© Whole Foods magazine
Why Foods Alone Are Failing Us
Significant declines found in the nutritional values of vegetables and fruits.
An Interview with Donald R. Davis, Ph.D. – Part 2
By Richard A. Passwater, Ph. D.
This is the second in a two-part interview with Dr. Don Davis of the University of Texas at Austin. Dr. Davis has worked at the Clayton Foundation Biochemical Institute since 1974. He also has been director of the Roger J. Williams Nutrition Institute (1987-1990). In addition to his research at the Biochemical Institute at the University of Texas, Austin, Dr. Davis also serves on the editorial boards of the Journal of Applied Nutrition (editor-in-chief 1986-1991) and the Journal of Advancement in Medicine. Also, he formerly served on the editorial board of Journal of International Academy of Preventive Medicine (1983-85). He has published more than 90 technical articles and letters.
He also had the good fortune to work with Dr. Roger Williams, and to be mentored by this true nutrition pioneer, for many years.
Recently, Dr. Davis made some front-page news when he presented the results of studies conducted almost two years ago at a meeting of the American Association for the Advancement of Science. Attendees at that meeting heard Don report on how the “nutritional content of vegetables and fruits has declined over the past 50 years—in some cases dramatically.”
When we left off last month, Don was just about to describe the evidence on which he bases these conclusions.
Passwater: What type of data did you review in your presentation?
Davis: I reviewed three kinds of evidence—dilution effects, historical comparisons, and comparisons of organic and conventional foods.
I discovered and learned about dilution effects while preparing our 2004 report. Surprisingly to me, agricultural scientists have known for over 25 years that yield-enhancing methods such as fertilization and irrigation tend to decrease the concentrations of some nutrients in foods. The crops grow faster and larger, but in some cases they are not able to acquire or synthesize enough nutrients to keep up with the increased plant size, so the available nutrients are diluted in concentration. I found an important review of these dilution effects published in 1981.
Then I was fortunate to discover two recent reports of a newly documented kind of dilution effect. I call it the genetic dilution effect, because it involves increasing yields by selective breeding. I call the previously known dilution effect the environmental dilution effect, because it is based solely on environmental methods, mainly fertilization and irrigation.
Genetic dilution effects are reported so far for only two foods that I know, broccoli and wheat. When high- and low-yield varieties are grown side-by-side—so they have the same environment of climate, soil, fertilization and irrigation—the high-yield varieties strongly tend to have lower concentrations of minerals. Only minerals have been studied so far.
Passwater: That is very important as well as very interesting. Scientists and the public alike should know more about this.
Davis: This genetic dilution effect is an emerging discovery, and very few scientists know about it yet. If it applies to other foods besides broccoli and wheat, as seems likely, then it is something we need to think carefully about. How much are we willing to sacrifice in broad nutritional quality in order to obtain high yields? That will be an issue of public health policy. Some hope that suitable organic growing methods might make it possible to achieve both high yield and high nutrient levels. Extensive research will be required to evaluate this possibility.
Passwater: What data did you present?
Davis: I cited three historical comparisons of nutrients in foods. The 1997 report used UK data from the 1930s and 1980s for 7 minerals in 20 fruits and 20 vegetables. Our 2004 report used U.S. Department of Agriculture data published in 1950 and 1999 for protein, carbohydrate, fat, five vitamins and four minerals. We studied 43 garden crops—39 vegetables, three melons and strawberries. A November 2005 report compared data from the 1930s to about 2000 for eight minerals in UK foods and six minerals in U.S. foods—mostly for a large number of fruits and vegetables, but also for some nuts.
In each study, about half of the studied nutrients showed large enough declines to be statistically significant, plus smaller, suggestive declines in a few other nutrients. In our study of U.S. foods, the statistically significant median declines ranged from 6% to 16% for protein, phosphorus, calcium and iron, plus 20% for vitamin C and 38% for riboflavin. There was also a suggestive decline of 6% in ash, which represents mainly potassium in our foods. Those are the median declines. Individual foods varied widely, and about a quarter of our foods and nutrients showed apparent increases. We think the increases are mostly due to random genetic variations in the varieties cultivated now compared to earlier.
