When it comes to losing weight, tracking calories is the most accurate and reliable method. Ideally, you want to aim for a 0.5-1% body weight loss per week. To achieve this, you need to have some idea of your maintenance calorie level. You can use a calculator such as the Mifflin-St Jeor, which generally provides a good baseline. Then, you’d adjust your calorie consumption based on the actual weight loss you see on the scale to achieve that 0.5-1% body weight loss per week.

While this approach provides a concrete guide to lose weight, your activity levels and calorie consumption will vary day to day. This means some days you’ll lose more weight than others, and over the course of cutting weight, you might reduce daily activity, thus lowering your calorie consumption. This is where a device like the Apple Watch could help with tracking. It could potentially replace the Mifflin-St Jeor calculator for determining your Total Daily Energy Expenditure (TDEE) and setting that baseline. But the question remains: Is it accurate, and can we use it as a reliable baseline?

To investigate this, I’ve conducted a personal experiment. For the past 30+ days, I’ve weighed myself daily, worn the Apple Watch to track energy expenditure, and meticulously tracked all calories I’ve consumed. Let’s start by looking at my energy expenditure graph:

This chart reveals some interesting patterns. For instance, my energy expenditure started increasing significantly around April 2024, which aligns well with when I began exercising more frequently. We can also see that the day-to-day variation is quite large. While the resting energy (shown in blue) remains relatively static, the active energy fluctuates considerably. Let’s take a closer look at the data:

These numbers reveal that the total average during the weight cut period was about 2963 calories in expenditure. However, the difference between a workout day and a non-workout day (strength training and usually some cardio, 4x a week) is quite significant. Assuming these energy expenditure estimates are accurate (which we’ll examine shortly), this data suggests that varying calorie consumption based on workout and non-workout days could be beneficial, especially if maintaining a calorie deficit is challenging.

Now, let’s address the main question: Are these estimates even accurate? Several studies have examined this issue with varying results, sometimes showing overestimation and other times underestimation.

A study published in the Journal of Personalized Medicine, “Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort”, evaluated seven commercially available wrist-worn devices against gold standard measurements. The study found that no device measured energy expenditure within an acceptable error range (<20% compared to the gold standard). Median error rates for energy expenditure across tasks ranged from 27.4% to 92.6%, depending on the device. This highlights significant limitations in the accuracy of wrist-worn devices for estimating energy expenditure.

Another study focusing specifically on the Apple Watch in patients with cardiovascular disease, “Accuracy of Apple Watch Measurements for Heart Rate and Energy Expenditure in Patients With Cardiovascular Disease: Cross-Sectional Study”, found that it systematically overestimated energy expenditure. The Apple Watch’s estimates were, on average, more than double the actual energy expenditure measured by indirect calorimetry. This substantial overestimation suggests caution when using the device for precise calorie tracking, especially in certain populations.

A third study, “Accuracy of Heart Rate Watches: Implications for Weight Management”, evaluating multiple wrist-worn devices, including the Apple Watch, found that while these devices can provide accurate heart rate measurements, their estimates of energy expenditure were considerably less accurate. The percentage error for energy expenditure ranged from 9% to 43% across devices. The researchers concluded that the poor estimates of energy expenditure limit these devices’ usefulness for monitoring energy balance and as weight loss aids.

It’s worth noting that Apple likely tweaks and updates its algorithms to improve accuracy over time. Additionally, accuracy may vary depending on the watch model and other factors such as the accuracy of personal data input (weight, height, age) or the correct selection of workout type.

To assess the Apple Watch’s accuracy in my case, I compared my actual weight loss to the estimated weight loss based on the watch’s energy expenditure calculations:

As we can see, actual weight varies due to factors like water weight fluctuations, which are influenced by carbohydrate consumption. The Apple Watch seems to both overestimate and underestimate at different points during the cut, without a clear consistent bias. I set my calorie consumption to 2300 calories per day, and the weight fluctuations causing error are well within the margin of error I could be making in measurement, especially on days when I estimated calories for meals eaten out.

Based on the studies, anecdotal experiences (such as Andrej Karpathy’s biohacking post), and comparisons to traditional calculators like Mifflin-St Jeor, I would lean towards the watch slightly overestimating rather than underestimating calorie expenditure. However, considering that a typical weight loss plan aims for a deficit of about 500+ calories per day, the Apple Watch’s estimates appear sufficiently accurate to use as a baseline, at least according to my data. As we saw from the referenced studies, energy expenditure measurements from wearable devices generally lack extreme precision. While my experience suggests reasonable accuracy for moderate activity levels, it’s important to note that for individuals with significantly higher daily expenditures due to intense or prolonged exercise, the margin of error could potentially increase. For those with more typical activity patterns similar to mine, the Apple Watch likely provides a reasonable estimate. Nonetheless, it’s crucial to monitor your actual weight loss and adjust your approach accordingly, regardless of your activity level.

In conclusion, while the Apple Watch may not be perfect in its calorie tracking, it provides a reasonable estimate that can be useful for setting a baseline in your weight management journey. As with any tool, it’s best used in conjunction with other methods, such as regular weigh-ins and adjustments based on actual results. Remember, the key to successful weight management is consistency and adaptability, regardless of the tools you use to track your progress.