Eggs of free-range hens in northern Nigeria are a good source of docosahexaenoic acid for pregnant and lactating women

Robert H Glew; Lu-Te Chuang

doi:10.4103/2276-7096.123575

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ORIGINAL ARTICLE

Year : 2013 | Volume : 15 | Issue : 2 | Page : 76-81

Eggs of free-range hens in northern Nigeria are a good source of docosahexaenoic acid for pregnant and lactating women

Robert H Glew¹, Lu-Te Chuang²
¹ Departments of Biochemistry and Molecular Biology and Surgery, School of Medicine, University of New Mexico, Albuquerque, New Mexico, USA
² Department of Biotechnology, Yuanpei University, Hsinchu, Taiwan

Date of Web Publication

24-Dec-2013

Correspondence Address:
Robert H Glew
Department of Surgery, MSC 10 5610, 1 University of New Mexico, Albuquerque, NM 87131-0001
USA

Source of Support: None, Conflict of Interest: None

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DOI: 10.4103/2276-7096.123575

Abstract

Background/Purpose: Although docosahexaenoic acid (DHA) is critical for the normal development of the central nervous system, especially in utero and during the 1 ^st year of life, the concentration of this important polyunsaturated fatty acid in the milk fat of Nigerian women is low relative to international standards. There is a compelling need to identify dietary sources of DHA for pregnant and lactating women in sub-Saharan Africa and elsewhere in the world where populations live inland far from the sea and therefore lack access to DHA-rich ocean fish. Hen's eggs represent a potentially useful source of DHA.
Methodology: Since the DHA content of eggs varies greatly with the hen's diet and knowing from studies elsewhere in the world that free-range (FR) hens produce eggs enriched for DHA, we compared the fatty acid compositions of the yolk of eggs from FR and "commercial" hens in northern Nigeria.
Results: The percentage of DHA in the FR eggs was 2.6-fold higher (P < 0.001) compared to commercial eggs. Two FR eggs per day could satisfy about three-fourths of the daily recommended intake of DHA for pregnant and lactating women. The FR eggs also contained 2.8-fold (P < 0.001) more α-linolenic acid, more than 10-fold more eicosapentaenoic acid, and 4.1-fold more of the healthful conjugated linoleic acids than commercial eggs.
Conclusion: Eggs produced by FR hens could be useful in improving the fatty acid nutrition of pregnant and lactating women in Nigeria, and ultimately their infants who derive much of their nutrition from breast milk.

Keywords: Conjugated linoleic acids, docosahexaenoic acid, n-3 polyunsaturated fatty acids, Nigeria, yolk lipids, α-linolenic acid

How to cite this article:
Glew RH, Chuang LT. Eggs of free-range hens in northern Nigeria are a good source of docosahexaenoic acid for pregnant and lactating women. J Med Trop 2013;15:76-81

How to cite this URL:
Glew RH, Chuang LT. Eggs of free-range hens in northern Nigeria are a good source of docosahexaenoic acid for pregnant and lactating women. J Med Trop [serial online] 2013 [cited 2023 Jun 6];15:76-81. Available from: https://www.jmedtropics.org/text.asp?2013/15/2/76/123575

Introduction

Optimal development of the mammalian central nervous system, especially during the third trimester of gestation and the 1 ^st year of life, is strongly dependent on the availability of the two essential fatty acids, linoleic acid (18:2n-6) and α-linolenic acid (18:3n-3), and the longer n-3 and n-6 polyunsaturated fatty acids (PUFAs), docosahexaenoic acid (DHA; 22:6n-3) and arachidonic acid (20:4n-6), respectively. ^{[1],[2],[3],[4],[5]}

Infants born in economically advanced countries can usually obtain adequate amounts of these critical n-3 and n-6 fatty acids from the mother, assuming she elects to breastfeed, or from dietary supplements and complementary foods. In contrast, in many regions of Nigeria and other parts of sub-Saharan Africa, breastfeeding is the sole means by which the nutritional needs of nearly all infants during the 1 ^st year of life are satisfied. ^[6],[7] However, it has been documented that lactating women in underdeveloped countries often produce milk that is deficient in linoleic acid and DHA. ^[8],[9] For example, in a study we conducted in 2006 in northern Nigeria ^[10] we found that the percentages of DHA in the milk of indigenous Nigerian women was about 0.11%, which is at the low end of the range of DHA values reported by Yuhas et al. when they analyzed 50 breast milk samples from women in nine countries, including the US, China, Philippines and Chile. ^[9]

