Complex equations with multiple input values

That's not a lot of information you are giving us.  For starters, which Excel product (Excel 365, Excel 2019, etc.) are you using, on which platform (Windows, Mac, the web...)?  Sometimes, the solution requires an Excel function or feature that is not present in older products.

The Excel formulas will depend on a number of factors; see the Calculations worksheet in the attached* file for my guesses.  Collecting those factors into one sheet, separate from the reference data, makes things clearer.
And without further information, I will assume that a larger Illness Factor should result in a larger MER.  (I multiply the RER by the Illness Factor to get the MER.)

There should be no reason for the users to enter a shorthand notation for the food product; selection from a dropdown list of the product names (possibly including container sizes) should be made available.  Excel's Data Validation feature allows this.  If any of your offered diets involve multiple food products, some design changes would probably be needed.

But this should give you a start, and help you develop ideas for the design.  Read the _Info worksheet for more information and references to documentation/tutorials.

* The "attached" file might not be attached.  Following a change to this forum's software in November, files that we attach frequently disappear.  If the attach does not work, I will put the file on OneDrive and post a link to it.

Edit: retrying the attach

Edit #2: That did not work, so get the workbook here: 2026-01-07 CT08 feeding calcns.xlsx