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Fueling for endurance athletes - concrete strategies from a few inputs.

Endurance athletes have to take in carbohydrates, fluid and electrolytes deliberately in training and competition - too little causes a performance drop, too much or the wrong thing causes GI trouble. The right amount depends on many factors at once: duration, intensity, sport, heat, gut tolerance. Blanket rules of thumb like '60 g of carbs per hour' ignore that, generic nutrition apps track daily balances instead of sessions, and AI coaches hand out recommendations you can't reason about.

Everything - the idea, domain research, the calculation logic, design, backend, web frontend and the native iOS app. AI was a development tool here, not an autopilot.

• For the first time I combined a backend, a web frontend and a native iOS app in one project. The clean layering paid off with the second client: the logic was reusable, only the transport layer was new.
• Payments across two worlds - Stripe on the web and Apple StoreKit on iOS - behind a single entitlement service: one source of truth for feature flags.
• A versioned mobile API with OpenAPI and a contract check in CI is the point where 'backend and app drift apart' stops being a problem.
• The first project that grew beyond a single Python script with a README - including its own domain, DNS, CI/CD and deployment.
• Next time: one clear deployment path instead of Docker and Vercel in parallel, and shared types between backend and iOS earlier.