Jan 30 2014

Nutritional Content of Produce

This is a frequent question I receive that I have yet to address – is the nutritional content of our produce diminishing over time? The claim that our produce is less nutritious is often used as a reason to justify routine vitamin supplementation, and various other dubious health claims, or recommendations to eat locally or eat organic.

The answer is a clear, “it depends” followed by, “it’s complicated.”

There are three levels to this question I want to address: is the nutritional content of produce decreasing over time; if so, what’s the cause; and what should we do about it?

The most often study I see cited is this 2004 study by Davis et al – they examined the nutrient content of 43 crops from 1950 to 1999. They found:

As a group, the 43 foods show apparent, statistically reliable declines (R < 1) for 6 nutrients (protein, Ca, P, Fe, riboflavin and ascorbic acid), but no statistically reliable changes for 7 other nutrients. Declines in the medians range from 6% for protein to 38% for riboflavin. When evaluated for individual foods and nutrients, R-values are usually not distinguishable from 1 with current data. Depending on whether we use low or high estimates of the 1950 SEs, respectively 33% or 20% of the apparent R-values differ reliably from 1. Significantly, about 28% of these R-values exceed 1.

While significant, these results are not as bad as presented by those citing the study. Of the 13 nutrients followed, 7 showed no change, and 6 showed a decline, ranging from 6-38%. It’s interesting that when looking at individual nutrients in individual crops, no significant difference was found. The declines only emerge when grouping the data.

So, while it seems there is a real decline in some nutrients (slightly less than half those tested), the overall impact on nutrition seems minor. You can still get plenty of vitamins and minerals from eating your vegetables.

Variables Affecting Nutritional Content

For the declines that were detected, what is the cause? This is where things get very complicated. Those citing a single dominant cause, for example soil depletion, never cite any specific evidence to back up that interpretation. The authors of the study suggest that the primary reason likely has to do with the varieties of crops that are being grown.

The best discussion of the topic I found was this paper from Harvard:

By the time fruits and vegetables reach your kitchen counter – whether from a stall at a local farmers market, or the supermarket produce department – several factors determine their nutritional quality: the specific variety chosen, the growing methods used, ripeness when harvested, post harvest handling, storage, extent and type of processing, and distance transported. The vitamin and mineral content of fruits and vegetables depends on decisions and practices all along the food system – from seed to table – whether or not that system is local or global.

Blaming any single variable is fallacious. Just eating locally may not make a significant difference, for example.

There does seem to be a consensus that Davis’s explanation is likely a significant contributor to changes in nutritional quality. Farmers have favored over the years varieties that are most profitable and convenient – that grow quickly and produce large and pretty produce. However, these traits may come at the expense of things like flavor and nutritional quality.

Another factor for which there seems to be agreement is that for some produce picking them early so that they will transport better, and allowing them to ripen after picking, also can compromise nutritional content.

Therefore a local farmer’s market may contain heirloom varieties with inherently better flavor and nutrition, picked that day and fully ripened on the plant.

A factor which does not seem to matter is whether or not the produce is grown “organically.” I use the scare quotes because I am against the use of the false dichotomy that the “organic” label encourages – it is a suite of practices that should be looked at individually. In any case, decades of studies of organically vs conventionally grown produce has not demonstrated any significant difference in terms of health or nutrition.

Another variable that people often believe is important but isn’t is canned vs frozen vs fresh. A comprehensive study found essentially no difference in nutritional quality among these various methods. Canned goods, which often have a bad rap, were just as nutritious as fresh or frozen.

What about raw vs cooked? Heavy cooking can deplete some vitamins and minerals, but raw is not always best. Some vegetables are actually more nutritious if steamed – the light cooking breaks down cell walls and allows for greater absorption of nutrients.

What to Do?

What, then, is the average consumer to do with all this information. I think the bottom line is this – the most nutritious produce is the produce you will actually eat. Even the worst fruits and vegetables still have a higher nutrient density than other types of food. If affordable and convenient produce allows you to eat more, that is more important than slight incremental gains in some nutrients (but not others) by obsessing over the details. If precut veggies afford you the time to prepare veggies for dinner after a long day at work, then don’t worry about the slight decrease in nutrients that precutting causes.

But, if you want to maximize the nutrient density of your produce, there are some simple things you can do. First and foremost, eat a variety. The main difference to pay attention to is color, as produce color often reflects their nutrients. When picking individual fruits and vegetables, deeper colors may indicate denser nutrients.

If you have the time and resources, buying produce that was picked recently, allowed to ripen on the plant, and did not have to travel far will give a boost to the nutrient density. Such produce is also likely to be more flavorful, and if you enjoy them more you are likely to eat more (or get your kids to eat more).

If you have the time and inclination, growing some of your own produce can give you access to heirloom varieties that you can pick when fully ripe, right before you consume them.

Fresh, canned, or frozen are all fine – whatever is most convenient and cost effective for you.

I don’t think the organic label is worth paying premium prices for. This is a marketing scam or ideology, in my opinion. I do think we should incorporate best practices in farming, and not worry about arbitrary and highly politicized labels.

The Farming Industry

I do think, however, that the farming industry has been moving toward monoculture (reliance on single varieties at the expense of local varieties) and cultivars that are optimized for profit rather than flavor or nutrition.

Monoculture is a problem on multiple levels, but perhaps worst is that it leaves our agriculture vulnerable to blight and pests. If you plant millions of the same exact variety, something will evolve to eat it. The most sustainable solution is to utilize a large number of varieties. Even if farmers plant mostly the most profitable types of produce, they should mix in other varieties.

Another solution is for farmers to cultivate (by whatever method – I won’t go down that rabbit hole here) varieties that reintroduce improved traits for flavor and nutrition while maintaining the advantages of mass production. That would be a win-win. Obviously if this were easy, they would already be in wide use. It seems like this is worth some investment in resources.

Conclusion

I want to emphasize the bottom line – fruits and vegetables are the most nutrient dense foods we eat and should be a major part of everyone’s diet. Differences in the various types and methods of producing and processing fruits and vegetables have an overall small impact on their nutrient density. It would be counterproductive to sacrifice the amount of fruits and vegetables consumed (because of expense or invonvenience) because of attempts to have small gains in their nutrient density.

 

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