First, let me give a blanket apology for all of us GPs – this is the first time ever all four of us have NOT posted in the same week. I’m on the road this week with my high schooler checking out colleges, and I think the other three are out drinking beer and tipping cows somewhere. So our visiting GP veggie specialist extraordinaire has graciously stepped in to answer a reader’s question about the apparent decline in vegetable nutrition. Here’s Charlie:
Your United States Department of Agriculture tracks information about all kinds of things, like dry bean production and farm wage data. They also measure nutrient content of foods (not pesticide residues–that’s for the FDA). Some curious researchers have wondered if the nutritional content of vegetables has changed since the mid-20th century. The data exist, so why not look through them?
Authors of a well-cited publication from 2004 have done just that. Specifically, Davis, Epp, and Riordan did (J. Amer. College Nutr., 23:669-682). What they found, for example, is if you ate cauliflower in 1950, you probably ate more protein, phosphorous, iron, and thiamin than if you ate the same amount cauliflower in 1999. They measured the ratio of the nutritional concentration in 1999 compared to the concentration in 1950 [smartly, they adjusted 1950 moisture content to match that of 1999]. If ratio was 1, there was no difference in the concentration. If the ratio was 0.5, then 1999 cauliflower had half the nutrition of 1950 cauliflower. They had to use some statistical trickery (they didn’t know error or the number of samples from 1950), but some people might just call that ‘educated assumptions’. When these ‘educated assumptions’ must be made, I’m a big fan of being conservative with them–in this case, that means that if there is a tiny difference, the researchers wouldn’t catch it. Being conservative with statistics makes the differences that show up more robust. Even with the most conservative assessment, the authors show that 26% of the time when nutrients are studied in vegetables, the concentration was lower in 1999 than in 1950. However, 11% of the time, the concentration was higher in 1999.
The primary author of that paper published a summary of evidence in 1999 (HortScience 44:15-19). The average numbers for a bunch of studies show similar declines, but statistically, there seems to be a significant decline in specific nutrients in about ¼ to ⅓ of vegetables studied over time.
‘Jade Cross’ brussel sprouts
Why would this be happening? Well one reason might be dilution. The review article gave an example of raspberries: growing raspberries with more phosphorous fertilizer gave more yield (on a dry weight basis), and higher phosphorous concentration in the fruit. But the plants still took up the same amount of calcium (or only slightly more), irrespective of how many pounds of raspberries were produced. More pounds of raspberries with the same pounds of calcium removed from the soil means less calcium per pound of raspberries. That makes sense. The plants can make much of their own dry matter (photosynthesis!), but they can’t make calcium. I have some questions about using the dilution argument for the 2004 paper: if dry matter didn’t change, but concentration of macronutrients went down, the concentration of something else had to go up–but what? Is the decline in specific things large relative to the concentration of that thing but small relative to the total dry matter?
The dilution effect may be the cause sometimes, but what causes the dilution effect? Atmospheric CO2, or changes in production practices like irrigation, pest control, and fertility might be important, but I like the ‘breeding’ explanation. Breeders don’t care how much calcium the plant has. They care if it yields well (dry matter), is resistant to pests and diseases, is pretty or unusual, tastes good, etc. If a trait is not selected for in a breeding program, it might go away over time. So maybe the answer is to breed veggies that accumulate (or make, if it’s a vitamin) more nutrients, or to grow more of the existing varieties that might, by chance, already have relatively high nutrient concentrations (they do exist). There may be a market for selling broccoli that has certifiably more calcium in it, and for change to happen in the marketplace, it has to be profitable. For right now, you have no idea if the broccoli you buy is a low-calcium or a high-calcium variety because consumers don’t demand to know.
The un-interesting headline reads “some vegetables may be declining in average nutrient concentrations over time”. The interesting (and false)
headline would be “vegetables aren’t good for you anymore”. From the cauliflower example above: in 1999, a serving of cauliflower would have about 2.5% of your recommended daily iron, 6.3% of your phosphorous, 3.5% of your protein, and 4.8% of your thiamine. In 1950, it would have been 6.1% of your iron, 10.3% of your phosphorous, 4.3% of your protein, and 9.2% of your thiamine. Your vegetables aren’t devoid of nutrition, they’re good for you. Easter candy probably has none of those things. If you’re worried, have a multivitamin, or better yet, eat MORE vegetables. But vegetables grown in 1950 are rather old by now, I’d avoid them if I were you. Meanwhile, know that a) science is aware of the issue, b) it’s not universal.
