This has come up in my feeds a handful of times lately, so I guess it's semi-viral
Headline:
Vegetables half as nutritious as they were in the 1960shttps://www.yahoo.com/news/science/articles/vegetables-half-nutritious-were-1960s-092255734.html
QuoteThe decrease in nutrient and vitamin value is thought to be down to declining soil quality and crops being selectively bred for their ability to grow quickly, rather than there being any focus on their health benefits.
Maybe.
Followup to the headline:
Are carrots becoming less nutritious?https://www.bbc.co.uk/programmes/w3ct9998
It turns out there are many factors here.
One is the simple question of whether the older assays were accurate. Robin Marles found that in general the traditional values were based on a very small number of samples and old methods.
Much of the data from 1960 is actually from a 1960 table with assays from 1936. One big change to the 1991 assays they were compared to was an increased use of imported and "out of season" produce. They also did not adjust for water content. So the numbers are based on total weight, not dry weight, and modern produce typically has a higher water content.
https://www.sciencedirect.com/science/article/pii/S0889157516302113
[in general, nutrient decrease during refrigeration is something we don't discuss very much - 100 years ago people distinguished between "fresh" food and refrigerated food - we now assume almost all "fresh" produce has been refrigerated]]
The main assertion is that appears in almost all of these articles is that mineral depletion of the soil is the driver. The BBC interviewed Steve McGrath, a researcher at
https://www.rothamsted.ac.uk/
They have been researching British wheat since 1843, collecting the soils, crops and even the fertilizers. They found things stayed about the same until the 1960s or 1970s.
At that point, they found some big declines, including a 50% drop in zinc. Iron, magnesium and copper were down by 20-30%
The big change was down to a shift to high-yield, short-straw wheats. That results in lots more wheat and a lot less straw. They aren't sure what the mechanism is, but the "dilution effect" is real: when you increase wheat yields per acre, you tend to get less zinc per kg of final product.
[McGrath notes that the nutrients he studies are not very water-soluble, so it seems plausible that as you user more water and sunlight to build carbohydrate chains, the minerals might not leach into the water fast enough to make up for the change in yields - but so far they haven't pinned down the exact reasons]
But they do know that it is NOT a function of soil quality. If anything, mineral concentrations have increased over the 180-year timespan. A lot of the micronutrients are not particularly water-soluble and so they tend to accumulate. The bigger problems are things like loss of carbon, erosion and salinification. "But there is no data to support the idea that we're washing out all the nutrients."
Again, this applies only to wheat in England. Beyond England? He says that there is almost no data for soils.
His guess is that all the grains that have substantially increased yields probably have lower nutrient density per kg than they did in the past, but
1. This is generally not a problem if you have a balanced diet
2. It is not a problem with mineral depletion of soils
[Since people are also eating a lot more calories than they did 100 years ago, that also means that even at lower concentration, there has to be some offset]
This is a good rundown too.
https://hillarylinmd.com/guides/are-modern-vegetables-less-nutritious