Since maternal intake of DHA during pregnancy and lactation correlates positively with favorable mental development of their offspring, ^[4],[11] populations in sub-Saharan Africa (and probably many other underdeveloped parts of the world as well) would likely benefit from greater access to foods that contain useful amounts of DHA and other nutritionally important n-3 and n-6 PUFA such as linoleic acid, α-linolenic acid and arachidonic acid.

Although ocean fish such as salmon, mackerel and tuna are excellent sources of DHA and other nutritionally important fatty acids, economic factors and a lack of refrigeration place these sources of nutritionally important fatty acids beyond the reach of most pregnant and lactating women who live in impoverished communities or far from the sea. Chicken eggs offer a means of ameliorating this problem. It has been shown by many different groups of investigators that, depending upon the diet a laying hen consumes, the yolk of the egg she produces can accumulate nutritionally significant amounts of DHA. ^{[12],[13],[14],[15]} Furthermore, it is well-established that free-range (FR) hens produce eggs that may contain as much as five times more DHA than hens raised on various grains (e.g., corn, barley, wheat bran) or soybean in confinement in commercial feed lots. ^[16],[17] FR hens probably derive much of the DHA in their eggs from the seeds, insects and small animals (e.g., lizards) they consume. ^[18] Thus, if it can be shown that FR hens in Nigeria where the DHA content of human milk and infant nutrition are matters of concern do in fact produce eggs that contain useful amounts of this critical fatty acid, then the local population, particularly lactating women and breast-fed children up to a year or two of age, would have available to them a means for significantly improving their nutrition, at least with respect to DHA. Furthermore, increasing the concentrations of PUFAs in breast milk might reduce the risk of mother-to-child transmission of human Immunodeficiency virus. ^[19]

We therefore collected eggs from FR chickens in northern Nigeria and compared the fatty acid composition of the yolks of those eggs with the proportions of fatty acids in the yolks of eggs of hens raised in confinement on mainly corn, millet and soybeans. The data we report should provide a basis for optimism with regard to improving the quality of the milk produced by women in impoverished regions of sub-Saharan Africa that are far from an ocean.

Methodology

Eggs

The eggs that were analyzed in this study were obtained in August, 2007 from two sources on the Jos Plateau in north-central Nigeria. Three commercial eggs were obtained from three different hens in a large, commercial egg-producing operation in the city of Jos that raises hens in confinement, without access to pasture, on a mixed feed consisting mainly of maize, soybeans and millet; these eggs are designated "agric" eggs by local market-women and people who consume them. Three eggs produced by FR hens were obtained from semi-nomadic Fulani pastoralists living in a rural settlement on the outskirts of Jos. The fresh eggs were held in boiling water for 12 min and cooled to room temperature before the yolks were removed and placed in cryovials and transported on ice to the US where they were stored at-80°C for 3 weeks before being extracted with chloroform-methanol and analyzed for fatty acids. The yolks of the commercial eggs from hens raised in cages were pale yellow whereas, in contrast, the yolks of the eggs produced by the FR hens were a deep reddish orange.