The increases may be hard to reconcile with popular ideas that modern foods are greatly affected by depletion of soil minerals or organic matter. All historical comparisons are potentially confounded by multi-decade changes in analytical methods for some nutrients, but I doubt this is a large factor. There is also the problem that early values for iron may be too high if clinging soil was not fully removed, because early workers did not realize that there is far more iron in soil than in foods.
Figure 3: Average nutrient declines in garden crops between 1950 and 1999.
Passwater: Didn’t you also report that organically grown foods have greater antioxidant levels?
Davis: Yes, in my talk I cited recent evidence that organically grown fruits and vegetables have roughly an average 30% higher levels of antioxidants and other phytochemicals than their conventional equivalents. That was the finding of a 2005 review of about 15 controlled studies, available on the website of the Organic Center (www.organic-center.org), “Elevating Antioxidant Levels in Food Through Organic Farming and Food Processing”).
The implication is that our conventional farming methods have caused a decline in these substances, sometimes called secondary plant metabolites. Plants produce many of them to protect themselves from stresses such as sunlight, insects, bacteria and fungi. These substances are often antioxidants, and they seem to help protect our cells, too, and to be part of the health-giving value of plant foods.
Passwater: Why would organic farming improve these antioxidants?
Davis: An interesting hypothesis is that organically grown foods are made healthier for humans by limited exposure to insects and pathogens that help stimulate production of these secondary metabolites. I also cited a January 2006 report comparing organically and conventionally grown strawberries. The organic strawberries had slightly more of many phytochemicals and vitamin C. Interestingly, they were also better able to suppress the growth of two kinds of cancer cells in test tube experiments. We know that consumption of fruits and vegetables is associated with reduced rates of cancer and other diseases. But we don’t know exactly why. The reasons may involve both nutrients and phytochemicals, and it may turn out that organically grown foods are superior in this way.
Passwater: It is bad enough that people seem to choose to eat poor diets. In 1975, I traced the nutrient content of the Standard American Diet (SAD Diet) through several decades. The typical American diet has also been less nourishing than the official recommendations for many nutrients. Now, it is harder for even those who try to consume healthy diets to get proper amounts of nutrients. What do your studies show regarding dietary habits?
Davis: Well, I concluded my talk by documenting the large percentages of Americans who do not consume the recommended amounts of several nutrients. A major reason is that over half of Americans fail to eat the minimal daily recommendations of two servings of fruit and three servings of vegetables, and very few get half of their servings of grain foods in the form of whole grains. Instead, as a nation, we get over half of our calories from non-whole foods created by humans—added sugars, added fats and white flour and white rice. In effect, we get most of our daily calories from a large cookie.
Passwater: Back to the issue of nutrient declines, what do you believe accounts for them?
Davis: My guess is that the declines are mainly an unintended and mostly unrealized side effect of our high yielding varieties and intensive farming methods, or in other words, the result of environmental and genetic dilution effects. But we can’t rule out other factors such as soil depletion of organic matter and possibly some minerals. In the case of some secondary plant metabolites, the use of pesticides may remove a needed stimulus for their production. We are only beginning to learn about these factors, and it may be many years before we can answer this question with any confidence. Genetic dilution effects are a very recent surprise, and more surprises may lie ahead.
Passwater: Earlier you mentioned that it may turn out that organically grown foods are superior in that they contain a good combination of both nutrients and phytochemicals. Are foods grown “organically” richer in nutrients?
Davis: I’ve not been very impressed by the many claims regarding vitamins and minerals, except there is evidence for somewhat higher vitamin C levels. The question is difficult to study, because there are large, natural variations within both kinds of farming, making it hard to prove differences between the two. And sometimes, lower levels of moisture in organic foods muddle the issue. Low moisture is valuable in the important sense that consumers get less water and more real food for their money. But they also get more calories. If your question is about nutrients per calorie or nutrients per dry weight, which to me is the most fundamental question, then one must adjust for any differences in moisture content. Unfortunately, few comparisons of organic and conventional foods have made that adjustment, or even reported the necessary moisture data.