10 thoughts on “Our visiting professor takes on veggie nutrition”
Ahem! I resemble that remark. I was neither beer-drinking nor cow-tipping (though both would be preferable to current state of affairs) – was just waiting for someone else to post and then *boom!* it’s Friday again. Time flies… Regarding Charlie’s post – very interesting! I think there are a few breeders out there working on added nutrients (I posted about orange cucumbers a while back). Jim Myers at Oregon State is one…his tomato ‘Indigo Rose’ was bred for high anthocyanin content. Plus it’s CUTE!
Yes, yes, I love the breeding explanation too!
I’m not planting blue tomatoes because the goal of the breeders was to achieve nice color, not the taste!
Interesting, I’ve read stories about nutrient decline but never seen the studies to back it up. I’ll have to go check it out.
Part of the research I do is finding vegetables that have more cancer-fighting chemicals compared to others, and how to grow them so they have more. We’ve found that Brussels sprouts, for instance, have more glucosinolates than other brassicas we’ve looked at. And I applaud efforts to look for (or breed) increases in nutrients and phytonutrients in veggies. There is value in that. But I also think there is lots of value in getting people to eat vegetables in the first place. Many people don’t, and doubling the thiamine in cauliflower won’t affect people who don’t eat cauliflower. As for that cute tomato, it’s easy to find a vegetable with more anthocyanin in it at the store / market. Our eyes have good anthocyanin detectors. Evaluating relative glucosinolate, calcium, or thiamine content of veggies is impossible as a consumer.
Speculation is intellectually creative fun but it wouldn’t be very hard to compare older varieties to newer ones to see if it is breeding or farming that is the source of the decline.
I find the numbers here alarming in terms of the potential affect on our national health. We are talking about the diets of hundreds of millions and the affect of a decline of even a small percentage of nutrients could have huge consequences.
This article is very interesting but I believe the tone should perhaps be more serious. We already know that deficiencies and excesses in the American diet are killing and disabling us at a level usually described as epidemic. I believe the research shows that you can’t correct deficiencies in your diet by swallowing some pills.
Before I start, I need to declare that I am not a nutritionist or epidemiologist. So with that, I argue that the decline of a small percentage of nutrients in vegetables probably does not have huge consequences on nutrient deficiencies (in Americans). And it’s not because we all get enough nutrients, it’s because we do not even come close to eating the recommended amount of fruits and vegetables. This is especially true for low-income Americans. Epidemiological studies (not as good as controlled studies, mind you) indicate health benefits from incremental increases in fruit and vegetable intake. So a 5% increase in calcium in raspberries would be great, but I think a 5% increase in raspberry consumption would be great too, probably even better than incremental nutrient increases. The CDC publishes information about fruit and veggie intake, and relevant results are included in this interesting analysis (it’s about potential cost and effectiveness of price supports in encouraging fruit and veggie consumption in low-income households): http://www.ers.usda.gov/publications/err70/err70.pdf. As far as supplemental vitamins are concerned (and again, I’m not a nutritionist), but I think the NIH sums it up nicely: “… the people least likely to get enough nutrients from diet alone who might benefit from [multivitamins & mineral supplements] are the least likely to take them.” http://ods.od.nih.gov/factsheets/MVMS-QuickFacts/. Most of us in the US who garden and think about fruits and veggies and eating healthy probably do pretty well, with or without a multivitamin (too much vitamins are bad sometimes too…). For even further reading, there’s some interesting stuff about poverty and malnutrition in this article: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1187906/ and in the references therein.
The suggestion to take a multivitamin may not be a good idea:
Over the last few years I’ve read about a lot of debunking research as far as the efficacy of multiple vitamins but it would be illuminating if someone like Linda did a thorough review of current research
The example of cauliflower at the end of this article suggests a huge drop off of nutrient content- nothing subtle there at all. The fact that the majority of Americans don’t eat enough of their vegetables only makes the numbers more of alarming to me.
I would like to read a nutritionists take on this. A more than 50% drop off of certain essential vitamins, yikes! No wonder so much of
store bought produce is tasteless.
James: It would be hard to argue that a multivitamin wouldn’t benefit a child with pellagra, for example (though he would benefit from nuts and green leafy veggies too). But yes, the effects of excess nutrients from supplemental vitamins, and the fact that supplements do not contain other phytonutrients or fiber, is an important thing to note.
Charlie, I think it may be more complicated than simply the things missing in a vitamin pill- it’s also the way they all work together including how plant components affect absorption as well as possible unknown nutrients. Obviously certain debilitating deficiencies can be successfully treated with a supplement but that doesn’t mean that they are broadly affective in compensating for a diet lacking in adequate nutrition- even beyond the fiber and anti-oxidant issue. This is my understanding of current research based on casual although attentive reading in popular media. That’s why I’d like to hear from someone more knowledgeable.