And that's key. Calories are important, but most people will naturally eat less calories as they shift from carbs+sugar to foods with higher protein and fiber content.
Hell, knowing what people actually eat and comparing it to their actual health would be extremely helpful, especially combining with a massive data-set and big data.
Food studies are currently based on recall... and peoples recall is horrible. "What did you eat last Monday for breakfast, and how many grams of it did you eat?"
The inputs are garbage and the outputs are garbage. Linking actual food intake to healthout comes would be VERY powerful.
This is a real problem. The good news is that if its not healthy for an individual, its not healthy for their family. A diet comprised of no fiber, no protein and high added sugar is always unhealthy. Families tend to eat similarly. Its very rare to have someone eat well and someone to eat unhealthily.
It also excludes going out to eat. That being said, if someone is concerned about their health enough to use an app like this, they SHOULD be eating at home more. They only way to know what is in your food is to make it yourself.
I had the same goal and developed FoodCoach (TheFoodCoach.app). It tracks nutrition (not calories) based on a picture of a grocery store receipt.
The high level goals are to increase protein, increase fiber and decrease sugar. Super simple. Tracking takes <5 minutes per week, for the entire family. With the receipt it recommends foods to buy more or buy less to help the user hit their protein, fiber and (low) sugar goals.
I used no-code Appgyver and have been very disappointed in its robustness. The backend is all hooked up. Everything works, but I've learned I'm not good at marketing. There many better (and free) sources of information to obtain nutrition data.