Passwater: How about antioxidant nutrient content?
Davis: I’m more hopeful about higher levels of antioxidants and other phytochemicals in organic foods. The evidence is newly emerging and therefore spotty, but promising. I mentioned the recent review that suggests an average 30% advantage for organic fruits and vegetables. But those who want direct evidence for human benefit will have a long wait ahead.
Passwater: The old “food tables” include only a relatively small number of nutrients. Is it reasonable to conclude that other nutrients not included in the food tables are also declining?
Davis: Yes, that is likely. In the reports I mentioned of genetic dilution effects in broccoli and wheat, the concentration of every nutrient studied was inversely correlated with yield, either reliably or suggestively. For broccoli it was two out of two—calcium and magnesium. In wheat it was six out of six—iron, zinc, copper, phosphorus, sulfur and selenium. There are no exceptions so far.
Passwater: Should we be alarmed?
Davis: I am happy when people become interested and concerned about their nutrition, because there is so much room for improvement of American diets. For those who want to improve their nutrition, I would emphasize two basic facts: One is that our current fruits and vegetables are still our richest sources of many nutrients and phytochemicals, and dozens of studies show that the more of them that we eat, the healthier we are. The other key fact is that most Americans freely choose to get most of their calories from the non-whole cookie ingredients I mentioned. Those added sugars, added fats and milled grains are all more broadly and deeply depleted of nutrients, by human hands, than are fruits and vegetables. Everyone who eats cookie ingredients has it in his power today to “undeplete” his diet.
Passwater: So what is the answer?
Davis: By far, the top priorities are to eat more fruits, vegetables and whole grains, and to eat less of the cookie ingredients. These key steps can be taken by anyone now, without changing our farming methods or doing more research. Other steps can help, too. Emphasizing fruits, berries, melons and vegetables with intense colors can be useful, because intense colors usually indicate high levels of protective phytochemicals (but not if the colors are artificial, of course). It is no accident that we are instinctively attracted to brightly colored foods. Organic foods seem useful too, though from the narrow perspective of nutrient content, cost-effectiveness is a legitimate issue. I hope that the rapidly growing production of organic foods will bring prices down. I hope, too, that conventional farmers will increasingly move back toward organic and sustainable methods, in their own interest.
Passwater: What about reversing dilution effects?
Davis: Overcoming and reversing dilution effects are probably our most uncertain and distant ways to improve nutrition. They are for the future, not here now. We don’t know how well we may be able to broadly restore nutrient levels while maintaining high yields. The alternative approach is to sacrifice yield and presumably cost by going back to less intensive farming methods, including lower-yielding, higher-nutrient varieties of crops. All this will take much research. Part of the problem is that we pay farmers by their yield. If we paid them by the concentration of nutrients and phytochemicals in their crops, improvements would come naturally, but slowly.
Passwater: That’s an interesting thought. Whole foods are our best source of nutrients, but does it make sense to also consider dietary supplements?
Davis: Yes, if people realize that supplements cannot substitute for whole foods. Well-chosen supplements are an easy and safe way to ensure intakes of vitamins and minerals that many Americans lack. But they are not practical sources of protein, fiber or potassium. And they can’t begin to substitute for the many thousands of potentially beneficial phytochemicals in whole foods.
Passwater: Exactly what you and many of us have been saying for decades. Eat a good diet and supplement scientifically. What are your other research interests and where is this research taking you from here?
Davis: I’m working on a follow-up study of possible nutrient declines in about 100 additional foods, including grains, beans, fruits and animal foods. I’m also working on a study of distinctive nutritional biochemistry in mothers of babies born with spina bifida and other neural tube defects.
Passwater: Very interesting and useful. Thank you Dr. Davis for sharing your research with us. WF
© 2006 Whole Foods Magazine and Richard A. Passwater, Ph.D.
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