Q: In a recent column about minerals, you mentioned that the extensive use of iron pans in the past added a lot of iron to our diets. I still use iron pans often, and wonder if I'm in danger of getting too much of the mineral. Can it be toxic?

A: Iron toxicity, or hemochromatosis, is a disease characterized by excessive iron deposits in nearly all body tissues. It has been associated with the use of cooking utensils only among the Bantus of Africa, who not only cook with iron pots but also brew beer in iron drums. They consume anywhere from 10 to 20 times the normal iron intake.

In this country, iron overload occurs most often as a result of a metabolic error that prevents some individuals from blocking the absorption of excess amounts, which gradually accumulate. The potential for problems among this group contributed to the decision by the Food and Drug Administration several years ago not to raise the levels added to flour and bread products.

Iron toxicity has also been found among alcoholics, apparently as a combined effect. First, the iron content of some wines is high. In addition, alcohol seems to increase the efficiency of absorption and to inhibit the liver's normal defense mechanisms.

Finally, iron overload has been observed in individuals who over long periods have taken medicinal iron preparations that they didn't need.

Q: What is the difference in nutritional value between white potatoes and sweet potatoes?

A: The caloric density of sweet potatoes is about 50 percent higher than that of white potatoes. Exactly how much difference that makes depends on the size of the potatoes. White potatoes contain about 20 calories per ounce and sweet potatoes about 31 per ounce. A four-ounce baked white potato would yield 80 calories, while the same amount of sweet potato would have about 124. This is not a big difference, especially if you enjoy sweet potatoes. On the other hand, if you prepare potatoes twice that size, the white potato would provide 160 calories and the sweet potato 248, a rather considerable amount.

The major nutritional difference between sweet and white potatoes is visible to the naked eye. The source of the bright orange color is carotene, which the body converts to vitamin A. Even a small sweet potato would provide more than the Recommended Dietary Allowance. White potatoes, by contrast, contain none. Beyond that, both supply some protein, a little iron, some B vitamins and some vitamin C. Both are also good sources of potassium, although white potatoes have the edge.

Sweet potatoes are believed to be native to Central and South America and were introduced to Spain by Columbus. How they got to the United States is uncertain. During Shakespeare's time, crystallized slices of the tuber were sold as aphrodisiacs.

Q: I have a lactase deficiency, but am able to drink small amounts of milk without ill effect. To make sure I get all the calcium I need, I also eat a lot of yogurt. Is the calcium from the milk absorbed as well as that from the yogurt in people who have this condition?

A: A recent study addressed exactly that question and, fortunately, found no difference in the extent of absorption from the two sources in a small group of people with lactase deficiency. These individuals absorbed at least as much calcium as a healthy control group.

One possible explanation for the difference between these findings and earlier studies, which detected a negative effect of undigested lactose on calcium absorption, relates to differences in amounts of lactose consumed. The earlier studies used drinks containing far larger doses of lactose, or milk sugar, than the typical dietary portions of dairy products used in the current study.

Several studies have shown that osteoporosis is more common in people with lactase deficiency. Indeed, individuals who have the problem report lower-than-normal calcium intakes.

So it appears wise and useful for those with the condition to pay special attention to getting enough calcium from milk as tolerated and, alternatively, from yogurt and hard cheese.