DEAR DR. BLONZ: I enjoy my generally vegetarian diet, but I remain a bit confused about beta carotene in plant foods. As I understand it, it plays a role as an antioxidant and also becomes vitamin A. The multivitamin I take contains vitamin A, and I wondered if that works the same as beta carotene. -- P.C., New York
DEAR P.C.: Population studies examining groups for links between diet and disease consistently find the incidence of such ailments as cardiovascular disease, diabetes, certain eye and digestive diseases and cancers goes down as the consumption of vegetables increases. The "vegetable effect" is likely due to a combination of what's in plant foods and the fact that the more vegetables you eat, the less room you have for less-healthful fare.
With each whole plant food, you get a unique team of substances designed to cope with assaults from animals, insects, microbes and the elements. Protective barriers and phytochemical substances have been perfected through natural selection to increase the odds for the plant's survival. (Read more on this at b.link/65pzas.) It's not a perfect system, of course, as unexpected weather and other factors can cause crop failures.
Beta carotene is a carrot-colored member of the carotenoid family found in plant foods. Chemically, beta carotene can be thought of as two pieces of vitamin A stuck together. If beta carotene is present when there's a need for vitamin A, the body activates an enzyme to break the large molecule into the active vitamin. Normally, vitamin A is found in dairy products, eggs, organ meats and fortified foods. If a sufficient amount of the preformed vitamin is present, dietary beta carotene is available to perform other functions.
Beta carotene's primary non-vitamin role is as an antioxidant. Obviously, we need oxygen to stay alive, but in the wrong place at the wrong time, oxygen can do serious damage to the body: It can destroy cells and is believed to be involved in many ailments. Antioxidants, such as beta carotene, can stifle those free-roving bits of oxygen that may be up to no good.
Vitamin A also has some ability as an antioxidant, but there's an important distinction between it and beta carotene. When vitamin A intake exceeds the body's needs, the excess gets stored in the liver. Vitamin A can produce toxic side effects if the dose is high enough, as in levels five to 10 times the recommended daily allowance (RDA). (Read more on this at b.link/np9ym3.) Even with beta carotene, excess consumption is not advised, but there, the most likely problem is that the body will develop an orangeish tinge. (Astaxanthin, a related carotenoid compound, is actually used in tanning pills.)
Considering beta carotene's positive qualities, you'd think there would be a recommended daily allowance, but such is not the case. The RDA system of nutrient recommendations was established to prevent deficiencies. The system wasn't designed to consider that a nutrient such as beta carotene might have beneficial effects beyond the ability to step in as an essential vitamin. It is, however, more testimony to the value of including various colors of vegetables in your diet.
Send questions to: "On Nutrition," Ed Blonz, c/o Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO, 64106. Send email inquiries to questions@blonz.com. Due to the volume of mail, personal replies cannot be provided.