Fatty Acid Analysis

Total lipids from the egg yolks were extracted according to the procedure of Folch et al. ^[20] Briefly, approximately 0.25 g of egg yolk was extracted with 20 ml chloroform: methanol (2:1, v/v) at 4°C for 24 h. The extracted lipids in the chloroform phase were separated from the aqueous phase by adding 4 ml of 0.9% (w/v) NaCl solution. The chloroform phase was evaporated at 40°C using a stream of nitrogen. The crude lipid fraction was then treated with 2 ml of 1% (v/v) sulfuric acid in methanol and 0.5 ml of dimethylsulfoxide for 20 min at 95°C to generate fatty acid methyl esters. ^[10],[21] The fatty acid methyl esters were extracted into hexane and separated and quantified by gas-liquid chromatography using an Agilent 6890 gas chromatograph (Agilent Technologies, Palo Alto, CA, USA) equipped with an on-column automatic injector, flame ionization detector, HP-88 capillary column (100 m × 0.25 μm film thickness) (Hewlett-Packard, Sunnyvale, CA) and Chemstation software. The operating conditions were as follows: Carrier gas, helium; injector temperature, 250°C; detector temperature, 280°C; temperature program, initially 175°C, raised to 220°C at a rate of 2°C/min and held at 220°C for 20 min. The fatty acid methyl esters were identified by comparing their retention times to those of known standards donated to us by Nu-Chek (Elysian, MN, USA) and quantified using the principle of internal standardization.

Statistical Methods

Data were analyzed by Student's t-test to compare differences between the fatty acid percentages of the two groups of eggs. Descriptive statistics and group comparisons were made using the Number Cruncher Statistical Software (version 6, Kaysville, UT, USA). The hypothesis was that two mean values were equal was tested at a significance level of P = 0.05.

Results

The main aim of the study was to determine if the yolk of eggs of FR hens in northern Nigeria contained nutritionally useful quantities of DHA. Another purpose of the study was to compare the fatty acid composition of yolks of eggs of FR hens with that of the yolk of eggs of hens raised in commercial feedlots in Nigeria and in egg-processing plants elsewhere in the world. The yolks of the eggs of the FR hens were deep reddish orange, the color of tomato skin, whereas the yolks of the commercial eggs were pale yellow. The total fatty acid content of the egg yolks from the two sources was not different (P > 0.05): FR eggs, 270 mg/g wet weight; commercial eggs, 256 mg/g wet weight.

As shown in [Table 1], the percentages of DHA (22:6n-3) and eicosapentaenoic acid (EPA) (20:5n-3) in the yolk lipids of eggs from the FR hens was 2.6-fold and more than 6-fold higher, respectively, than the proportions of DHA and EPA in the yolks of eggs produced by commercially-raised hens in Jos. The percentage of docosapentaenoic acid (22:5n-3), which is a precursor of DHA, was also higher in the yolk of eggs of the FR hens (0.17% vs. 0.09%, P = 0.001). The contribution the n-3 essential fatty acid α-linolenic acid (18:3n-3) made to the fatty acid total was also higher, by 2.7-fold, in the eggs of the FR hens (0.69% vs. 0.25%, P=0.02). Although the mean percentage of the n-6 essential fatty acid, linoleic acid (18:2n-6), was 30% lower in the yolk lipids of the eggs of the FR hens compared to mean percentage of linoleic acid in the commercially-produced eggs (10.7% vs. 14.6%), the difference between the two values was not statistically significant. There was no difference in the proportion of arachidonic acid (20:4n-6) in the eggs from the two different sources; in both cases, the percentage of arachidonic acid was in the 2.65-2.70% range.

Table 1: Fatty acid compositions of egg yolk lipids by weight percentage

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Since conjugated linoleic acids (CLAs) have health benefits, including promoting infant growth, ^[22] we were interested in knowing the CLA content of the egg yolk lipids of the FR and commercial hens. Together, the two CLAs, c9, t11-18:2 and t10, c12-18:2, accounted for 0.21% and 0.04%, respectively, of the fatty acid total in the yolk lipid fraction of the eggs of range-fed and commercial hens and the difference in these two values was significant (P < 0.001). Thus, eggs of the range-fed hens appear to be a much better source of healthful CLAs than eggs produced by the commercial hens. On the other hand, the combined proportions of trans fatty acids (t11-18:1 and t9, t12-18:2) in the eggs yolks from the two sources were not significantly different (0.24 and 0.22%). It is generally accepted that trans fatty acids are detrimental to human health.

