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Which of the several calorie/energy expenditure formulas available is the most reliable? I know of the Harris-Benedict formula and a couple others, but don't know which is the most accurate.

The Harris-Benedict is the traditional one used but it's very dated now. I know there have been a few more since then but saying that it's the one I was taught to use for my degree so it must be accurate enough. Have you tried using them all? There can't be much difference in the calculations they produce, use them all and get an average? Nothing can ever be that accurate anyway, they're all estimates. The only real way of finding out how many calories you're burning is by collecting the air expired and this isn't exactly possible if you're planning to do day to day things!