"At what age should my baby begin strained foods?"

It's a good question. Gerber Products Co., the number one manufacturer of baby foods in the U.S., asks it on the back of its rice cereal boxes. The ad is an eye-catcher, according to Gerber's vice president for nutrition sciences, Dr. George Purvis. And not surprisingly, most new parents are anxious to know when to introduce their infant to the world of "real" foods.

The Gerber ad, billed as a "nutrition report," tells parents to look for certain "signs" when the baby has doubled birth weight, or weighs 13 pounds, that will help them determine the "right time" for feeding solid foods: drinking more than a quart of formula a day; demand for breast-feeding more than eight to 10 times during a 24-hour period; and "apparent hunger."

"These signs will generally appear when babies are 4 to 6 months old," Gerber says, but emphasizes that each baby is unique and suggests parents consult their health adviser "when you see a sign."

The American Academy of Pediatrics' Committee on Nutrition recommends 4 to 6 months as the optimal time for solid food introduction, but makes no mention of the "signs" in the Gerber ad. In fact, the AAP statement on the feeding of supplemental foods, along with experts in pediatric nutrition, say a baby's readiness for solid food depends in large part on neuromuscular development.

According to Dr. Samuel J. Fomon, professor of pediatrics at the University of Iowa, Iowa City, and a leading authority in pediatric nutrition, before solid foods are introduced, babies should be able to sit up with support and respond to the caretaker's feeding efforts, leaning forward to indicate they want more and turning their heads to indicate they've had enough. "Until that time, you would be running the risk of forced feeding," he observed.

Like breast-feeding, solid food introduction has come full circle. Before the availability of commercially prepared baby foods in the late 1930s, parents waited for their children to get teeth, so children went right from the breast to table food, often at 1 year of age or older.

In the 1940s and 1950s, solids were begun at an earlier and earlier age, in part, according to the AAP statement, because of the desire of mothers to see their infants gain weight rapidly and because of the ready availability of convenient forms of solid foods. At one point, infants were getting cereal several days out of the womb.

In the last 10 years, however, coinciding with the resurgence of breast-feeding, many pediatricians have been telling parents their babies don't need supplementary foods until nearly the second half of their first year. Baby food manufacturers also are discussing the subject:

* H.J. Heinz Co., in a guide to infant feeding called "Bright Start," states, "Recommendations on the time to introduce cereals to babies now range from several weeks of age to 3 to 4 months of age." Heinz's consumer adviser, Fran Shoemaker, said the pamphlet, mailed to new parents, has had an excellent response.

* Beech-Nut Nutrition Corp. offers a toll-free nutrition hotline to new parents and receives approximately 300 calls a day. Staff operators tell new mothers who ask that the company recommends 4 months of age as the time to begin solid foods.

All the companies in their promotional material include the caveat that parents should check with their physician for advice on the matter.

Dr. Myron Winick, professor of pediatrics and nutrition at Columbia University College of Physicians and Surgeons in New York City, in his book, "Growing Up Healthy--A Parent's Guide to Good Nutrition," says the American mother "has for years been under subtle pressure to use commercially prepared infant foods too early and too long. This is an example of an excellent product, which fills a real need, being marketed for inappropriate uses." Commenting on the Gerber ad, Fomon said, "Obviously, Gerber would like to sell baby food. For good nutritional advice, you probably wouldn't turn to a baby food manufacturer. It's like asking egg manufacturers if eggs are good for you."

Purvis defended the guidelines in the ad, saying they are derived from published, up-to-date sources. The firm has been questioned on the promotion in the past and has written a standard response emphasizing that the guidelines should be considered as a "total feeding program," and should not be applied individually, although the ad itself fails to make this point explicit.

Gerber cites a 1976 AAP statement on iron supplementation to support the guideline about limiting infant formula consumption to one quart per day. The limit to a quart of formula a day "is something some pediatricians believe," said Fomon, "but mostly, it's convenient for the manufacturers to believe that." A 1979 article on supplemental feeding coauthored by Fomon and frequently cited by AAP and other authorities says numerical limits can't be set on the amount of formula or breast milk an infant takes.

