Recently I heard the tail end of a radio program about nutrition and blindness. It said that fortification of monosodium glutamate (MSG) was used as a means of preventing nutritional blindness in children in Indonesia. Can you tell me more about this?

In Indonesia, more than 50,000 preschoolers are afflicted with potentially blinding xerophthalmia, a severe vitamin-A deficiency. MSG is used universally in that country, and the idea of fortifying it with vitamin A was suggested long ago. Technical problems made it difficult to implement. The problem is that the vitamin, which is deep yellow, discolored the white MSG crystals, and people found this aesthetically unacceptable. It also tended to separate physically during processing, so it was not distributed evenly.

A technique developed by the United States Department of Agriculture's Office of International Cooperation (OICD) and a private company in Wisconsin called Coating Place, Inc., solved the problem. The vitamin is embedded in a matrix of fine MSG powder. In field trials in Indonesia in 1985, not only did the use of the fortified MSG significantly reduce the incidence of vitamin-A deficiency and xerophthalmia, but fewer children died in the area where it was distributed than in regions where it was unavailable. The addition of the vitamin was also found to increase the amount in circulating blood levels and in the milk of lactating mothers.

Based on the results of these trials, it is estimated that fortification can save as many as 100,000 lives and prevent 10,000 cases of blindness per year in Indonesia alone.

Since the field studies were conducted, the technique has been further refined. Currently the Indonesian government plans to begin MSG fortification in the three districts where the problem is most severe and to gradually extend it throughout the country.

OICD has worked with Helen Keller International, the agency that helped identify MSG as a suitable vehicle, to target appropriate foods in other countries where vitamin-A deficiency blindness is a problem. Among the programs to be launched in the near future are one for whole-wheat fortification in Bangladesh and another for rice fortification in the Philippines.

I have long wondered what ginseng is supposed to do. Can you enlighten me?

The claims for ginseng, a root used in Oriental folk medicine, include both curative and preventive properties, not to mention aphrodisiac powers. Popular literature in this country has promoted using the root in tea or as a liquid extract to boost energy, and capsules are claimed to promote disease resistance.

Ginseng does contain certain compounds shown to produce physiologic effects in laboratory animals. But there have been no controlled studies showing that it is of any benefit to humans. Moreover, there have been reports of adverse symptoms in people who have taken large amounts over long periods. Finally, analyses of ginseng products have found that many of those available contained very little or no ginseng at all.

Can you explain exactly what lactose intolerance is?

The digestion of lactose, or milk sugar, requires an enzyme called lactase, which is produced in the small intestine. Symptoms of lactose intolerance occur if the intestine does not produce enough lactase to break down the amount of sugar consumed.

The undigested lactose draws water in the intestine, increasing pressure and leading to bloating and discomfort. The undigested lactose then makes its way to the large intestine where it is broken down by bacterial fermentation to hydrogen, carbon dioxide and organic acids. Pain, gas and diarrhea result.

Lactose intolerance is not an all-or-none phenomenon. Different individuals can tolerate different amounts of milk and milk products without ill effect. Moreover, some people are affected by cold milk but not by warm milk, and by milk taken when the stomach is empty but not when it is consumed with a meal.

Other dairy foods differ in their effect. For example, some people who cannot drink much milk without experiencing symptoms may tolerate hard cheese, yogurt (if it does not contain nonfat milk solids added after pasteurization), and even small amounts of ice cream.

For those who suffer symptoms from small amounts of milk or milk-containing foods, excellent solutions are available. Commercial lactase preparations can be added to milk to break the lactose down to its simple sugars, glucose and galactose. And tablet and capsule forms of lactase may be taken with milk products at the time they are consumed, or sprinkled on lactose-containing foods.

What is the difference between sweet potatoes and yams?

Sweet potatoes and yams come from two different species of plant originating in separate corners of the world. What we commonly eat are sweet potatoes, which are native to tropical America. In fact, the oldest archaeological evidence concerning their origin comes from Peru and dates back to 750 B.C.

Yams, on the other hand, are native to Africa, and probably West Africa. It is thought that the African slaves introduced yams to the New World. True yams, whose flesh ranges in color from white to yellow, usually weigh between 2 and 8 pounds. But some can exceed 100 pounds.

Sweet potatoes are usually yellow to deep orange, but red or purple varieties exist. Sweet potatoes, as their color suggests, provide generous amounts of vitamin A. They are also a good source of vitamin C and contain a lot of potassium, as well as B vitamins, iron and other minerals.

They are calorically more dense than white potatoes. A medium sweet potato, weighing 3 1/2 ounces, provides 140 calories. And the addition of extra ingredients such as butter or margarine, brown sugar or maple syrup, and nuts can easily double or triple that tally.

For occasional holiday eating that might be fine. Unfortunately, after the holidays we tend to dismiss sweet potatoes as "too fattening." That is a shame. The natural flavor of sweet potato can be enjoyed without adding rich frills. A baked sweet potato tastes delicious as it comes from the oven. And it contains no more calories than a baked white potato of the same size with just a single pat of butter added.