Finding nutritious food can be a difficult task. The misleading packaging, empty claims of goodness and nutritional content, the complicated laws around food packaging in different countries coupled with a busy lifestyle can cause someone to make the wrong choices for themselves and household.
An app, Android and iOS based for the first iteration would allow barcodes to be scanned whilst shopping. A simple, nicely designed user interface with use of colours depicting levels of nutrition would tell you if you're about to make a mistake.
You want people to have to enter basically nothing, the entry to barrier has to be low, you're already asking users to scan barcodes of each product they buy which could become a chore in itself, don't add more to the already too much to ask nature of your app.
This is why a user would not enter their height, weight or any other info. This isn't a weight loss app, it's a nutrition companion app to help people make the right choices and just be informed. Simple graphics showing how much a particular product contributes to the recommended daily intake for a child, teen, man and woman is all you would need.
There are numerous applications out there trying to achieve similar goals, most require you to manually input information. This app in particular uses the premise of traffic lights for determining the nutritional content of food by entering the values from the nutrition label manually.