DEAR DR. BLONZ: If one has a good diet (low in red meat, sugar and fat; high in fruits and vegetables), is it necessary to take a multivitamin? My doctor claims that food is no longer as nutritious as it once was because of soil depletion, so we should take supplemental vitamins and minerals. Do you think this is true? -- H.H., via email
DEAR H.H.: I am skeptical of your doctor's statement. I find the "soil-depletion" argument to be very misleading. Fruits, vegetables and grains, whether grown organically or through conventional methods, require sunlight, water and nutrients -- including nitrogen, phosphorous and potassium -- to develop. The soil must contain the plant's essential elements and if one or more is not available in sufficient amounts, either the plants won't grow or they'll fail to develop normally (and it's unlikely they will ever make it to market).
A plant can absorb minerals from the soil even if it does not need them. For example, foods grown in selenium-rich soil are likely to contain more of this mineral than the same food grown in selenium-poor soil. A plant's mineral content can also vary according to weather conditions, the amount of time it has been allowed to mature and when it's harvested during the growing season.
The idea of "vitamin-depleted" soils makes even less sense. Plants do not get their vitamins from the soil; they are synthesized within the plant itself. The amount of a particular vitamin found in a fruit, vegetable or grain is determined by the plant's genes, not by the amount of this nutrient in the soil. After harvest, vitamin content can decline over time and some vitamins are destroyed during cooking.
It then follows that there isn't really a specific area of the country (or world) where a person can be sure that the fruits and vegetables will be the most nutritious. Variety is key wherever you live, and we are blessed with a transportation system that brings us food from many regions.
As for whether you think it is necessary to take a supplement, I think that is a personal decision based on your diet. I find nothing wrong with taking a multivitamin, but that in no way should be thought of as a substitute for a healthful diet based on fruits, vegetables and whole grains. I like the image of the diet as a musical composition. Whole foods represent a symphony orchestra of nutrients and phytochemicals, all working together. A supplement, by contrast, can be thought of as providing only a few of the instruments.
Send questions to: "On Nutrition," Ed Blonz, c/o Universal Uclick, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to email@example.com. Due to the volume of mail, personal replies cannot be provided.