Another noteworthy difference between the fatty acid compositions of the two sources of egg yolk was found in the percentages of total n-3 and n-6 PUFA: whereas the combined n-6 PUFA accounted for only 14.8% of the fatty acid total in the yolk lipids of the FR hens, they accounted for 19.2% of the fatty acids in the yolk lipids of the commercial hens; in contrast, the proportion of all n-3 PUFA was nearly 3-fold greater in the egg yolk lipids of the FR hens (2.84% vs. 1.10%, P < 0.001). There were no statistically significant differences in the proportions of total saturated fatty acids and total monoenoic fatty acids in the two sources of eggs.

Discussion

Appreciating the importance of DHA to the growth and development of the central nervous system and overall growth of infants and young children, ^{[1],[2],[3],[4],[5]} the present study was undertaken in an effort to identify a substantial source of DHA that was locally available and which would be affordable to the inhabitants of northern Nigeria, particularly pregnant women and breastfeeding mothers in rural as well as urban communities. In a previous study, we found that freshwater fish harvested from rivers in the region do not contain nutritionally significant amounts of DHA. ^[23] Knowing that the type of dietary fatty acids of the laying hen can markedly affect the fatty acid profile of the egg-yolk lipids ^{[12],[13],[14],[15],[16]} and in light of the fact that others have shown that the DHA content of FR hens is generally higher than that of commercially-raised hens, ^[16],[17] we wanted to know if FR hens in rural northern Nigeria, too, produced eggs with a relatively high DHA content. Indeed, our data show that, on a wet weight basis [Table 2], the yolk of the eggs of range-fed hens raised by the semi-nomadic Fulani in Plateau State, Nigeria contain nearly three times more DHA and 6-fold or more EPA than egg yolks derived from hens raised in confinement by commercial egg producers [Table 2].

Table 2: Content (mg/g wet weight) of selected fatty acids in the yolk of eggs produced by free-range and commercial hens

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How significant in nutritional terms might this three-fold increase in the DHA content of the eggs of the FR hens be? To address this question, we compared the amount of DHA a pregnant or nursing woman might derive from eggs of FR chickens to the recommended intake of DHA. Nutrition experts in several countries have recommended that the daily intake of DHA by lactating mothers should be 200-300 mg. ^[24],[25] Since the average weight of the yolk one egg of a FR hen in the present was 15 g, using the data in [Table 2] which show a DHA content of 5.07 mg/g of egg yolk (wet weight), we estimate that two eggs from FR hens would provide approximately 150 mg of DHA, or 75% of the recommended daily intake of 200 mg for pregnant and lactating women. Thus, according to this calculation, the eggs of FR chickens could satisfy most of the DHA most of the DHA needs of pregnant or lactating women in northern Nigeria.

In terms of fatty acid nutrition, the eggs of the FR hens we analyzed appeared to be superior to those of the eggs that were commercially-produced not only because they contained more DHA but also because the yolk of the eggs of the FR hens contained nearly three times more of the essential n-3 fatty acid, α-linolenic acid, and four times more of the healthful CLAs than the eggs produced by the caged hens in the present study that were fed a controlled diet of corn, millet and soybeans [Table 2]. CLAs have been reported to have a wide range of health-promoting effects, including anti-carcinogenic, anti-diabetic, and immune-stimulatory effects. ^[2],[22] CLAs have also been reported to enhance growth and development of infants. Noteworthy, too, in our data was the large disparity in the linoleic acid: α-linolenic acid ratio for the two kinds of eggs: This ratio was 16 for the eggs of FR hens and 60 for those of the commercial hens. There is consensus among nutritionists that the balance between these two essential fatty acids is important health-wise and it is generally accepted that the ratio of linoleic acid to α-linolenic acid in the diet should be in the range of 5-15. ^[8] Thus, from the standpoint of fatty acid nutrition at least, there are a number of reasons why the consumption of eggs of FR hens instead of commercial eggs should be promoted in northern Nigeria, especially by nursing mothers regardless of whether they reside in an urban or rural area of that that region of sub-Saharan Africa and who are known to produce a milk that contains DHA levels that are among the lowest that have been reported for populations in developed and underdeveloped parts of the world. ^{[2],[8],[9],[10]} It has been shown that inclusion of the yolk of four omega-3-enriched eggs per week in the weaning diet of breast-fed infants for 3-6 months increased their erythrocyte DHA concentration 30-40%. ^[26] A subsequent study showed that maturation of visual acuity was accelerated in breast-fed term infants when their weaning food was supplemented with DHA-enriched egg yolks. ^[27]

