How to load a .env file's environment variables into both Bash and Fish shells on startup?

I want to have a .env file with a bunch of name-value pairs, like:

VAR1=xxxVAR2=yyy# sample C include dir use caseINCLUDE_DIR=/etc/include/xxx

I want these variables to be available in both bash and fish shells, such that I can access them in either shell at any time like echo $VAR1.