Peruse the infant formula aisle, or check out the options for prenatal nutritional supplements, and you’ll find that nearly all these products boast a “brain nourishing” omega-3 fatty acid called DHA. But despite decades of research, it’s still not clear that DHA in formula boosts brain health in babies, or that mothers need to go out of their way to take DHA supplements.

A systematic review of studies published this month by the Cochrane Collaboration concluded there was no clear evidence that formula supplementation with DHA, or docosahexaenoic acid, a nutrient found mainly in fish and fish oil, improves infant brain development. At the same time, it found no harm from adding the nutrient. The findings are consistent with a review of the effects of omega-3 supplements in pregnancy and infancy published by the Agency for Healthcare Research and Quality last fall that found little evidence of benefit.

Still, many experts believe there is value in including DHA in formula. “Even if you can’t easily prove it, because it’s hard to prove developmental outcomes, it makes sense to use it,” said Dr. Steven Abrams, a professor of pediatrics at Dell Medical School at the University of Texas at Austin. “It’s probably a good idea to keep it in there, and it’s certainly safe.”

During pregnancy and the first few years of life, DHA accumulates in the brain and retina of the eye and plays an important role in neural and vision development. Breast milk contains DHA in varying concentrations, depending on how much is in the mother’s diet, and some DHA can be made in the body from precursor omega-3 fatty acids, although this process is inefficient.

Studies from the 1990s found that formula-fed infants had lower levels of brain DHA than breast-fed infants and suggested that adding DHA to formula improved cognitive and vision development. The results were compelling enough that in 2002, the Food and Drug Administration approved the addition of DHA to infant formulas.

“And then the marketplace took over,” said Dr. Abrams. “People chose the formula that was advertised as having this brain substance, DHA, and soon the formula companies just decided to put it in everything.”

Today, all major formula brands sold in the United States contain DHA, usually in the form of a purified oil derived from algae, an ingredient that adds to the cost of formula.

“If they claim feeding this formula will make your child smarter or have a higher I.Q., that is not a rightful claim,” said Dr. Karen Simmer, a professor of newborn medicine at the University of Western Australia and an author of the Cochrane review.

The review combined data from 15 randomized controlled trials into a meta-analysis including nearly 1,900 children, many tracked from infancy into mid-childhood. Some studies found small improvements in vision or cognition, but many did not, and when the results were pooled, there was no clear pattern of benefit from DHA added to formula.

Other studies have focused on fish oil or DHA supplementation during pregnancy, when omega-3s can cross the placenta to the developing fetus. The supplements do seem to increase the length of pregnancy by about two days and result in slightly heavier newborns, but this doesn’t translate to a reduction in premature birth, the A.H.R.Q. report said.

Some studies have found that when mothers take fish oil or omega-3 supplements in pregnancy, their children are less likely to have eczema, food allergies or asthma, at least for the first few years of life, but this research is also inconsistent. And as with formula, prenatal DHA supplements don’t seem to make for smarter children. That conclusion was confirmed in a recent randomized controlled trial published in JAMA, which found no effect of prenatal DHA supplementation on children’s I.Q. at age 7.

This type of confusion is common in nutrition research, says Carol Haggans, a registered dietitian and spokeswoman for the National Institutes of Health Office of Dietary Supplements, which publishes advice for consumers about omega-3s and other supplements. In the case of DHA, she said, “There’s a good theoretical basis for it, and then you have the observational studies that show that fish and other types of seafood are beneficial.” But when researchers try to package a nutrient like DHA in a supplement and test it in randomized controlled trials, the same benefits often aren’t observed.

One reason for the conflicting results is that people have different baseline levels of DHA, depending on how much fish they eat, and genetic variations can affect fatty acid metabolism. “If you don’t have a deficient population, then it probably doesn’t matter” if people take a supplement, said Dr. Susan Carlson, professor of nutrition at the University of Kansas Medical Center.

But in the United States, many women don’t seem to be getting enough DHA. Women of childbearing age consume an average of 60 milligrams of DHA per day, but many experts recommend at least 200 milligrams per day during pregnancy and breast-feeding.

That amount of DHA can be obtained by eating eight to 12 ounces of fish per week, as recommended for pregnant or breast-feeding women in the Dietary Guidelines for Americans. Focus on fish varieties high in omega-3s and low in methyl mercury, like salmon, trout, herring and anchovies, the guidelines say.

“It’s preferable to follow those dietary guidelines and get omega-3s from food first, but dietary supplements can be an option,” particularly for women who don’t eat fish, said Ms. Haggans. In the amounts found in prenatal supplements, DHA has only minor side effects, but it’s always best to talk this decision over with a health care provider, she said.

And while experts may disagree on the value of DHA supplements, they seem to agree on the value of fish. “Fish has a lot of different nutrients besides DHA that are probably very good. A lot of iodine comes from fish, selenium comes from fish, and these are all key brain nutrients,” said Dr. Carlson.

“The best thing you can do is eat a balanced, healthy diet, and part of a balanced healthy diet is to eat fish twice a week. And then, when you introduce food to your child, to include some fish in their diet,” Dr. Simmer said.