Most fitness apps seem obsessed with calories. Go for a run, and your Apple Watch tells you how many calories you burned. Scan a barcode and MyFitnessPal tells you how many calories are in the food you are about to eat.

So what exactly are calories, and does counting them really help you achieve your fitness goals?

Have you ever noticed that some of your workout data is missing from the Health app on your iPhone?

Apple’s Health app is designed to provide a central hub for all your fitness apps to save and share their data. You might assume this means all your Active Calories are added together, regardless of which app you use to log them. But the truth is not that simple — although you can tweak some hidden settings to customize what you see.