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Nutrient levels in food have dropped to the point they cannot sustain human life as it, espcially when processed. In the 1930s we began adding vitamins to food to replace that which was lost in processing or absent in the first place - Niacin (B3), Thiamine (B1) and Riboflavine were added to all flour in which cured the Pellagra epidemic by the 1930s. But in 1998 we had to add Folate to do something about the level of birth defects from lack of a vitamin found in primarily leaves, which is where the names "Folate" and "Folic Acid" come from: foliage.

"Since the advent of mandatory folic acid fortification in 1998, neural tube birth defects have dropped by 20 to 30 percent, and studies have shown that far fewer people have low levels of folate in their blood."
https://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/vitamins/vitamin-b/

"Cooking vegetables can result in thiamin losses ranging from 11 to 66% WW, depending on the commodity and cooking process"

"Next to vitamin C, thiamin is the least stable of the vitamins to thermal processing"
Nutritional comparison of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds
Joy C Rickman, Diane M Barrett and Christine M Bruhn
http://ucce.ucdavis.edu/files/datastore/234-779.pdf

loss

Dirt Poor: Have Fruits and Vegetables Become Less Nutritious?

Because of soil depletion, crops grown decades ago were much richer in vitamins and minerals than the varieties most of us get today

A landmark study on the topic by Donald Davis and his team of researchers from the University of Texas (UT) at Austin’s Department of Chemistry and Biochemistry was published in December 2004 in the Journal of the American College of Nutrition. They studied U.S. Department of Agriculture nutritional data from both 1950 and 1999 for 43 different vegetables and fruits, finding “reliable declines” in the amount of protein, calcium, phosphorus, iron, riboflavin (vitamin B2) and vitamin C over the past half century. Davis and his colleagues chalk up this declining nutritional content to the preponderance of agricultural practices designed to improve traits (size, growth rate, pest resistance) other than nutrition.

Mercola writes:
"Note that while heat processing tests lowered the vitamin C in sweet corn and tomatoes while raising the antioxidant activity, scientists tested two other types of vegetables - a root (beets) and a legume (green beans).

The antioxidant activity of beets stayed constant despite an 8 percent loss of vitamin C and 30 percent loss of dietary folate. The interesting result came with the green beans: the vitamin C and folate content in the green beans remained, while noting a 32 percent reduction in phenolic compounds during processing, including a 20 percent decrease in antioxidant activity."

http://pubs.acs.org/doi/abs/10.1021/jf034861d,
Antioxidant Activity of Processed Table Beets (Beta vulgaris var, conditiva) and Green Beans (Phaseolus vulgaris L.),
Sept. 2012