An estimated 5 per cent of babies born in Hong Kong are underweight, according to Health Department statistics from 2011, and thus face a greater risk of disease, significant disabilities or even death.
But the children can avoid this fate, says a nutrition professor, if mothers increase their intake of an omega-3 fatty acid simply known as DHA (docosahexaenoic acid), a nutrient primarily found in fish.
Dr Susan Carlson, a professor based at the University of Kansas Medical Centre who has researched the nutrient for about 30 years, was in Hong Kong recently to present her latest findings from an ongoing six-year study on the benefits of DHA during pregnancy.
She says consuming DHA supplements results in an "overall longer gestation period as well as greater infant size". "This appears to be due to a significant reduction in early pre-term and very low birth weight infants, which was an important outcome of the study."
The World Health Organisation says health risks, including a longer period of hospitalisation after birth, are higher among babies delivered weighing less than 2.5kg.
Carlson was invited to Hong Kong by DSM, the nutritional products maker that provided the supplement capsules in Carlson's study.
DHA is an omega-3 fatty acid (or "good" fat) that is found in fatty tissues in the brain and retina. It is also found in varying amounts in human breast milk, fish oils and marine algae. The body can convert alpha-linolenic acid from plant oils, and breast and formula milk into the nutrient.
DHA is essential for infant brain, eye and nervous system development. It is particularly important in the third trimester of pregnancy, when significant brain growth occurs. Very premature infants may be deficient in DHA because they miss out on this period in utero when the fatty acid accumulates in tissue.
Yet, the findings are hotly debated. "I would say that it's pretty controversial," Carlson admits, saying researchers have conducted three reviews of available studies and found that DHA did not benefit infants' cognitive development.
Still, since 2001, Carlson's studies with colleague and neuroscientist John Colombo have influenced infant formula makers to begin adding DHA using marine oil extracts.
However, Melissa Liu, with the risk assessment section of Hong Kong's Centre for Food Safety, notes there is insufficient evidence to confirm the benefits of DHA in infants and children.
"As such, the European Union only accepts claims mentioning that DHA intake contributes to the normal visual development of infants up to 12 months [old], while claims mentioning function of DHA intake on brain development of non-breastfed infants or eye development for infants and children above 12 months are not accepted," says Liu.
"In fact, there are concerns that for nutrients added to formula products, including DHA, their structures and functions may not be the same as those present in breast milk because they are extracted from cows' milk or other ingredients."
Even the Codex Alimentarius Commission - a WHO body that sets international food standards - "does not consider DHA to be an essential composition of infant formula and follow-up formula products", after taking into account that DHA can be synthesised in the body from alpha-linolenic acid.
Much of the interest in omega-3 fatty acid intake and pregnancy began in the early 1980s, when Danish researchers published a study in The Lancet saying women living on the Faroe Islands, a Danish territory, had babies that were 194 grams heavier and had gestation periods four days longer than those in Denmark.
The Faroese diet had substantially more omega-3 fatty acid and less omega-6 fatty acid (from eggs, poultry and plant oils) than the Danish diet, and fatty acid content in red blood cells was significantly higher in the Faroese women. To test the effect of the fatty acids on pregnancy, the researchers randomly assigned women who were 30-weeks pregnant either a fish oil supplement, olive oil or no supplement.
For the fish oil group, the women's pregnancy was four days longer than the other groups, and their babies weighed 107 grams more than the olive oil group and 43 grams more than the control group.
Published last month in the peer-reviewed American Journal of Clinical Nutrition, Carlson's latest study involved 350 women who were enrolled when they were 14-weeks pregnant on average. The women were randomly split into two groups, consuming orange-flavoured capsules daily that contained either DHA (600mg dose) or a placebo, until delivery. The dose of DHA was equivalent to that found in about three to four ounces of oily cold-water fish such as salmon, Carlson says.
It was found that the prenatal algal DHA supplements increased DHA blood levels in both the mother and baby. In addition, the newborns were heavier, longer and had a larger head circumference compared to those from the placebo group.
There was a much lower incidence of low birth weight in the DHA group (which had 3.9 per cent) compared to the placebo group (9 per cent), and there was zero very-low-birth-weight babies (less than 1.5kg) in the DHA group while the placebo group had 3.4 per cent.
There was also a lower incidence of pre-term births (born before 34 weeks) in the DHA group. And if babies were pre-term, those whose mothers took DHA spent an average of 7.8 days in hospital, about 30 days fewer than those born to the placebo group.
"If every woman in the United States took DHA supplements, this could reduce early pre-term births by 80,000 a year," says Carlson. "That's US$9 billion saved on hospital stays each year."
Breast milk is still best, Carlson notes, but breastfeeding mothers' diets - especially vegetarians - may lack DHA. Supplements - whether from algae or fish oil, which are of the same quality - can be taken by both pregnant and breastfeeding women.
There are other factors that might affect mothers' and infants' DHA levels, including differences in the body's ability to synthesise the nutrient, the rate of maternal to fetal transfer of DHA and the status of other nutrients required to synthesise it. Carlson's studies have all been based on the US population and she identifies the need for more country-specific studies.
But can too much DHA be bad? Carlson thinks not. "Some of the women in one part of China where they consume the most fish have 2.5 per cent DHA of total fatty acids in their breast milk, and nobody suggests that's a problem. Japanese women have about 1 per cent. In the US, it's about 0.2 per cent; in Europe it's 0.3 per cent."
She also contends that studies junking DHA's benefits have a few problems. "One is that people stop studying the children when they're 18 months, and most of them are not working with developmental psychologists like I am, and so they use a very standard global test called the Bayley Scales of Infant Development. [The test] is designed to predict problems in the babies' development, not whether they're doing any better," she says.
"We have published studies that show in the first year of life these children have higher cognitive function using tests which are mainly based on attention. There's quite a bit of evidence from researchers who have looked at children at age four, five and six, and found benefits when they didn't find them earlier. I personally think that there are some children who benefit," she says.
With evidence still murky, Codex recommends DHA in infant formula should not exceed 0.5 per cent of total fat content. It should also be present in at least the same concentration as arachidonic acid (a type of omega-6 fatty acid) and lower than eicosapentaenoic acid (omega-3). These levels do not pose major risks, says Liu.