Fomon said there's no evidence, as implied in the Gerber ad, that babies will nurse less frequently when offered solids. More frequent nursing occurs during rapid growth spurts in the baby's development; it's nature's way of increasing the mother's milk supply.

In drawing up the guideline, Gerber said it relied on "practical experience" that "has indicated that adequately fed, normal breast-fed infants will demand food at intervals greater than three hours." In the letter, Gerber conceded that "the very young infant often will require breast-feeding more frequently than every three hours." However, when considered with the other guidelines, the firm explained, "it is obvious that foods other than breast milk should not be introduced too early."

To back up the claim that "apparent hunger" may signal a readiness for solid foods, the Gerber letter says only this: "An infant who is continually dissatisfied or hungry may be attempting to communicate the desire for additional foods. Who is in a better position to assess the wants and needs of an infant than the mother?"

To the lay person, it might seem logical that solid foods would satisfy a baby's hunger better than milk. Part of the magic associated with the early introduction of solids, according to Dr. Thomas A. Anderson, a colleague of Fomon's at the University of Iowa, is the assumption that a feeding of cereal will help the baby sleep through the night. For many parents, the promise of eight uninterrupted hours of rest is tantalizing enough to try anything short of leaving home. But studies have shown that the cereal feeding makes no difference in the baby's nighttime "snacking" habits.

"Just because the food looks semi-solid doesn't mean it has any different effect on the baby's satiety," Fomon commented, pointing out that many strained baby foods have about the same caloric density as milk. Milk is made up of 12 percent solids; some of the strained foods actually have less.

There are several reasons cited for delaying supplemental foods to infants:

* The young infant can't signal to the feeder when he or she has had enough, so there is the risk of overfeeding and subsequent obesity that may last a lifetime. Because of the lack of hard evidence supporting this link, Fomon calls it a "nebulous" argument, "but it's the only one that holds much weight."

* An extrusion, or tongue thrusting, reflex used for sucking causes the young infant to push the food right out of his mouth. This reflex disappears at 4 to 5 months of age. At the same time, the ability to swallow nonliquid foods and to chew is developed.

* Babies are thought to be more prone to allergies at a younger age. In addition, in the first months, the gastrointestinal tract has not matured and babies may have difficulty digesting some proteins, fats and carbohydrates. One study by Fomon found that a small percentage of babies under six weeks of age had trouble digesting starch.

But the best reason for delaying solids is that the milk from well-nourished mothers or from infant formula provides all the known nutritional requirements for the first six months of life.

Says Dr. Alvin Mauer of St. Jude's Hospital for Children in Memphis, and chairman of the AAP Committee on Nutrition: "There's no reason to feed infants early."

Despite this, Mauer believes the majority of infants get started on solids before 4 months of age. A 1978 survey of 268 infants younger than 6 months of age in western Massachusetts revealed that by 4 weeks, 57 percent had received cereal; by 3 months, it was 87 percent. However, a survey of 154 infants in 1979 by Purvis and two Gerber colleagues indicated that babies on the average are getting less of their total energy intake from supplementary foods than they did in 1972.

Social custom and pressure from well-meaning friends and relatives is largely to blame for the continued practice of early supplementary feeding, Mauer believes. Interestingly, in a 1978 Pennsylvania State University survey, women who were breast-feeding tended to delay introduction of solids beyond their physician's recommendation, while women who were bottle-feeding introduced the foods earlier than their doctor had advised.

Surveys of physicians show there are some who continue to advocate early introduction of supplementary foods. "It's one of those habits that's difficult to change," noted Anderson.

Fomon says he's observed some breast-feeding women deferring the introduction of solid foods beyond the time their babies are 6 months old. By the time babies are 8 or 9 months old, they often go right to finger foods, skipping strained foods entirely.

Beyond 6 months, however, a baby's iron stores are thought to run out, and pediatricians usually recommend an iron supplement or iron from fortified cereals. Deferring solids too long may risk iron deficiency, Fomon said, adding that there is no advantage to deferring solid foods beyond 5 or 6 months of age.