Scientists at the Children's Nutrition Research Center (CNRC) in Houston are growing food plants with a key nutrient "tagged" so that they can measure the body's absorption of beta-carotene and track its conversion into vitamin A.

The CNRC is administered jointly by USDA's Agricultural Research Service and Baylor College of Medicine.

Why track beta-carotene? Much of the body's supply of vitamin A is derived from beta carotene, yet little is actually known about how the body converts this important precursor into vitamin A. One important question to be answered: Do cooking techniques--such as heating or adding spices and oils--affect the vitamin's availability to the body?

To track the nutrients' journey through the human body, the scientists are "tagging" them with safe, non-radioactive forms of elements such as iron.

Iron and other elements occur in several forms, or isotopes, that have different atomic weights. Plant physiologist Michael A. Grusak at the Houston center found a way to grow vegetables in solutions with isotopes that are heavier than their common forms, so they're easily detected.

Grusak is now able to track the progress of beta carotene in spinach as it's converted to vitamin A in the body. He grows the spinach in a formula containing labeled isotopes of hydrogen. As the plant grows, it incorporates the labeled isotopes in making beta-carotene.

The beta-carotene in spinach grown this way provides the same nutrition, once it's in the body, as regular beta-carotene.

In the current study, the special spinach is fed to volunteers. Researchers then test the volunteer's blood to find out how much beta-carotene the body actually absorbs and converts to vitamin A.

Scientific contact: Michael A. Grusak, Children's Nutrition Research Center, ARS, Houston, Texas, phone (713) 798-7044, fax (713) 798-7078, mgrusak@bcm.tmc.edu.