The supply of adequate nutrients to the developing fetus and the nutritional profile of breast milk depend directly on the quality of a woman's diet. While in general, eating a well-balanced diet assures good nutrition, during the perinatal period, the adequacy of certain individual nutrients becomes very important. Adequate intake of folic acid, for example, is critical immediately preceding conception and during early pregnancy for the prevention of neural tube defects. Current research has also revealed the importance during pregnancy and lactation of a lesser-known nutrient, docosahexaenoic acid (DHA), to support fetal/infant neural growth and development.
DHA is a Polyunsaturated Fatty Acid (PUFA) belonging to the omega-3 (n-3) fatty acid family. While many individual fatty acids comprise the n-3 family, the most prevalent include alpha-linolenic acid ( ALA ), eicosapentaenoic acid (EPA), and DHA. Even though these fatty acids are members of the same "family," they are not interchangeable. ALA is considered a "short-chain"n-3 fatty acid containing 18 carbon atoms and 3 double bonds. EPA and DHA are "longchain" n-3 fatty acids (LC n-3 PUFA) containing more carbon atoms and double bonds (20:5 n-3 and 22:6 n-3, respectively) than ALA. ALA is found primarily in seeds and seed oils such as flax (linseed) or canola and in bakery products such as multigrain breads. ALA can serve as a substrate for the production of LC n-3 PUFA in the human body; however the amount of DHA produced by the body from ALA is highly variable and generally not sufficient to meet metabolic needs (Burdge 2004), thus requiring that direct sources of DHA be consumed, especially during pregnancy. DHA and EPA are found in marine products or in the meat and eggs of animals fed n-3 PUFA-enriched diets. DHA plays a unique role in the body that cannot be duplicated by any of the other n-3 fatty acids
• DHA is critical for optimal growth and development of the fetal nervous system
• Maternal intake of DHA may increase length of gestation closer to full term
• Infants born of mothers who supplement DHA during pregnancy have more mature nervous systems at birth
• Infants born of mothers who supplement DHA during pregnancy have better visual acuity in later life
DHA represents 20% of the total and 95% of the n-3 fatty acid content of the brain and 50% of the fatty acids in the retina. The placenta preferentially accumulates DHA throughout pregnancy and particularly during the third trimester when the "brain growth spurt" begins. The brain growth spurt is the most rpid period of nervous system development beginning at the third trimester and continuing throughout the first year of life. While the placenta will take care of shuttling DHA to the fetus, a reliable dietary supply of preformed DHA must be available. (Readers are referred to a review by Lauritzen et al. (2001) for a comprehensive discussion of the role of DHA in fetal and infant nervous system development).
It appears that ensuring an adequate supply of DHA to the fetus may begin long before the third trimester. Recent research has indicated that that the levels of DHA in a woman’s blood during the first trimester predict the levels of DHA in her blood and her infant’s blood at delivery even if she has increased the DHA content of her diet throughout pregnancy (Otto et al. 2001a, Smuts et al., 2003). The significance of maintaining higher DHA blood levels throughout pregnancy to delivery is two-fold:
(1) it may lengthen pregnancy closer to full term and
(2) it may significantly enhance the function and development of the infant nervous system.
Early epidemiological studies have suggested a relationship between LC n-3 fatty acid intake and the length of gestation. Recently, Olsen and Secher (2002) conducted a prospective study of seafood intake during early pregnancy in a cohort of 8729 pregnant women living in Denmark . Quintiles of LC n-3 intake were defined as 0 mg up to 38 mg, > 38 -92 mg, > 92 -146 mg, > 146-215 mg/day. All analyses were adjusted for previously identified covariants including the following: sex of the infant; maternal smoking; alcohol consumption; maternal age; parity; height; and pre-pregnant weight. Gestational age was significantly (P<.001) incerased by an average of 3.3 days from the lowest quintile of LC n-3 PUFA intake to the highest. A similar pattern was seen for adjusted birthweight across intake quintiles with a. 84 g increase in weight between the lowest and highest quintile. Based on the quintiles of estimated LC n-3 PUFA intake the authors estimated the working range of dose response was mainly from zero intake up to 150 mg LC n-3 PUFA per day. The authors suggested that consumption of LC n-3 PUFA below 150 mg per day is a strong risk factor for preterm delivery and low birth weight.
