But why? It makes no sense to take in a substance you would get from food anyways but isn’t absorbed by the body if it’s just by itself?

It does get absorbed by the body if it's by itself. The key is that, whether it's in your food or a supplement, your body will not take in more than it needs. So unless you're deficient in a particular vitamin, any extra won't get absorbed, and you'll just pee it out. So supplements are good for people with specific vitamin deficiencies, to make up for that lack, but for anyone without a deficiency, they just give you expensive urine.

I speculate a lot of people (in the US) don't eat particularly well.

i.e. the lack of fruit/veg/nuts/legumes/fiber and the amount of fat/sugar/salt/carbs/indirect-crazy-chemicals in food.

My main concern is that when I get "old" I have "X" becz I didn't eat enough "Y". e.g. eat things and potentially take something to help/avoid glaucoma.

At worst (if I'm not stupid) I have expensive urine. With a bit of luck I've reduced my chances of getting cancer/heart-disease/stroke by a few points.

I mean, maybe, but that's definitely a question to ask your doctor. Instead of self-diagnosing with a deficiency that you may not have and then spending money to treat it in a way that does nothing if you were wrong, ask your doctor, get a blood test, and get a real diagnosis to know if you need supplements.

Especially since, as stop50 mentioned in this thread, there are some vitamins that you can actually overdose on without meaning to as they get stored in fat and accumulate. Then you're causing yourself harm based on guessing you might have a deficiency. Doctors, medicine, blood tests: they all exist for a reason. Get the information before taking action, don't make guesses about your health.

Well some vitamin excess causes harm, for instance high vitamin c intake is associated with kidney stones

Sure, that's what I mean about not being stupid.

Well, combine that fact with the fact that the majority of Americans are deficient in at least one vitamin, and the study seems to be saying:

Vitamin supplements don't help, except for most people.

This is not correct. An individual nutrient might not be as well absorbable by itself: it might need to be packed with other chemical from REAL foods to be absorbed.

Eg we know that different form of magneisum supplementation are absorbed in a very different % based on which substances they are "packed with".

It becomes even more complicated because certain foods might block nutrient absorption or certain diets might cause a loss of certain nutrients much more (eg high carb vs keto).

While you're right that that some nutrients are better (or worse) absorbed based on what other nutrients you digest at the same time, that doesn't really change the fact that you won't absorb/use more of any given nutrient than you need, so without a deficiency, supplements don't help.

This is only true for vitamins that disolve in water. fat disovable vitamins (vitamin d for example) are stored in the body and can overdose more easily.

True, but I think that means "still not helpful, and possibly even harmful for those"... so still no good reason to take vitamin supplements without a deficiency.

I'm agreeing to that.

Then it still sounds like it is completely beneficial.

It doesn't work like that because real food has millions of god-know-which chemicals that act in tandem with the micronutrients being absorbed.

If those chemical are not present the absorption in the gut will not work or might not be as effective.

This "eat and it will be absorbed" is a severe error: the absorption process is an INTERACTION of many things, not just individual stuff being absorbed independently.

This is why it's a severe error to isolate stuff into a new processed food and claim it will have the same results as the foods that it was made from: the processing will remove god knows which chemicals and will then eg modify it by heat which modifies stuff even further: thus you end with a totaly different substance in the end.

This is why saying stuff like "we grow meat from bacteria to replace real meat" is ridiculous since real meat is a lot more than just "protein".

I wonder if the same applies to naturally grown fruits and vegetables today. Because most varieties of fruits and vegetables are selectively bred over millennia to produce bigger, sweeter, more shelf stable versions such that many modern versions would be unrecognizable to those from the past.

It probably does, must fruits and veggies that we know of didn't exist ~500 years ago. All modern fruits are sugar bombs compared to "old" fruits. Old fruits were more bitter and much smaller.

Quick changes through breeding might change their nutritional composition also, not just sugar.

A great example is Weston Prices's book "nutrition and physical degeneration" where he clearly shows that whenever processed foods (sugar and white flour based products) entered isolated cultures their teeth got destroyed asap. And destroyed teeth are only the most visible side effect, god knows what else is happening to our bodies.

We don’t know this. This is speculation on your part (for what it’s worth, it does sound feasible).

As pointed out by vlod, eating supplements with your meals should solve this, at least partially.

Then taking vitamins and suppliments during my main meal should alleviate that?