Q. What part of the animal are sweetbreads, and what is their nutritional value?

A. Sweetbreads are the thymus gland of a young animal. (The gland actually disappears as the animal gets older.) While nutrient tables do list values for lamb sweetbreads, it is calf sweetbreads that generally appear on restaurant menus. Three and a half ounces of raw calf sweetbreads contains 95 calories, about 75 percent of them from protein and the rest from fat. Sweetbreads are relatively low in fat but have a lot of cholesterol. That same portion provides 250 mg of cholesterol, as much as you would get from a single raw egg.

If you are counting your calories and/or watching your fat intake, it is a good idea to ask for the details of preparation before ordering sweetbreads in a restaurant. Recipes that depend heavily on butter, oil and cream -- as they commonly do -- will significantly alter the low-fat, low-calorie nutritional profile of sweetbreads.

Q. Is it true that eating too much liver can cause vitamin A toxicity?

A. Yes, but only in unusual situations. Acute vitamin A toxicity has occurred in Arctic explorers who ate polar-bear liver. It also can result from eating the liver of large fish, which may have high concentrations of vitamin A. A case of toxicity reported recently in the medical literature was traced to the consumption of shark liver.

Toxicity also can happen when individuals consume smaller amounts over a longer period of time. Several years ago, liver toxicity was diagnosed in a pair of 7-month-old twins who were given four ounces of chicken liver a day for four months, even though they were getting enough vitamin A from other sources.

A brief report in the Journal of the American Medical Association recently outlined five cases of "liver lovers' headache," or pseudotumor cerebri; that is, headache and related findings mimicking a brain tumor. PTC, as it is called, often occurs in obese young women, and it often goes unexplained. The fact that it also occurs in as many as half of all those with hypervitaminosis A led doctors from the Medical College of Virginia and the University of Iowa to examine the dietary habits of 50 patients with this problem.

They found five cases, two in men and three in women, in which the symptoms were associated with the consumption of large amounts of liver. All were mildly to severely obese. Two of the patients admitted to buying between 6 and 24 pounds of liver a week.

Dietary histories indicated that intakes in four cases ranged between 60,000 and 87,000 International Units (IUs), but one person consumed as much as 341,000 IUs of the vitamin a day. The Recommended Dietary Allowance (RDA) for vitamin A is 4,000 IUs for women and 5,000 IUs for men. Thus, these individuals were consuming many times what they needed -- in one case, perhaps as much as 17 times the RDA.

Liver is an excellent food. Eating it occasionally is fine. But as with many other dietary behaviors, excess leads to problems.

Q. In reply to a recent question, you said that one could easily compare the cost of the lean portion of ground beef at various fat levels by dividing the cost per pound by the percent of lean. But how does the amount of lean left in cooked meat of different fat contents compare?

A. A recent study by researchers at the USDA-Agricultural Research Service Meat Science Research Laboratory in Beltsville found limited differences in cooked meat prepared from raw ground beef containing three different ratios of fat to lean.

The researchers cooked beef patties containing 18.4 percent, 21.5 percent and 27 percent fat, which they regarded as within the range of extra lean, lean and regular fat content. Three methods were used: broiling, roasting in a convection oven and grilling. All patties were cooked to medium well-done, or until the pink color had just disappeared.

While those with the highest fat content provided the lowest yield, differences within a method were small. The largest spread in final yield, which occurred when the meat was grilled, was only 3.5 percent. Differences in caloric content, accounted for almost entirely by fat, also were small, ranging from 21 calories per 100 grams, or 3 1/3 ounces (275 to 296 calories), to a 36-calorie difference (284 vs. 320) in the roasted meat.