I tried the intermittent fasting app Elon Musk used to lose 20 pounds to see if it’s worth the hefty price tag

Suffice it to say, I’m always on the hunt for a new book, podcast, or Instagram account that I feel will make me healthier, leaner, more energized, and age backward. So when the editor of Fortune Well asked me to undergo an intermittent fasting regimen, touted by Tesla CEO Elon Musk, for five days, I was game. 

I’ve long been an accidental devotee of intermittent fasting. A typical day for me consists of a 6 a.m. wake-up time, work, a long jog as I listen to the day’s trending news, more work, a lunch-time pilates class at a nearby studio (don’t tell my boss), and—you guessed it—more work. By the time my stomach starts rumbling, it’s around 3 p.m, and by then, I’m only about three hours away from dinner. Is this healthy? I’m sure I’ll have my doubters. But it works for me, and rarely do I have the dreaded mid-day slump despite long-held abstinence from coffee. Moreover, I swear by this way of eating for the same reason Musk does: It keeps me lithe and lean.

In late August, the outspoken CEO told his nearly 108 million followers that he keeps fit by “lifting a little.” But what’s really helped him lose almost 20 pounds, and feel healthier, is periodically fasting. And it should come as no surprise that the tech billionaire relies on technology to stay on track. “The Zero app is quite good,” he tweeted.

I figured the app might be worth exploring if the wealthiest person in America swears by it. So, I did.

The setup

After downloading Zero, the app walked me through several prompts, requesting that I input my age and sex and select what brought me to Zero. For example, mental clarity, weight management, and gut health. I chose mental clarity, a longer and healthier life, and weight management.

While I’m not looking to lose weight, I wanted to see just how much I could lose in the next five days, given that Musk attributed part of his weight loss to fasting. The app then builds plans for you based on your chosen priorities. My two programs centered on healthy weight loss, which required 16 hours of fasting and an eight-hour eating window, and a circadian circuit, 13 hours of fasting and an 11-hour eating window. I’m a glutton for punishment, so I picked the 16:8 fast (It’s also the most popular selection on the app).

You can also customize your experience to see trends, such as weight fluctuations over time, sleep hours, resting heart rate, caloric intake, and glucose and ketone levels. Additionally, you can view weekly metrics, such as average activity hours, fat burn, and fasting. I did not use these features (I have other apps for that), but I liked the daily “coaching” reminders to help me stay on track. My day one reminder was to “skip the snacks to hit your goals.” Duly noted.

The experience

The first day was relatively easy. For context, I’d just returned from a trip to the south of France the day before, an excursion characterized by a seemingly endless supply of cocktails and carbs. Food was the last thing on my mind, and I exceeded my fasting goal by an extra eight hours, refraining from eating for a full 24 hours. I had a hearty dinner of greens around 7 p.m. and called it a night. The app congratulated me on a successful day one and informed me of the fat-burning phase I was in throughout the day. I entered my catabolic zone four to 16 hours into fasting, meaning my primary fuel source was glucose from liver glycogen and fatty acids from fat stores. (Or so the app said.)

Day two was more of the same. I typically skip breakfast, preferring to work and work out in the early morning. I woke up feeling rested and recharged and went for a 5-mile jog with my dog. I weighed myself that morning and was down half a pound. My fast technically ended around noon, but I continued until about 2 p.m. when I started to feel peckish. I had a small snack, jumped back into work, then ate dinner around 9:30 p.m. 

On day three, I had a business lunch I’d scheduled weeks ago, which fell within my fasting window. Normally I’d eat something light so I’m not awkwardly sitting empty-handed while my guest is scarfing down a meal. I asked if we could push the meeting to 2 p.m., which fell within my eating window, and she graciously accepted the time change.If you’re someone who often travels for work or has business meetings centered on meals—breakfast, lunch, or dinner—you’ll have to plan around your eating window or stick to water, which can feel uncomfortable in a 1:1 meeting.

I had piles of work to get through that day and didn’t eat again until around 10:30 p.m., which meant my 16-hour window started late. Not ideal, and it also meant that my fasting period lasted well into the next day. By 12 p.m., I was famished, and it didn’t help that I’d doubled up on my workout that morning. However, I’d lost another half a pound the past two days for a total of 1.5 pounds in three days, and I gritted through the hunger. But when my eating window opened, I found myself indiscriminately snacking as I prepared dinner.

By day five, I’d figured out the perfect system: eat earlier in the day so that most of my fasting time fell during my sleeping time. And since I prefer workouts on an empty stomach, I got them done during the fasting window, too. I also found myself coming into the office late on Wednesday, so I wouldn’t feel tempted by the free lunch that hits our cafeteria at noon. Luckily, I didn’t have any late dinners or drinks set up this week, but I imagine it would be a nuisance to navigate (I’d either nurse water or move the get-together to an earlier time). At the culmination of five days, I’d lost 4.5 pounds, much of which I’m sure was water weight, and truly felt a renewed sense of mental acuity.

The takeaway

Though the app offers a free seven-day trial, it costs $69.99 annually, which is pricey. If you’re simply using the app to time your fasting and eating windows, you’re better off setting a timer on your smartphone. 

The app does come with other functionalities to track your calorie intake, weight, and the like. But several other apps offer these functions for free, including Happy Scale, Lose It, and My Fitness Pal.

It’s also worth noting that I have never suffered from disordered eating, and many nutritionists advise against calorie counting or any food restriction for those diagnosed with an eating disorder. But for someone like myself, who’s data-obsessed yet still eats intuitively, for the most part, I found the app to be a helpful way to self-regulate, stay on track after a period of indulgence, and monitor progress. That is, if you’re willing to shell out nearly $70 annually to gauge how many hours you’ve gone without sustenance each day.