about 28%.

479

The Samsung Galaxy Watch was about

78%, or Ring four was about 86%.

480

And Apple Watch Series 10 92%

Which is the closest to lab result,

481

underestimating it by only about 8%,

and then the Garmin vivo active six

482

was 112%, so it actually overestimated

caloric expenditure by about 12%.

483

So then there was a systematic review

done that looked at 158 publications

484

covering nine brands of fitness wearables.

485

And among the studies, no brand

fell within the acceptable accuracy

486

limits for energy expenditure.

487

There was an overall tendency to

underestimate the energy expenditure,

488

and only about 18% of the comparisons

fell within plus or minus that 10%

489

error, 53% fell below that 10% error.

490

So the bottom line, the metric

resting heart rate has excellent

491

accuracy for these fitness

wearables, about two to 5% error.

492

Step count is pretty

good, eight to 10% error.

493

Heart rate variability is good for.

494

But it . Varies by device.

495

Calorie expenditure is poor.

496

27 to 93% error range across various

studies and across various devices.

497

Sleep duration had moderate

accuracy, often greater than 10%.

498

Overestimation and sleep staging was

pretty weak, about 79% accuracy at best.

499

So what does all this mean?

500

What I found very interesting

was the algorithm switching

501

with different activities.

502

I should have probably assumed that

based on, especially the R ring, where

503

if I did a strength training day and

it was an upper body strength training

504

day and I put high intensity compared

to low intensity or moderate intensity,

505

it definitely changed that caloric

expenditure variable that was going

506

into that equation a little bit more.

507

So I thought that was interesting

in how they pull from different

508

data from these different sensors.

509

Weight, different data, a little

heavier depending on the activity,

510

which I think is kind of cool.

511

But how would I recommend to use calories?

512

Like why are we even tracking

them on these fitness trackers if

513

the accuracy is relatively low?

514

Across a lot of studies, I think we're

probably getting closer and closer,

515

and I think that's probably the goal.

516

But if your goal is something like.

517

Understanding intensity of your workout

or looking into different trends

518

of your workouts mixed with your

activity level throughout the day.

519

Even just walking, I think calories can

give you a good trend, but I wouldn't

520

necessarily look at it and use it as a

way to base your dietary modifications

521

on if you're trying to go more into a

caloric deficit or a caloric surplus.

522

Again, there's high amounts of error

in these calculations, in these

523

estimations, in these formulas that

these fitness wearables are using.

524

Based on the research, there's error

at every step, which only compounds the

525

amount of error in that final answer.

526

So.

527

I wouldn't necessarily use it as the

subjective data to base any changes

528

in my nutritional patterns or exercise

patterns off of, but I think if we look

529

at trends, it might be a nice way to

have just another variable to track.

530

Overall, I feel like calories is not

something I look at or rely heavily on.

531

For data.

532

I like to use heart rate

or VO two or my heart rate

533

variability, like how I recover.

534

My VO two max, I like to use some of those

variables to gauge certain performance

535

improvements and training improvements.

536

So I don't necessarily

like to use calories.

537

That's not saying that you shouldn't, but

this just gives a little bit of a rundown,

538

a little bit , of the science behind how

different fitness trackers are calculating

539

calories and the accuracy amongst them.

540

So I hope you enjoyed this episode.

541

This was a fun little

investigative journalism episode.

542

Again, I hope you learned a little

bit about caloric expenditure, how

543

calories are counted, how the different

formulas involved in estimating caloric

544

expenditure and your basal metabolic

rate and mets metabolic equivalents.

545

That might be something

new for, for you guys too.

546

And a reminder on VO two, like

we got to incorporate a lot of.

547

Fun exercise physiology stuff

in today's episode, but I

548

hope you guys learned a lot.

549

I hope you're able to bring this up in

conversation with some of your friends

550

who are like, oh, my move calories are at

this, and you'll be like, you know, hang

551

on, that's cool, but maybe look at this

variable instead, given the inaccuracy

552

in some of these fitness trackers

and their ability to count calories.

553

So maybe a fun conversation topic to bring

up With your other health and wellness

554

e friends, but I enjoyed this look into

fitness trackers and caloric expenditure.

555

I hope you guys learned a lot

and I'll see you guys again.

556

On the next episode of

Wellness is the pod.