Debunking the 20-Minute Exercise Myth!

Dear Davey,

I remember a “myth” from a few years back when I started exercising: When doing cardio exercise, you have to work for at least 20-30 minutes to get into the fat burning zone. In other words, it takes at least 20 minutes to start losing weight.

It sounds like a myth that is not true, but I just wanted to confirm.



Good nose! You were able to sniff out a very pervasive myth. And yes, it’s totally untrue.

The myth probably started based on a misinterpretation of actual science. During the early stages of cardio, your body does rely more on carbohydrates than fat. The longer you exercise, the more your body shifts from carbohydrates to fat. But really, you’re getting a tremendous benefit from each stage of exercise.

Moreover, the 20-minute myth ignores the bigger boost that your metabolism gets from exercise. Much of the exercise-induced calorie-burning happens when you get off the treadmill – especially if you maximize your cardio with interval training.

Need some solid proof to finally put this myth to rest? Researchers from the University of Pittsburgh School of Medicine looked at two groups of exercisers. Group 1 performed a single session of exercise for 20 – 40 minutes. Group 2 performed the same amount of exercise, but broken up into 10 minute sessions over the course of the day. According to the 20-minute exercise myth, group 2 shouldn’t see any weight loss. But, just the opposite held true; participants in group 2 lost an average of 20 pounds, compared to 14 pounds in group 1.

And for what it’s worth, my current cardio routine usually lasts 15 minutes.

The bottom line: The 20-minute myth is entirely untrue.

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  1. My nutritionist says 30 mins to maintain weight, 60 mins to lose weight. I guess this doesn’t take intensity into account, but these numbers are a lot higher than 20.

    • Perhaps your nutritionist means 60 minutes in total of cardio and strength training. If you do cardio for longer than 45 minutes, you can run the risk of burning muscle – which may result in a slower metabolism and actual weight gain.

      Besides, if you are doing cardio correctly and really pushing yourself – 60 minutes would be impossible!

      • Ow,thank you for this post. Usually when I do cardio I do it at my best….and there´s no way I can picture myself maintaining that intensity for 60 minutes….unless there´s an ambulance waiting to catch me at the end.
        Plus i find it really boring to do steady cardio for an hour (or more,once your body get´s used to it).
        I know a lot of trainers and nutritionist out there who don´t keep up with the new scientific discoveries and give wrong info- i heard a few days ago about someone being made to do 200 crunches at the gym :/

  2. The “fat burning zone” is when your body switches from an anaerobic to an aerobic metabolism. Lifting weights and short-burst exercises like sprinting are anaerobic: they break down carbohydrate stored in muscles (glycogen) into glucose and then metabolize it anaerobically, with a yield of 2 ATP per glucose molecule. It’s fast energy, but not efficient.

    You know when your muscles have run out of gas (glycogen). You start to slow down. For me, it happens maybe five minutes into a run. Then I get my ‘second wind’. What’s this? It means my body has switched from an anaerobic to an aerobic metabolism. I have adrenalin on board, which is causing my fat cells (adipocytes), especially the ones inside my gut, to release fatty acids, the molecules that make up fat, into my blood stream. These fatty acids enter my muscle cells where they are taken up by thousands of mitochondria, ‘the powerhouse of the cell’ as my high school bio teacher called them, and are metabolized aerobically, with a whopping yield of approximately 96 ATP per fatty acid. That’s a lot of heat, which is why I start to sweat.

    Take home lesson? As soon as you start sweating profusely you’re exercising aerobically and burning fat! Mind you this marker doesn’t hold if it’s 90 degrees out with 90% humidity, but at room temperature it’s a pretty good one. The current recommendations are to do something that makes you sweat profusely but not too winded to carry on a brief conversation (whistle, not sing) for forty minutes to an hour, four times a week.

  3. The benefits that occur in the brain usually don’t start until you’ve been exercising for 25 minutes. Obviously any amount of exercise will start using the energy in your body because it takes energy to move but in the brain there are factors that really, in my opinion, should be utilized. They all, of course, start at different times but as I understand the general time is 25 minutes. For example here’s a few of them: VEGF (Vascular Endothelial Growth Factor), which is the heart creating new capillaries because your muscles need more oxygen. Essentially once you’ve created them your muscles get more oxygen outside of exercise too. IGF-1 (Insulin-like Growth Factor), which is activated in the brain to help memorize and remember things. It is theorized that during evolutionary times we would use this when we were hunting (basically continuously moving) and we needed to remember where food was. Our brain would start this so that we could remember where to go to get it, nowadays however we can use it for memorizing things before a test or learning new things (best used immediately after exercising, but not during). FGF-2 (Fibroblast Growth Factor), is the body rebuilding tissue and lowering wound healing times created from exercise. Then more in the brain you have BDNF (Brain-derived Neurotrophic Factor), which in essence makes your brain grow like crazy. Basically inside each neuron you have this chemical and when it’s released it sprouts new dendrites (it basically reaches out to other neurons), which makes the neuron very strong and good at communicating. I’m pretty sure BDNF release until 25 minutes but I guess I could be wrong. It’s really worth checking into because having an adaptable, strong brain is incredibly important at old age. Davey, I really highly suggest reading Spark by John j Ratey because it’s what got me into exercise. It talks about how exercise fixes imbalances in the body. Literally chapters are called: Depression, Anxiety, Hormones (in women), Addiction- and it goes through how exercise repairs the mind. It’s easy to understand and I recommend Spark above pretty much any other book! Check it out sometime! 🙂