Design a database schema for a fitness app.

Focus on identifying the core entities that will be part of the fitness app, such as Users, Workouts, Exercises, and Nutrition. Consider the relationships between these entities; for example, a User can have multiple Workouts, and each Workout can consist of multiple Exercises. Think about the attributes that each entity should have, like User details (name, age, fitness goals), Workout details (date, duration, type), and Exercise specifics (name, repetitions, sets). Also, address how you would handle data normalization to avoid redundancy and ensure efficient querying. Finally, consider any potential scalability issues as the app grows in user base and features.