Q: I was surprised to read in your column that cholesterol is found only in animal fats and that avocados, therefore, have none. Is this a new finding? I have read and heard many times that there is cholesterol in avocados, coconut and palm oils. This would be good news because it is almost impossible to find cookies or crackers without either coconut or palm oil.

A: No, this is not a new finding. Avocados, coconut oil and palm oil do not contain cholesterol because, as you point out, they are plant foods. But there is a basic difference between the oil in avocados and that extracted from palm and coconuts. The oil in avocados is unsaturated, while both palm and coconuts oils are highly saturated. In fact, they are so saturated that at room temperature they are solids. This is why individuals trying to follow serum cholesterol-lowering diets are advised to avoid them.

Q: For many years I have eaten a grapefruit at breakfast and a banana at lunch. Recently a good friend of mine who, like me, is over 80, was told by his doctor to stop eating bananas because he had too much potassium in his system. I then began to worry about whether this amount of fruit was too much for me, also. What should I do?

A: Without a complete medical picture we cannot second-guess the reasoning behind the advice your friend's doctor gave him. Possibly it was related to evidence of decreased kidney function. But we do know that a specific recommendation of this type would not be made strictly on the basis of age.

Similarly, without knowing whether you have any medical conditions requiring dietary modifications, our opinion must be guarded. In general, there are several excellent reasons for continuing to eat plenty of fresh fruits -- and lots of vegetables as well. First they provide generous amounts of vitamins and minerals necessary for essential body processes. Second, they supply carbohydrates for energy.

And some, especially certain vegetables, contain appreciable amounts of protein. Fruits and vegetables also contribute to fluid intake. And finally, they provide the fiber that helps maintain normal bowel function.

Q: You have mentioned that water hardness can affect the natural pigments in food and result in color changes. Can you explain what is meant by water hardness?

A: Hard water is simply water that contains natural salts. There are two types of "hard" water. In one, called "temporarily hard," the calcium, magnesium and iron bicarbonate are precipitated when water is boiled. Over a long period of time, deposits of these mineral salts build up. They are likely to be found in a kettle used exclusively for boiling water. The second type, "permanently hard" water, contains calcium, magnesium and iron sulfates that do not precipitate on boiling. But they do form insoluble compounds with soaps, making them less effective cleaning agents.

Some people who live in hard-water areas decide to install water-softening systems to remove these minerals. One type of water softener involves the exchange of these mineral ions for sodium ions. The result is a water supply that can be quite high in sodium.

Q: Does adding chocolate to milk decrease the absorption of calcium from the milk?

A: The idea that the oxalate in chocolate will tie up the calcium in milk refuses to die. That hypothesis was disproved over 40 years ago in animal studies in which substantial amounts of low-grade cocoa given together with milk did not affect the utilization of calcium in the milk.

In studies conducted a number of years later, college students were fed enough chocolate to cause cramps and nausea, but calcium utilization was still not affected. Moreover, the oxalate content of the cocoa used in that study was higher than would occur in most of the cocoa used in foods for human consumption.

Q: A friend recently provided me with a special supplement designed to improve endurance performance in athletes. In addition to vitamins and minerals, it contains both amino acids and essential fatty acids. Can you tell me whether there is any scientific rationale behind taking such a mixture?

A: We know of none. Indeed, a recent study evaluated the effects of a mixture similar to the one you described on well-trained male volunteers. The study subjects were divided into two groups of nine pairs matched on the basis of how far they ran each week.

One pair was given three capsules a day of the supplement while the other was given three identical-looking placebos, or "sugar pills." Neither subjects nor investigators knew who was receiving the preparation being evaluated.

Both before taking the supplements and at the end of a four-week period, subjects performed two sets of exercise. In one they exercised to exhaustion, and in the other they exercised for one hour at a predetermined speed requiring them to work at 65 to 70 percent of their capacity. The results of tests performed both before and after the supplementation period showed that maximum oxygen uptake, a measure of endurance capacity, did not rise at the end of the study.

Similarly, there was no change in the way the body responded to submaximal endurance exercise. Blood levels of free fatty acids, which spare glycogen and stave off fatigue, did not rise in those who were taking the supplement. In other words, the study failed to provide support for taking a supplement of this type to improve athletic performance.