Plant research fine-tunes nutrition recommendations

A unique partnership between plant scientists and human nutritionists has led to a better understanding of the role edible plants play in meeting human requirements for vitamin A. Vitamin A deficiency is a major cause of blindness in developing countries, while a less severe shortfall can cause night blindness.

"Beta carotene and other carotenoids are the main source of vitamin A for many people around the world," said Dr. Michael Grusak, a USDA plant scientist and director of the CNRC Plant Physiology lab. "Yet, scientific understanding of how effectively dietary carotenoids are converted into vitamin A has been very limited."

Beta carotene is the most potent "provitamin A" carotenoid in the human diet. Best known as the orange pigment of carrots and apricots, beta carotene is also found in spinach and broccoli. Less-potent carotenoids are found in many other fruits and vegetables. The only source of preformed, or 'ready to use,' vitamin A in the human diet is animal fat.

Nutritionists have traditionally used a "conversion factor" of 6-to1 to calculate the amount of vitamin A available in the diet. However, the analytical techniques used to set this factor could not differentiate between plasma vitamin A made from carotenoids and that released from body stores, Grusak said. Those studies also used high doses of pure beta carotene, not food, as the source of carotenoids.

"Nutrients in plants are often much less available to the body," he said.

To test the conversion factor, Grusak constructed a hydroponic chamber, which allowed safe, non-radioactive "tracers" to be incorporated into the carotenoid pigments of growing spinach and broccoli plants. Once harvested, the vegetables were sent to Tufts University in Boston, where they were cooked and fed to volunteers. Blood vitamin A tests later revealed that only about half as much of the carotenoids had been converted into vitamin A than was predicted by the 6-to-1 factor.

"Bioavailability studies, such as these with beta carotene, extend our understanding of how specific food components contribute to human nutrition and health," Grusak said.

As a result of these studies, a new 12-to-1 beta-carotene conversion factor was included in the updated vitamin A recommendations released by the National Academy of Sciences (NAS) in March 2000. The NAS updates U.S. nutritional standards, commonly known as the RDAs, every 10 years.

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Breakfast-skippers risk iron shortfall, poorer grades

Teens who start their day without breakfast are twice as likely to have diets low in iron -- a shortfall that could be hurting their grades.

"Breakfast supplies more than just the energy kids need to get through the morning," said Dr. Theresa Nicklas, a professor of pediatrics at Baylor College of Medicine in Houston. "Teens who eat breakfast are also two to five times more likely to consume at least two-thirds the recommended amounts of most vitamins and minerals, including iron."

In a study involving over 700 ninth graders in Louisiana, Nicklas found that 19 percent skipped breakfast, including 20 percent of white and 36 percent of non-white girls. The diets of one in three breakfast-dodgers had a significant iron shortfall -- twice the rate of their breakfast-eating peers.

Intakes of other vitamins and minerals, including zinc, calcium, and folic acid, were also much higher among the breakfast-eaters, while fat consumption was lower. The study results were published in the Journal of Adolescent Health.

"It's important for parents to realize that the nutrients teens miss when they skip breakfast are rarely recouped during other meals," Nicklas said.

Iron-deficiency anemia has long been known to have a negative affect on behavior and learning. And, even marginal iron levels were linked to poorer math scores among adolescent girls in one recent study. In other studies, eating breakfast has been shown to improve memory, grades, school attendance and punctuality in children.

Girls, in particular, are at risk for low iron because they have increased needs. And while some teens skip breakfast to cut calories, this practice is rarely effective. Instead, research suggests that meal-skippers often eat more high-calorie, salty and low-fiber snacks.

"Low iron is certainly not the only possible reason for poor math scores among some adolescent girls," Nicklas said. "But, eating breakfast can help concentration, which is important for academic success."

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Q&A -- How can I help my daughter get more iron?

There are two ways to pump up your daughter's iron status: first, offer more iron-rich foods. Second, use the following dietary "tricks" to squeeze more iron out of the foods she already eats:

• Include a little meat with your favorite iron-rich vegetables. The form of iron found in plant foods isn't as easily absorbed as that found in meats. But, absorption can be improved by simply adding a piece of grilled chicken or slice of marinated beef to a bean burrito or spinach salad.
  
• Use vitamin C-rich foods to pump up iron absorption at mealtime. Top oatmeal with sliced strawberries, serve orange juice with iron-fortified cereal, toss red and green pepper strips into whole-grain pastas and broccoli "slaw" with a peanut-butter sandwiches.
  
• Avoid tea or coffee beverages at mealtime. These beverages contain tannins that tie up iron, reducing how much can be absorbed.