Most calorie-tracking apps treat Apple Health as a one-way export: log a meal, the nutrition data flows out to Health, done. Fewer apps read anything back in. That direction matters more than it sounds like it should.
The one-way version
A one-way sync means your calorie budget is set once, usually from a generic activity-level question during onboarding ("sedentary," "lightly active," and so on), and stays fixed regardless of what you actually do that day. Wear an Apple Watch, walk 15,000 steps, and log a heavy gym session, and a one-way app has no idea any of that happened. Your target calories on a high-activity day and a couch day are identical.
What flows in with two-way sync
Arcal reads weight and energy burned from Apple Health, from a smart scale, an Apple Watch, or whatever else is already writing to Health, and nutrition data flows back out in the other direction. That means your calorie budget can reflect what you actually burned that day instead of a static guess from onboarding, and your weight trend reflects your actual scale data without manually re-entering it into a second app.
Why more apps do not do this
It is more integration work for a feature most users never notice unless they specifically look for it, and one-way export is enough to check the "syncs with Apple Health" box on a feature list. Two-way sync is the difference between an app that logs what you eat and one that actually knows how the two sides of your calorie balance, in and out, are moving. See the full feature list or join the waitlist.