Q: I have two questions about sweet substances. First, what is the difference between corn syrup, which I keep on the shelf for rare occasions such as making pecan pie or fudge, and "high fructose" corn syrup, which seems to be used in many foods nowadays? Second, what type of sugar does maple syrup contain?

A: The basic ingredient in both kinds of corn syrup is cornstarch, but the way it is treated results in different forms of simple sugars. Traditional corn syrup was first produced in the 1920s. Treating a watery cornstarch mixture with acids, heat and/or enzymes produced a viscous sweetener containing glucose molecules of different chain lengths. It is less sweet than sucrose.

In the process for making high-fructose corn syrup (developed during the 1970s), enzymes are used to convert glucose from cornstarch into fructose. High-fructose corn syrup contains from 40 percent to 100 percent fructose. Because it is sweeter than sucrose or table sugar, it is possible to use less to produce the desired level of sweetness.

As for your second question, maple syrup, which is produced by boiling the sap of mature sugar-maple trees, is almost pure sucrose.

Q: A friend told me that the amino acid lysine is a useful supplement for treating genital herpes infections. Is this true?

A: Lysine has been promoted for the treatment of herpes, but there is no supporting evidence. The origin of the claim dates back to laboratory observations first published in 1964. That research, which was conducted for the purpose of determining amino-acid requirements of the virus, found that lysine exerts a partially inhibitory effect on the growth of the virus, while arginine seems to promote replication.

But clinical trials conducted over a 10-year period proved inconclusive. Moreover, these studies were flawed in design. The amount of lysine that subjects consumed in their diets was not reported, nor was any attempt made to control their dietary intake of arginine or lysine.

Limitations of these studies were further underscored by a small pilot study conducted by Susan Algert, M.S., R.D., and co-workers at the University of California (San Diego) Medical Center and the University of California (La Jolla), reported in the Journal of the American Dietetic Association. In their study, consumption of lysine and arginine for a group of individuals with herpes and another group did not differ significantly. Furthermore, the fact that intakes of lysine and arginine within both the subject and control groups varied considerably raises questions about the validity of studies which claimed that lysine was effective.

In those studies, there was no way to measure the effect of supplements taken over and above usual dietary intake, because dietary intake was not assessed. All this adds up to a lack of scientific proof that lysine helps to prevent or treat genital herpes.

Q: Can you tell me how much food is produced by individual American families for direct consumption at the present time?

A: According to USDA estimates, only about 2 percent -- and most of that is produce raised in family gardens. This represents a massive shift from the last century, when an estimated 1/3 of all food was produced for individual family consumption.

Even though our country started out with an agrarian economy, nowadays many people have never had the experience of picking a fresh vegetable, let alone milking a cow. It is interesting that the disappearance of the family cow as a source of dairy products has taken a somewhat more gradual course than you might think. As recently as 1910, more than 1/3 of the population still drank milk from the family cow. The amount of milk consumed by households who owned a cow was twice that of those who bought their milk.

As recently as 1920, 28 percent of American families still owned a cow. And as late as 1940, as many as one in five families still produced their own milk. In the next 40 years, that figure gradually slipped to less than 1 percent.

Q: A friend recently returned from France inspired to prepare dishes she had eaten there which she regarded as both "healthful" and delicious. One of these, quite good, was lentil salad with vinaigrette dressing. What is the nutritional value?

A: Lentils are members of the legume family and are generous sources of vegetable proteins. A cup of cooked lentils has just over 200 calories and almost 16 grams of protein, a little more than you would get in 2 ounces of meat. Of course, that protein does not contain the same balance of amino acids as the meat and is therefore not of quite the same quality.

But amino-acid shortages in the lentils can easily be made up from other foods eaten at the same meal. Lentils also provide B vitamins and minerals, and are a particularly good source of iron, with a cup containing about 23 percent of the Recommended Dietary Allowance for an adult woman.

We cannot say for certain how many calories a cup of lentil salad will contain: It depends on the type of dressing used. In one recipe the lentils were tossed with an oil-and-vinegar dressing seasoned with Dijon mustard. The ratio of oil to vinegar was 7 to 2, resulting in a dressing with about 100 calories per tablespoon. Two tablespoons of the dressing would provide as many calories as the lentils themselves.

We find it quite acceptable, and prefer the taste, when more vinegar and less oil are used. A mixture of 7 tablespoons of oil to 4 of vinegar, or part vinegar and part lemon juice, reduces the dressing to 80 calories per tablespoon. Lentil salad, prepared with a vinaigrette using raspberry wine vinegar, is often served with small amounts of warm duck meat. But with the holiday season bearing down upon us, and leftover turkey destined to be in abundance, you will find that dark turkey meat makes an excellent alternative.

Lentils, by the way, are the oldest of the cultivated legumes, dating back as far as 7000 B.C. to Southwest Asia.