Grandjean and coworkers (2001) in their study of a cohort of 182 singleton full-term births in the Farøe Islands analysed the concentration of fatty acids and seafood contaminants in blood samples as predictors of gestational length and birthweight. The majority (90%) of the population was reported to consume at least 3 fish dinners per week. All results were adjusted for previously identified covariants. Gestational length showed a strong positive association with the concentration of DHA in umbilical cord serum phospholipid. When all fatty acids were entered as independent variables in regression analyses, DHA was the best predictor (P<0.001) of gestation length. An increase in the relative concentration of DHA in cord serum phospholipid by 1% was associated with an increase in gestation of 1.5 days. There was no relationship between EPA and gestational length and a significant negative relationship between EPA and birthweight (P<0.015). An increase in relative EPA concentration ot 1% was associated with a decrease in birthweight of 246 g. These results indicate a relationship between length of gestation and maternal DHA status even in a high seafood consuming population.
Most recently, Smuts and coworkers (2003) reported the results on gestational outcomes in a randomised, double-blind, controlled clinical trial supplementing pregnant women (n=291) in the US with DHA from DHA-rich eggs. All results were controlled for previously identified covariants. Women in the DHA egg group consumed 137 mg of DHA per day and women in the ordinary egg group consumed 30 mg DHA per day. Neither egg contained detectable EPA. After controlling for potential confounders the length of gestation in the DHA supplemented group increased significantly by 6 days (P<.009). Infants from the DHA group were an average of 83 g heavier than those born of others in the control group. The birth weight increase was not statistically significant in this study but consistent with the findings of Olsen and Secher (2002). The above studies suggest that women consuming between 137-150 mg of DHA per day may observe increases in gestational length and possibly birth weight.
Maternal DHA status during pregnancy and at delivery appears not only to influence the length of gestation but the maturity and subsequent function of the infant nervous system. Cheruku et al. (2002) have reported that sleep patterns of full-term infants born to US mothers with higher plasma phospholipid DHA at delivery are suggestive of greater infant central nervous system maturity. Helland et al. (2001) have also reported higher EEG maturity scores for term neonates with higher concentrations of DHA in umbilical plasma phospholipids. DHA status at delivery has been linked to early maturation of central visual pathways (Malcom et al., 2003). This pattern continues to be evident in later life as visual stereoacuity has been found to be significantly enhanced among 3.5 year olds whose mothers reported consuming high DHA diets during pregnancy and who exhibited significantly higher red blood cell phospholipid DHA at delivery (Williams et al. 2001) compared to infants born of mothers reporting a low DHA diet during pregnancy. Maternal intake of DHA during pregnancy has also been reported to significantly enhance mental processing scores of children at 4 years of age (Helland et al., 2003). Most recently, Colombo and coworkers (2004) reported a significant enhancement of cognitive development at 18 months among children whose mothers had higher DHA status at birth.
DHA is a normal constituent of mothers' milk but amounts vary dramatically in response to diet. Importantly, in breast milk, DHA occurs in concert with the LC n-6 fatty acid, arachidonic acid (ARA). The combination of ARA and DHA are necessary components of infant formulas, however, as human milk ARA levels appear well regulated and not readily affected by dietary variations only the DHA content of the maternal diet need be examined. DHA levels have been reported to be as low as 0.1% of total fatty acids in breast milk and as high as 1.5%. Currently, it is uncertain which level of breast milk DHA is optimal for the support of infant neural development, but infant formulas containing 0.2% of total fatty acids as DHA have consistently resulted in benefits to infant neurologic outcomes compared to infants fed formulas without DHA. It is critical that infant formulas contain both ARA and DHA. Since human milk DHA levels respond readily to maternal diet it is fairly easy to enhance the DHA content of breast milk through maternal supplementation.
Studies suggest that significant increases in breast milk DHA content occur with as little as 200 mg supplemental DHA per day. It is important to note that the longer a woman nurses her baby, the more critical her need for DHA in her diet to prevent post-partum declines in milk DHA content (Otto et al., 2001b). Maintaining higher levels of breast milk DHA appears to have a significant impact on infant development. A dose-response relationship between infant visual acuity and breast milk DHA has been described when breast milk DHA is > 0.35 wt% DHA (Horby Jorgensen et al., 2001). No other breast milk PUFA (EPA, APA, ALA , or linoleic acid) has been correlated with visual acuity (Horby Jorgensen et al., 2001). Most recently, research from Baylor University presented by Jensen et al. at the 2004 Pediatric Academic Societies' Annual Meeting reported that children whose mothers were supplemented with 200 mg of DHA during the first 4 months of lactation had a significant improvement in sustained attention at 5 years of age compared to children whose mothers were supplemented with a placebo. This study indicated that the amount of DHA available to the neonate is important since infants of supplemented mothers received approximately 2-fold more DHA (about 0.4 wt% of milk fatty acids) compared to infants of unsupplemented mothers (about 0.2 wt% of milk fatty acids).
