Thank you all for your comments. It's great to have people reading the articles. :-)

The effect of training intensity can get complex, and research is always shedding new light on the subject. The running article was intended to keep it simple (like me!), and look at primary energy sources during the activity itself.

My understanding is that burning fat for fuel is a chemical process that requires a lot of oxygen. If you work too hard, your body has to use alternative fuel sources requiring less or no oxygen.

Noi, as you've said, initially carbs are used for fuel regardless of intensity, once this supply is diminished the intensity level will dictate where energy comes from during the exercise.

I also completely agree regarding intense training pushing up your metabolic rate (consuming more calories) for a prolonged period, considerably longer than the time spent exercising. During the intense training, fat can't (to any great extent) be used. However, once you get your breath back you will begin to burn fat after you cease exercising because your metabolic rate is running so fast (and you have sufficient oxygen coming in).

So you can burn fat as a result of training intensely too!

I personally keep all my running at an intense level because my body fat is already low enough, and I can cut a bit more training intensely as you've highlighted.

Those that are more over weight may find a longer, slower intensity easier to achieve (physically and psychologically). I personally believe a mixture of both methods works the best. Intense training (including weight training) will aid fat loss by increasing your lean muscle mass, and further boost you metabolic rate - meaning you can walk around burning fat when you're not even training!

That's my take on it anyway! Like you say, it get's us all talking about our experiences…