Tuesday, February 03, 2009

Evidence on declining fruit and vegetable nutrient composition

Research by Donald R. Davis who retired from Biochemical Institute at The University of Texas (in my hometown of Austin) and in conjunction with the Bio-Communications Research Institute in Wichita Kansas, has summarized three kinds of evidence that points towards the decline in the nutrient value of fruits and vegetables in the US and UK over the last 50 to 100 years.

The report, published in February 2009 Journal of HortScience, reviews a highly cited research study titled "The dilution effect in plant nutrition studies" published in 1981 by Jarrell and Beverly in Advances in Agronomy. Jarrell and Beverly found that fertilized plants contained larger absolute amounts of minerals than the unfertilized plants, but these amounts were sufficiently diluted by the increased dry matter that all mineral concentrations declined, except for phosphorus, which is the common fertilizer.

Next, Davis looked at historical food composition data derived from three quantitative reports. While these studies are limited by their ability to be compared due to variation in methods, they found:
apparent median declines of 5% to 40% or more in some minerals in groups of vegetables and perhaps fruits; one study also evaluated vitamins and protein with similar results.
Finally, Davis evaluated studies of collections of cultivars of a single food that was grown side by side for purposes of comparing their nutrient content. The foods were broccoli, wheat and maize. The side by side allows the elimination of using historical data in the form of averages (as was done in the previous section), which allows all environmental conditions (soil, fertilization, irrigation, pest control, climate, harvest, sampling, and analytical methods) to be held constant. He found:
plantings of low- and high-yield cultivars of broccoli and grains found consistently negative correlations between yield and concentrations of minerals and protein, a newly recognized genetic dilution effect.
In conclusion,
Further studies are needed to assess the generality of dilution effects among foods and to greatly expand the numbers of nutrients and phytochemicals considered. Side-by-side comparisons of multiple cultivars in multiple environments can provide rigorous answers to the many remaining uncertainties. They are also well suited for testing proposed environmental and genetic methods to overcome dilution effects. Specifically, we would like to find ways to decrease the inverse correlation coefficients between yield and nutrient concentration or to decrease the negative slopes in plots of nutrient concentration vs. yield.

Over three billion of the world’s population is malnourished in nutrient elements and vitamin,including in developed countries. Vegetables and fruits are among the richest sources of many nutrients. Thus, declining nutrient concentrations in horticultural products are most unwelcome. Past and ongoing efforts to increase yields, combined with apparent broad tradeoffs between yield and the concentrations of perhaps half of all essential nutrients, work against recent efforts to increase one or a few micronutrients in individual foods.


Anonymous said...

By yield, do they mean per plant/acre or per fruit? If the latter, it seems reasonable to assume that larger fruits would usually result in a lower concentration of nutrients per gram. Most things have diminishing marginal returns. I guess the question, though, is how you maximize nutrients overall for the consumer. Sure, per pound they may be purchasing fewer nutrients, but does it keep the cost lower overall by using less acreage/plants/resources, and does this result in cheaper nutrients and an easier way to distribute nutrients to consumers than lower yielding plants with smaller fruits that are more highly concentrated?

Those with a garden know that you often starve your plants as they're fruiting to concetrate the flavors. Tomatoes and other berries are best when they get as little water as necessary once they start producing.

But this also results in much fewer pounds of fruit per plant. Trade-offs.

Anonymous said...

Cost, size, and flavor seem less important on the topic of sustenance and nourishment. I understand that consumers want more flavor and lower cost; however, most methods may be doing more harm to each person than good in the long run. Perhaps we need our palates to become more sensitive to flavors, distinctions, textures, and to listen to our body. There is pleasure in eating; however, the point is growth, health, and nutrition.

To each his own preference and yet we're affecting people who have little awareness and less choice until messages are more widespread and practices changed.

Each vegetable, fruit, nut, seed growing from the earth has specific chemical make up and the nutrients impact people differently, at various stages in their life. Eating for the sake of eating or for communal practices while popular makes less sense than eating for health and letting food be a natural healer, medicine, as people have directed for ages.

Nada said...

Extramsg may be right that, in a straightforward way, the lower cost/yield ratio may mean that it still costs less to raise x% of your RDA of vitamin C in the form of broccoli now than it did in 1850. His/her point that there's a economy of scale with getting more fruit from a single plant is especially insightful. But, I think there are a number of factors that may make this fact irrelevant.

First, America is currently producing too much food. Vast storehouses of food, especially grains, are destroyed or left to rot every year. (Observe the American Corn Growers task Washington with the task of, in their words, "curbing overproduction".) The overproduction that doesn't go to waste is often used to flood foreign markets and put local farmers out of business, actually decreasing global arable land. (See also BMJ and CARE's whitepaper on food aid.) A lot of people are going to warn me that, even though food is being overproduced now, in 10 or 20 years we might be in a "food crisis" due to climate change or overpopulation. The real crisis will come when our production is centered on only a few major crops in only a few major areas, when overproduction muscles out global agricultural diversity. Then, a small climate change, a new pest, or a new disease could swiftly wreak havoc. Not to mention - the ridiculous amount of pesticides and fertilizers we use to sustain these hyperproductive monocultures are demonstrably causing environmental harm. We're borrowing capacity from the future, when we may need it much more.

Apologies - I'm rambling. My point is, we probably don't need all the extra production we have. We could cut production and still exceed demand.

The second factor is that it's a little naive to expect eating more food to make up for lost nutrient density. First, it's often not possible or reasonable to expect a person to eat enough of the food to make up for lost nutrients. How would you feel about eating twice as much broccoli? Few people in America get anywhere near enough produce as it is, and that's assuming they're eating nutrient-rich varieties. Besides, your system can only handle so much roughage, heh heh. Second, it can actually be damaging to expect people to eat more food to get the same number of nutrients - we're in the midst of an obesity crisis the last time I heard. I doubt anyone in America is fat from doubling up on broccoli, but wheat? Sure.

(I remember reading someplace about a study showing that cravings for high-fat and high-sugar foods were much worse in malnourished people, but I'm having trouble finding a decent source. But, o brutal irony!, your appetite is not all that discerning. If plants aren't supplying your nutritional needs, you may find yourself eating extra chocolate and french fries as well as extra wheat.)

At any rate, the very last thing America needs is to eat more nutrient-light food (nice summary - follow through to the NHANES report).

Amanda Crowe said...
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Michael LaBelle said...

Good article and good comments all around. To a great degree, the available mineral content of the soil determines the nutrients in the plant. Adding plant available minerals, coupled with increasing the soil biology to facilitate the availability of those minerals will go a long way to ensuring that plants have the building blocks needed to produce the vitamins we expect from our fruits and vegetables. Without minerals, the plants CANNOT make the nutrients you want nor will they be able to take up the minerals you need to maintain your body.