How do you explain people like myself, who don't take any supplements, don't have any vitamin deficiencies, and are in good health?
The study you linked finds varying decline in vitamin and mineral content of common crops over time, not a study showing that people who don't take multivitamins have vitamin deficiencies. And it has methodological problems highlighted by a followup study [1]: "Comparisons of food composition data published decades apart are not reliable. Over time changes in data sources, crop varieties, geographic origin, ripeness, sample size, sampling methods, laboratory analysis and statistical treatment affect reported nutrient levels."
[1] https://www.sciencedirect.com/science/article/pii/S088915751...