The fatty acid compositions we report for the yolk lipids of eggs produced by commercial hens in Nigeria are similar to what has been reported by other investigators. For example, the fatty acid profiles for "supermarket eggs" in Greece reported by Simopoulos and Salem ^[13] and eggs produced by hens fed a maize, soybean and canola meal-based diet by Rowghani et al. ^[15] in Iran are similar to the fatty acid profile of the yolk lipids of the "commercial eggs" that we analyzed in the present study. Furthermore, the percentages of DHA, arachidonic acid and α-linolenic acid Gao and Charter ^[12] reported for the yolk lipids of eggs produced by chickens fed a "regular diet" agreed closely with the corresponding values we report herein for the "commercial eggs" we obtained in Nigeria. The content of trans fatty acids (e.g., t11-18:1) and CLAs (e.g., c9, t11-18:2) we found for the commercial eggs in our study agree with corresponding values reported by other investigators. ^[28]

A limitation of the present study was that we sampled hens' eggs from just one community in northern Nigeria and only during the rainy season. In light of the fact that the fatty acid composition of eggs is strongly affected by the hen's diet, it would be interesting to compare the content of DHA and the other nutritionally important fatty acids in eggs produced by animals in different regions of the country and at various times of the year, that is during the hot, dry season (November-February) as well as the rainy season (May-September). In addition, we did not take note of the strains of hens that provided the eggs for this study.

With regard to future studies, we would like to know if supplementing the diets of lactating women in rural communities in northern Nigeria with eggs from FR hens could significantly increase their DHA status and the DHA content of the milk these women produce. If the DHA content of the breast milk of Nigerian women could be enhanced substantially by increasing their intake of eggs from range-fed chickens, then this could have positive implications for the overall health and cognitive development of the infants they nurse. In this regard, federal and local public health officials bent on encouraging pregnant and lactating women to increase their intake of eggs from FR hens would certainly have to overcome local taboos against consumption of eggs during the perinatal period. ^[29],[30] Egg taboos are imposed during pregnancy and lactation in many different cultures in Nigeria and other countries in sub-Saharan Africa on the grounds that eating eggs will cause the fetus to become too large or the baby to be born with one or more congenital defects or to acquire criminal tendencies. Follow-up studies should also include comparisons of the content of other lipid-soluble nutrients such as carotenoids in eggs produced by FR versus commercial, caged hens in different regions of the country. A similar intervention with the eggs of FR eggs would also be valuable in the US Southwest, New Mexico in particular, because most pregnant and lactating women who account for the three major ethnic groups in that state have a DHA intake below 100 mg/day. ^[31],[32]

Conclusion

In summary, we have demonstrated that in the region of northern Nigeria where our study was performed the eggs of FR hens provide considerably greater quantities of healthful DHA, α-linolenic acid and CLAs than eggs from commercial producers.

Acknowledgments

The authors are grateful to Nu-Chek, Inc. for providing fatty acid methyl ester standards. This study was supported by a Fulbright Scholar Award to RHG from the US State Department.

