The calorie counter on Garmins is notoriously inaccurate anyway.
I believe the algorthym on the 500 is supposed to be better.
But its still going to a very rough estimate at best for cycling, and it would be totally off running for the reasons stated above.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.