Building BODY: My Personal Health Data Vault

How I used AI to build a private, deep-dive health tracking tool that goes far beyond what Apple Health offers.

It was about two months ago, late at night after a long day at work, when I hit a wall of frustration. I was staring at my iPhone, scrolling through endless lists of heart rate numbers and sleep durations. I have an Apple Watch, and while the Apple Health app is great for a quick glance, it felt shallow. I could see that I slept poorly last night, but I couldn’t easily see why.

The “story” of my body

I’m fully aware that there’s something a bit freaky about collecting this much data on yourself—it’s almost like trying to read your future in coffee grounds. But at the same time, there’s something deeply fascinating about seeing yourself reflected in numbers. It’s an experiment, really. And because modern technology does so much of the heavy lifting for us now, the logging happens almost invisibly in the background; I don’t have to manually write everything down anymore. I just let the sensors do their job, and then I get to see what the patterns reveal.

And there’s a ton of data that Apple collects. From steps, floors climbed, activity energy, and workout time, to hours spent standing, heart rate, and resting heart rate—and thanks to nutrition apps like Yazio and a digital scale, I also collect approximate data on my dietary intake, such as calories, carbohydrates, protein, weight, body mass index and more, through the connection to Apple Health. So why should only this corporation know that about me?

I wanted to see the connections. I wanted to see the “story” of my body, not just a collection of isolated data points. I found myself wishing for a way to aggregate everything—nutrition, sleep architecture, heart rate curves, even my daily mood—into one place where I could actually perform deep analysis. I didn’t want to rely on another subscription service that owns my data – other than Apple; I wanted to own the data myself. And I wanted to store the data for the long term.

That was the moment BODY was born.

Taking control of my own biology

BODY is my personal health intelligence engine. It isn’t a fitness app in the traditional sense; it doesn’t ping me to “get moving” or shout at me about my step count. Instead, it acts as a highly secure digital vault and analyst.

The concept is simple: I wanted a system that collects, organizes, and analyzes my personal health, fitness, and daily well-being data to help me see long-term trends and connections. Using an app called “Health Auto Export,” my iPhone sends my health metrics from my Apple Watch several times a day to my Mac Mini at home using a REST API. From there, BODY takes over. It turns raw, messy numbers into a coherent narrative of my physical life.

Beyond the step count

What makes BODY different from the apps already on my phone is the depth of the information it tracks and how it connects the dots.

Instead of just telling me how many hours I slept, it looks at my “sleep architecture”—the different phases of sleep my brain goes through—and compares that to my recovery levels and heart rate. It can tell me if my rest is actually helping me recover for the next day’s activities. It also looks at my body composition, pulling data from my smart scales to track muscle, bone, and fat mass, and matches that against my energy balance—comparing what I eat against how much I actually burn.

One of my favorite features is how it handles training. It doesn’t just count calories; it records full heart rate curves and specific segments of my workouts, like a long run. It calculates my “training load,” which is a way of measuring how much stress I’m putting on my body compared to my long-term fitness level. This helps me see if my performance is actually improving over months or years.

It even adds a human element. I can log my mood and my general “presence” throughout the day. Over time, the tool starts to show me things I never noticed: “You tend to feel more stressed on days when your sleep architecture shows low deep sleep,” or “Your training intensity seems to drop whenever your calorie intake stays too low for more than three days.”

A piece of the fIOx puzzle

As I mentioned in the overview article of this series, everything I build is part of the larger fIOx system. BODY isn’t a standalone silo; it lives on my Mac Mini and is part of that central ecosystem. Because of the way I’ve structured fIOx with a central API/MCP and a Web UI, all the data BODY collects is instantly available to me through my own interface, reachable securely from anywhere via my Tailscale VPN. This means I can check my long-term health trends on my laptop or even interact with the data through the AI components I’m building.

My health status report

Right now, BODY is doing a great job of acting as a watchful assistant, flagging anomalies or “stale” data when my sensors stop reporting. But I’m not done. I’m currently experimenting on the “smart” aspect of the tool. I use Large Language Models (LLMs) to have actual conversations with my data. Instead of looking at a graph, I let the LLM analyze my health status. And the system already talks to me:

"Look, Martin, when I look at your numbers, I see a very dynamic picture. It’s great to see how you’ve massively ramped up your activity. You’re currently taking over x steps a day, which is a huge increase compared to your usual, rather low numbers of around x steps. Your energy expenditure from this activity has also increased noticeably. The fact that you’re also regularly incorporating rowing and running into your daily routine shows that you’re really putting a lot of effort into this right now.

But what also concerns me is the balance between your hard work and your recovery. Your body seems to be under a lot of strain right now. Your resting heart rate has risen slightly, and your body fat percentage has also crept up a bit compared to last month, even though your weight has remained almost stable. I’m particularly concerned about your sleep duration: At only about x hours, you’re still well below what your body actually needs to recover from intense workouts like rowing or long runs.

If you keep this up, you risk burning out in the long run. If the workload remains this high but your recovery—especially your sleep—is so short, your nervous system will eventually rebel. This could lead to you feeling constantly tired, a drop in your athletic performance, and a weakened immune system. Your body needs rest periods to repair your muscles and recharge your energy for the next day."

It’s a work in progress, a “spare-time build” that is slowly becoming a central part of how I understand my own body.