Is fasted cardio the best for burning fat?


Have you heard the saying that “abs are made in the kitchen”? I agree with that for the most part. As an RD, I’m all about healthy and balanced eating. But I’m also a strong proponent of pushing yourself at the gym. There have been various studies and theories on what exercise is the best for weight management and body fat reduction, though unfortunately no consensus. I did my own investigation into one popular trend: fasted cardio.

What’s fasted cardio? It’s a fancy way of describing when athletes do long, but low intensity cardio, such as slow running, or biking for upwards of 45 minutes before having a meal in the morning. It’s popular with bodybuilders and physique competitors, who want to be super lean for their shows. (Side note: I found that most of them have the same prep routines: fasted cardio, 6-7 meals a day, supplement stacks, weight lifting at night, diet cycling between bulk/cut, and endless gym selfies). The theory is that fasted cardio helps you lose body fat. Is it true?

The idea behind fasted cardio is that on an empty stomach, our bodies have inadequate amounts of glucose in our blood to access for immediate energy, which means our bodies quickly transition from burning sugar to burning fat. The “low and slow” idea is that after an extended period of time of slow exercise, your body will reduce its usage of glycogen and increase it’s breakdown of body fat. Makes sense, right?

Kinda. Although it’s a solid way to burn extra calories in the morning, it’s not a silver bullet. There are two important things to note. First, fasted cardio does reduce glycogen stores, which also reduces water within our muscles. Many may think this of “leaning out”, but it most likely is due to water loss rather than fat loss. Second, fasted cardio is an inefficient way of burning fat. You have to go for an extended period of time (studies have suggested 45 minutes is when you make the switch from glycogen to fat, so to actually burn fat you’d have to continue exercising for quite a bit longer). Plus, some people just don’t like exercising on an empty stomach, which is completely valid.

My take away is to modify your focus on losing body fat, and focus your energy on building and sustaining lean body mass/muscle. The latter can lead to the former. Consider HIIT, or high intensity interval training, which helps build/tone muscle while helping to burn more calories during the day. Muscle growth/maintenance is important because well after your time at the gym, your body is still burning energy. At rest, you’ve raised your metabolic rate and are tapping into your fat stores for energy. This combo can help reduce overall body fat better than fasted cardio.

I like cardio like the next guy, but weight training and HIIT takes the cake for me 🙂

Happy lifting, friends!

Thomas Ngo RD


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