Are the nutrition content for staples (eg. milk, bread, eggs, apples) likely going to change any time soon? As for the processed foods, they might change from time to time, but I doubt they undergo drastic changes that the nutrition profile is entirely different. On the off chance that it does happen, you'll likely notice it on your tongue, and can update your brain accordingly. If you do any sort of research, you'll probably start to notice such processed foods aren't healthy, so you shouldn't be buying them in the first place.

>Or the time in your day you have to prepare food may suddenly shrink; do you then have the time and spare cognitive cycles to make a good decision under stress? (You probably don't. Not, like, you specifically, but in aggregate.)

>[...]

>It is difficult for me to know these things, let alone to in all cases exert the self-control to act on them

People aren't getting diabetes because they accidentally ate mcdonalds one time. It's from bad diet over a long period of time. For that reason your objection doesn't really make sense.

As mentioned in a sibling comment, while this is theoretically an issue, it doesn't happen often enough in practice that it causes a burden

>Ethical consumerism is a constant treadmill. Even knowing which brands are owned by which corporations is an absolutely exhausting affair.

Why are you mixing ethics into this? We were talking about making healthy purchasing decisions, not boycotting [insert brand here] because they punch kittens or whatever.