I've thought about making something like this in the past.

I've used the "Nutrients" iOS app for tracking nutrition, but not in the way it's meant to be used. Maybe my usage patterns could help you writing your app.

I tend to use the app to get a pulse on the nutrient density of the foods I consume (or feed my daughter). I would make simple meals, and plug them in, and then try to get all the nutrient bars filled up. It was harder than I expected. It was a puzzle to figure out because foods have different levels of each nutrient. I want to avoid adding onto nutrients I'm already consuming enough of.

I tried to use the Nutrients app to search for foods dense in some nutrient I was lacking, but I often found Google searches to be better for this. The way the app ranked foods wasn't useful to me. Was it measuring nutrients by weight? What if I wanted to rank by price, or by region? I don't care that raw Moose Liver has lots of Riboflavin.

I preferred using the app to determine my grocery list because I don't like recipes. I want to know how to cook things individually (pasta, rice, eggs, asparagus, etc) with salt + (butter or oil), and then figure out how to assemble meals on my own. With recipes, I would often have leftovers I didn't know what to do with. I could look up more recipes, but I couldn't see how this would make me a better cook since I didn't know what I was doing or why. I was inspired by Samin Nosrat's Salt Fat Acid Heat approach to cooking. This way I could get nutrition and flavor simultaneously.

This all got really complicated, and I eventually figured I wouldn't reach the end of it. For example, rice grown in different regions has different levels of arsenic. I'm not concerned about arsenic specifically, but the finding got me more curiously interested in toxins, and soil differences around the world. I got into nutrition thinking I could be convinced of one specific diet over another, but I soon found myself looking into differences between soil in different regions.

After I used the app enough, I got a sense of some of my blind spots, and used that to adjust my diet intuitively.

Some changes that more-or-less stuck:
- More sun for Vitamin D
- More Avocados
- Omega-3 from fish oil
- Nutritional Yeast for B Vitamins
- More greens (especially for magnesium)
- Spinach in smoothies
- Less sugars, carbs, and bread
- Parmesan cheese for calcium
- More beans

I have decent intuition around green means chlorophyll molecule means there's a magnesium atom in there, and some others. The minerals are easy enough for me to get enough of. I can usually get enough Vitamin C. I don't have good intuition around Vitamin K, E, Niacin, Riboflavin, Folate. Beans have lots of Folate. This makes sense, but lots of other foods I regularly eat have it too.

I'm inspired to get back into this and start tracking again.

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BACKGROUND:

After my daughter was born, I was suddenly extremely interested in nutrition. I worried what might happen if my daughter started missing important nutrients. However, it was hard to get trustworthy information on nutrition. Important debates weren't settled. I wasn't confident that I could trust things like the food pyramid. Like you, I felt more confident about using micro and macro nutrients as a way to decide what to eat, but also to compose meals that were nutritionally complete. This is something I didn't see much focus on. People would tout some specific food as "healthy" without putting it in context.

From there, I still wanted to cover my bases for unknown unknowns. If I added more traditional foods, I'd be able to cover for it. As an outsider, I don't know how likely it is that we've discovered all the nutrients we need. For example, I recently saw a research paper asking if Lithium is a micronutrient. Maybe there were foods that had nutrients that weren't discovered, or maybe different people need different levels of the same nutrients. Maybe microplastics are a bigger problem than we imagine. It's hard to account for everything. I wanted a baseline I could start from. I looked into traditional slavic foods. I found that potatoes were more recents, for example, so I wouldn't use them to cover for unknown unknowns. However, cabbage and buckwheat are both nutritionally rich and slavic staples. Maybe I could use this finding to trust dishes that feature these ingredients.