Q: Mike i dont understand something.It is said that after 20 mins of cardio the body starts burning fat.If it so then the calories we burn before we reach the 20 mins where are from?muscles?water?If it isnt fat then it has to be something right?

Also the hiit cardio lasts less than 20 mins.Then how it burns fat?Is it maybe cause it is a mixed form of aerobic and anaerobic exercise?

I ve also heard that its good in order to lose fat to make 4-5 times a week 30-40 mins cardio at brisk pace or 3 times a week hiit.Is it ok?

A: The statement 'your body starts burning fat after 20 mins' is very misleading. Firstly, at no point does your body burns ONLY fat or ONLY glucose (glycogen) as well as ready creatine phosphate store (which is the primary source of energy for beginning of any exercise) -- it's always a mix of all three.

Secondly, even for duration of low-intensity exercise (such as cardio) either liver glycogen or muscle glycogen store is contributing about a third of the energy.

Thirdly, fat stores become the dominant energy source after the first two (not 20!) minutes of any continuous exercise.

Finally, it's a bad approach to think of any one exercise or activity in isolation as burning so many calories. Your body is a very very complex machine and nothing happens in isolation. That cardio will not only burn calories while you're doing it, but it will also affect how your body burns calories for many hours afterwards. That's why I always say that counting calories is a subpar approach to losing fat. Instead you should just settle on a good training program (such as the ones I give here) and stick to it while simply adjusting your diet to keep weight loss on a slow steady pace of around one pound/week.

So now the answer to your second question becomes clearer... 4-5 cardio sessions of 30-40 mins per week is 2-3 hours of cardio per week total -- in my opinion is a little on the high side for most people, but not unreasonable. I would rather do 2-3 sessions of HIIT and 30-60 minutes of regular cardio per week. But either one should be OK. I recommend trying both and see which you like more -- some people respond better to regular cardio than HIIT. However if you are not doing any weight training at all, then I would consider HIIT a must.