DEAR DR. BLONZ: You always speak about the importance of eating whole foods, but I keep seeing articles about the fact that our soils have become depleted of minerals. This depletion makes our foods less adequate to provide the nutrients we need than they were in decades past. In the survey course I took in agriculture when I was in college, I do recall that plants require certain nutrients to grow to their proper size, shape, color, form and taste. If this is the case, then it would seem that all the produce I see at the marketplace would have to have had their essential nutrients or they would not have grown -- or at least they would not look very good. Are those articles based on solid information, or is what I recall from the class more correct? -- O.F., Carlsbad, Calif.
DEAR O.F.: Your class recollections are on solid ground. Plants act as miners, pulling the mineral elements out of the soil that they need to grow. If a mineral needed by the plant is unavailable, the plant will not grow. It is incorrect to think that today's fruits, vegetables or grains will provide any less of their essential nutrients than the same varieties would have provided in the past. Plants synthesize their own vitamins, so these will be there as the plant grows.
Another layer to this question is the fact that nonessential minerals present in the soil can also end up in a plant. Whole foods grown in iodine-rich or selenium-rich soils, for example, can have more of these nutrients than the same type of food grown in soils with lesser amounts of these minerals.
Aside from the essential elements, what gets pulled in can vary from plant to plant, and mineral to mineral -- even among different varieties of the same fruit, vegetable or grain. The plant's overall nutrient content can also vary according to the time of the growing season and the length of time the plant has had to grow. This means that a plant picked green may not have the same total nutrient content as one allowed to ripen "on the vine."
It is difficult to speak with any statistical certainty, because we don't have records of nutrient content from the produce of a hundred years ago. Based on what we know, however, it's likely that the amounts would be comparable. Nutrient data tables, such as those at the USDA database (ndb.nal.usda.gov), present an average of what would be expected in a particular food.
DEAR DR. BLONZ: If the body can only absorb a limited portion of the minerals in a supplement, say 50 percent, and a supplement claims to supply 100 percent of the RDA, does that mean that you have to take two times the dose to actually absorb the RDA? -- S.M., San Francisco
DEAR S.M.: Dietary recommendations take the body's efficiency of absorption into account. They reflect the average amount an individual should be eating every day to satisfy the requirement. If, for example, science determines that the average body should have 100 milligrams of a certain mineral every day, and research indicated that we only absorb about 50 percent this mineral when consumed orally, then the recommended daily allowance (RDA) for that mineral would be 200 milligrams per day.
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