That's the sales pitch for multivitamins and the like; they talk you into a problem but they provide the solution.
Certain vitamins your body will dispose of safely (like vitamin C), but there's others that just build up until they kill you; there's the case of Xavier Mertz [0] who was theorized to have died from an excess of vitamin A after eating his sled dogs' livers.
But the basic rule is: just eat normally / "a balanced diet"; you likely don't need supplements unless you have absorption issues or stick to a specific diet (e.g. vegetarianism / veganism, soylent / diet shakes, or the same thing every day, like the 14 year old who only ate chips and ended up (partially) blind due to a vitamin deficiency [1])
The question is one of optimization. There are arguably optimal doses of particular nutrients (i.e. Vitamin D) that would be extremely hard to achieve through "a balanced diet". Also most people do not and many can not eat a "balanced diet", as that requires a ton of effort to maintain.
To put it another way, if the benefits of exercise were available in pill form on grocery store shelves, everyone would be taking that pill. Saying "you could save some money by just exercising normally!" is making a lot of assumptions about what people are willing and able to do, and frankly you'd be a fool to waste time exercising if the benefits were achievable through a magic pill.
Supplements aren't quite that magical, but they make optimal, hard-to-achieve doses achievable, even on a bad diet. Granted they can also be misused, or doses can be so low/ingredients not bio-available such that they have no impact. They're a tool like anything else, and can be used correctly or incorrectly. Part of what bugs me about the meta-analysis saying "supplements don't reduce all-cause mortality!" is that they lump wildly different supplements with different make-ups, taken in different doses on different schedules from different starting diets with different amounts of discipline/regularity. Of course at a population level the number of people using them wrong probably outnumber the people who do their research and maintain a disciplined approach. Your average person has issues getting their car's oil changed and understanding the difference between synthetic and conventional oil. A skilled individual could properly apply and see benefits from supplements, just as a skilled individual could successfully change their own oil.
No, I'd say there's high consensus it's one that satisfies all of your vitamin and mineral RDA's. Also very high consensus it at least includes vegetable staples like broccoli, cucumbers, onions, tomatoes, and green salads with some way of obtaining proteins like meats or beans.
Even that balanced diet will consist of less and less minerals and vitamins as time goes on and growers optimize for vegetable, fruit VOLUME instead of vitamin/mineral contents.
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The level of decline varied depending on the specific nutrients and the type of fruit or vegetable, but it generally ranged from 6 percent for protein to 38 percent for riboflavin. In particular, calcium dropped most dramatically in broccoli, kale, and mustard greens, while the iron content took a substantial hit in chard, cucumbers, and turnip greens. Asparagus, collard greens, mustard greens, and turnip greens lost considerable amounts of vitamin C.
Further studies since then have backed up the case that nutrient levels are dissipating. Research in the January 2022 issue of the journal Foods found that while most vegetables grown in Australia had relatively similar iron content between 1980 and 2010, there were noteworthy drops in certain veggies. Declines in iron content, ranging from 30 to 50 percent, occurred for sweet corn, red-skinned potatoes, cauliflower, green beans, green peas, and chickpeas. By contrast, Hass avocados, mushrooms, and silverbeet (another name for chard) actually gained in iron.
Grains have also experienced declines, experts say. A study in a 2020 issue of Scientific Reports found that protein content in wheat decreased by 23 percent from 1955 to 2016, and there were notable reductions in manganese, iron, zinc, and magnesium, as well.
Food storage and preservation also has a lot to do with it as well. Oxidation makes a lot of nutrients mostly unavailable, and degradation can happen quickly. Minerals in vegetables aren’t chelated, so while they sit in refrigerated storage, they are still rapidly losing nutrition.
Certain vitamins your body will dispose of safely (like vitamin C), but there's others that just build up until they kill you; there's the case of Xavier Mertz [0] who was theorized to have died from an excess of vitamin A after eating his sled dogs' livers.
But the basic rule is: just eat normally / "a balanced diet"; you likely don't need supplements unless you have absorption issues or stick to a specific diet (e.g. vegetarianism / veganism, soylent / diet shakes, or the same thing every day, like the 14 year old who only ate chips and ended up (partially) blind due to a vitamin deficiency [1])
[0] https://en.wikipedia.org/wiki/Xavier_Mertz
[1] https://www.acpjournals.org/doi/10.7326/L19-0361, https://www.cbsnews.com/news/british-teenager-went-blind-fro...