I think you are both right, but also both wrong. My understanding is that % wise you will burn a higher ratio of fat to carb calories walking. However you will burn a higher number of total calories running for the same time period. The ratio of fat/carbs is reduced but even so the total # of fat calories burned running will still exceed the total fat calories burned walking. I am sure someone on the board has the science to provide all of the specifics.

while it is true that you burn a higher percentage of fat (vs glycogen) at lower intensities (walking), the overall number of calories burned will always be higher when performing at higher intensities (running).

It depends ;-)

Basically, if you work out hard enough, you burn mostly glycogen...which can be replentished with post-workout fuel, and you won't have burned much fat due to the workout. However, if you DON'T ingest post-workout fuel, your fat reserves can be tapped to replentish your glycogen stores.

If you work slowly enough, glycogen isn't used up so fast because your fat-mobilizing enzymes are able to keep your blood glucose levels up sufficiently. But, if you ingest post-workout fuel, you are replentishing your fat stores instead of your glycogen stores.

IOW, your post-workout fuel replentishes whichever fuel storage source that was depleted by the exercise.

It IS as simple as calories processed and calories burned. SO, your goal should still be as simple as burn more than you process, and you will, over time, lose fat. Of course, if you burn too much too fast, you can lose muscle...that's why many people don't advocate running vigorously for more than a couple of hours at a time in training....it can be very destructive of protein, making it harder to recover from the workout, as the muscle requests rebuilding efforts...this takes more energy to do than just replentishing glycogen stores.

So, like I said, it depends....

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Quid quid latine dictum sit altum videtur
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Trijs has it right. Yes, while walking you burn a higher percentage of fat calories, but a smaller number total, so it's a higher percentage of a smaller number. Here is a basic illustration.

Let's say you burn 20% fat calories walking 30 min

In that 30 min walking you burn 100 calories total

So, you have burned 20% of 100, therefore 20 fat calories

Let's sat you burn 10% fat calories running 30 min

But, in that 30 min running you burn a lot more total calories, let's say 300

So, you have burned 10% of 300, therefore 30 fat calories

Please excuse the over-generalizations in the math, etc. But I think you get the point, you were both right, but you were more right!!!

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Portside Athletics Blog

The body uses fat as fuel more efficiently at lower HRs, because youi simply can't convert fat to energy fast enough to power the machine at high output. However for most of us, walking is too easy to really cause any cardiovascular improvement nor does mitochondrial density increase as a result of ultra-low intensity workouts.

The optimum range for training the body to convert fat is 20-30 BPM below AT for the majority of athletes. There are tests that can be performed to determine the best range for each individual.

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But its not like there are only two speeds, walking and a anarobic run. Definitely run, just not where all you do is consume glycogen (mostly anarobic).

Conventional advice is to find your LT, say at 80% HR and stay just below it. Most can do a pretty good run at this level of intensity and still target those "fat calories" and burn more total calories.

everybody here is right. for references, check "Sport Nutrition" my jeukendrup and gleeson. it's a good text that i'e used in the past for sports nutrition classes. also, do a simple pubmed search (if you have access to a university library) and look for VO2 ratios and the like. this concept is a pretty basic tenet in exercise sci.