Finally, maternal benefits are also evident when breast milk DHA levels are elevated. A survey of 23 countries found a 50-fold difference in prevalence rates of major postpartum depression across countries was associated with maternal LC n-3 PUFA status as evident by concentrations of DHA in mothers' milk (Hibbeln, 2002). Recent observations have confirmed that blood levels of DHA are indeed significantly lower among women suffering post-partum depression as compared to matched controls (De Vriese et al., 2003). More research needs to be completed, however, to determine if supplementation with DHA will reduce the occurrence or severity of post-partum depression.
Based on the information above, targeting foods rich in DHA during pregnancy and early life is obviously important. Various authorities have made specific recommendations concerning dietary DHA during pregnancy and lactation (see Table 1).
| Authority/Country | Recommended Daily Intake |
|---|---|
| Health Council of the Netherlands | 200 mg n-3 fatty acids from fish 1 |
| AFSS 2/France | 250 mg DHA |
| Health Canada | Up to 140 mg EPA and DHA 3 |
| Institute of Medicine/United States | Up to 140 mg EPA and DFA 3 |
| Scientific Advisory Committee on Nutrition/UK | 450 mg LC n-3 PUFA 4 |
1. n-3 fatty acids from fish include eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)
2. Agence Francaise de Securite Sanitaire des Aliments
3. expressed as a contribution to the recommended adequate intake (AI) for ALA
4. long-chain n-3 polyunsaturated fatty acids, specifically EPA, DHA and DPA n-3, during pregnancy
Accomplishing dietary enrichment with DHA, particularly during pregnancy, is not as simple, however, as increasing the fish content of the maternal diet. Seafood is the most abundant source of LC n-3 PUFA in the human diet, but it is also one of the most abundant sources of environmental pollutants, particularly methyl mercury, which can damage the developing fetal nervous system. Generally, a woman can be assured low-level exposure to fish contaminated with high levels of methyl mercury by avoiding large predatory fish such as shark. A complete list of fish recommended for limitation by Food Standards Australia New Zealand for women who are trying to become pregnant, those that are pregnant, and by young children are listed in Table 2. For agencies attempting to establish safe limits, tuna has been a point of controversy. The UK and Canada include tuna steaks, but not canned tuna, in their advisories to pregnant women. Canned tuna is believed to pose little risk for methyl mercury contamination because tuna used in canning are often younger than those used for steaks and therefore have not lived long enough to accumulate significant levels of methyl mercury.
| Type of fish | Suggested limitation |
|---|---|
| Billfish/swordfish*, broadbill and marlin | No more than one serving per fortnight. No other fish to be consumed that fortnight |
| Shark* /flake | No more than one serving per fortnight; |
| Orange roughy / sea perch | No more than one serving per week; |
| Catfish | No more than one serving per week; |
1. Limitations are indicated for women planning pregnancy, for those who are pregnant, and for young children to avoid excess exposure to methylmercury. Source: FSANZ 2004.
*Also limited by the US , UK and Canadian governments.
Safe alternatives to fish include DHA enriched chicken eggs (about 100150 mg DHA per serving) and dietary supplements containing DHA from marine microalgae (up to 200 mg DHA per capsule). Microalgae are grown independent of the ocean and have no opportunity to accumulate methyl mercury. Other foods that contain low levels of DHA but can contribute significantly to DHA intake based on their frequency of consumption include chicken meat (about 30 mg DHA per serving) and ordinary eggs (about 20 mg DHA per serving). It is possible, therefore, by using a variety of DHA containing foods to accomplish a meaningful level of DHA in the maternal/infant diet. Eggs are often a weaning food for young children, thus DHA-enriched eggs are a sensible way to increase the DHA content of the toddler diet.
Dietary sources of ALA are often touted as alternatives to marine products for increasing DHA intake. Products rich in ALA such as flaxseed, various seed oils, and grain products made with these seeds and seed oils DO NOT contain preformed DHA. They will only supply ALA , which is a very inefficient and unreliable substrate for the production of minute amounts of DHA in the human body. Of importance, Francois et al. (2003) have reported that supplementation of nursing women with large amounts (10.7 g/d) of ALA has no effect on breast milk or maternal plasma phospholipid DHA, indicating that support of maternal DHA status post-partum relies directly on DHA from the diet. To assure adequate DHA intake, foods rich in preformed DHA must be added to the diet. Vegans are recommended to seek marine microalgal oils supplements, preferably in vegan capsules, to supplement the DHA content of their diet.
In summary, women need to target safe sources of preformed DHA during pregnancy to assure a sufficient DHA supply to the developing fetus. Breast milk naturally contains DHA; however, levels may be low if preformed DHA is limited in the maternal diet. DHA is a vital nutrient for the support and optimisation of brain and eye development during pregnancy and early life.
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