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Question: Which of the two vegetables has more beta carotene, sweet potato or yam? I have been told that the darker the color, the higher the beta carotene. But diet books tell you to eat sweet potatoes, never yams. In case you want to know, the yam is darker in color.

- Confused

Answer: I would not use the color rule to judge the beta carotene content of a food. Color often fails the reliability test.

A cup of dandelion greens has 8.4 milligrams of carotene, a medium sweet potato 5.9 milligrams, a half-cup of cooked spinach 4.9 and a slice of cantaloupe 2. The color rule can be far from absolute.

Some have said beta carotene holds qualities that will protect against cancers and cataracts, to cite just two disparate claims for it. But a Finnish study has cast doubt on the anti-cancer claim, even challenging beta carotene's much-touted lung cancer benefit.

A large study now under way should tell us more about beta carotene. It is a bit premature to be offering blanket endorsement or skepticism.

In any event, experts figure the body needs about 6 to 15 milligrams of it daily, amply available from a range of vegetables.

I am leaving your "yams vs. sweet potatoes" question dangling. I cannot answer it. I have asked around, and no one knows why sweet potatoes should be favored over yams. In fact, I only recently learned the differences between the two delicious vegetables.

Question: Please tell me about surgery for emphysema. I thought it was a permanent condition until I heard doctors discussing surgery.

- Mrs. U.B.

Answer: In emphysema, the lungs become overexpanded. In their stretched condition, they fill the chest and push down on the breathing muscles - the diaphragm. Million of the lungs' air sacs have become pushed out of shape. The person cannot carry on the vital exchange of oxygen to refresh the blood.

The surgery discussed is in its infancy. In the procedure, the chest surgeon removes most of the most damaged lung section - which frees the diaphragm muscle to work more effectively, thus helping bring oxygen in by way of sections that are less seriously damaged.

I don't believe it will become standard surgery very soon. We first must learn how effective it has been and for how long in improving emphysema sufferers' health.

Of course, you are free to look around for any surgical trials under way.

For an in-depth discussion of lung problems, see the emphysema-bronchitis report I'm sending you. Others can order the report by writing: Dr. Donohue - No. 10, P.O. Box 5539, Riverton, NJ 08077-5539. Enclose $3 and a self-addressed, stamped (55 cents) No. 10 envelope.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him at P.O. Box 5539, Riverton, NJ 08077-5539.