References

1.	Crawford MA, Hassam AG, Williams G. Essential fatty acids and fetal brain growth. Lancet 1976;1:452-3. [PUBMED]
2.	Elias SL, Innis SM. Infant plasma trans, n-6, and n-3 fatty acids and conjugated linoleic acids are related to maternal plasma fatty acids, length of gestation, and birth weight and length. Am J Clin Nutr 2001;73:807-14. [PUBMED]
3.	Bouwstra H, Dijck-Brouwer DA, Wildeman JA, Tjoonk HM, van der Heide JC, Boersma ER, et al. Long-chain polyunsaturated fatty acids have a positive effect on the quality of general movements of healthy term infants. Am J Clin Nutr 2003;78:313-8. [PUBMED]
4.	Helland IB, Smith L, Saarem K, Saugstad OD, Drevon CA. Maternal supplementation with very-long-chain n-3 fatty acids during pregnancy and lactation augments children's IQ at 4 years of age. Pediatrics 2003;111:e39-44. [PUBMED]
5.	Jensen CL, Voigt RG, Prager TC, Zou YL, Fraley JK, Rozelle JC, et al. Effects of maternal docosahexaenoic acid intake on visual function and neurodevelopment in breastfed term infants. Am J Clin Nutr 2005;82:125-32. [PUBMED]
6.	el Bushra HM, Salih MA, Satti SA, Ahmed Mel F, Kamil IA. Infant-feeding practices in urban and rural communities of the Sudan. Trop Geogr Med 1994;46:309-12. [PUBMED]
7.	Pérez-Escamilla R. Evidence based breast-feeding promotion: The Baby-Friendly Hospital Initiative. J Nutr 2007;137:484-7.
8.	Smit EN, Martini IA, Kemperman RF, Schaafsma A, Muskiet FA, Boersma ER. Fatty acids in formulae for term infants: Compliance of present recommendations with the actual human milk fatty acid composition of geographically different populations. Acta Paediatr 2003;92:790-6. [PUBMED]
9.	Yuhas R, Pramuk K, Lien EL. Human milk fatty acid composition from nine countries varies most in DHA. Lipids 2006;41:851-8. [PUBMED]
10.	Glew RH, Herbein JH, Moya MH, Valdez JM, Obadofin M, Wark WA, et al. Trans fatty acids and conjugated linoleic acids in the milk of urban women and nomadic Fulani of northern Nigeria. Clin Chim Acta 2006;367:48-54. [PUBMED]
11.	Ryan AS, Astwood JD, Gautier S, Kuratko CN, Nelson EB, Salem N Jr. Effects of long-chain polyunsaturated fatty acid supplementation on neurodevelopment in childhood: A review of human studies. Prostaglandins Leukot Essent Fatty Acids 2010;82:305-14. [PUBMED]
12.	Gao YC, Charter EA. Nutritionally important fatty acids in hen egg yolks from different sources. Poult Sci 2000;79:921-4. [PUBMED]
13.	Simopoulos AP, Salem N Jr. Egg yolk as a source of long-chain polyunsaturated fatty acids in infant feeding. Am J Clin Nutr 1992;55:411-4. [PUBMED]
14.	Milinsk MC, Murakami AE, Gomes ST, Matsushita M, de Souza NE, et al. Fatty acid profile of egg yolk lipids from hens fed diets rich in n-3 fatty acids. Food Chem 2003;83:287-92.
15.	Rowghani E, Arab M, Nazifi S, Bakhtiari Z. Effect of canola oil on cholesterol and fatty acid composition of laying hens. Int J Poult Sci 2007;6:111-4.
16.	Lopez-Bore CJ, Arias RS, Rey AI, Castano A, Isabel B, Thos J. Effect of range-free feeding on omega-3 fatty acid and alpha-tocopherol content and oxidative stability of eggs. Anim Feed Sci Technol 1998;72:33-40.
17.	Petroviæ M, Gaèiæ M, Karaèiæ V, Gottstein Z, Mazija H, Mediæ H. Enrichment of eggs in n-3 polyunsaturated fatty acids by feeding hens with different amount of linseed oil in diet. Food Chem 2012;135:1563-8.
18.	Banjo AD, Lawal OA, Songonuga EA. The nutritional value of fourteen species of ddible iinsects in Southwestern Nigeria. Afr J Biotechnol 2006;5:298-301.
19.	Villamor E, Koulinska IN, Furtado J, Baylin A, Aboud S, Manji K, et al. Long-chain n-6 polyunsaturated fatty acids in breast milk decrease the risk of HIV transmission through breastfeeding. Am J Clin Nutr 2007;86:682-9. [PUBMED]
20.	Folch J, Lees M, Sloane Stanley GH. A simple method for the isolation and purification of total lipides from animal tissues. J Biol Chem 1957;226:497-509. [PUBMED]
21.	Yamasaki M, Mansho K, Mishima H, Kasai M, Sugano M, Tachibana H, et al. Dietary effect of conjugated linoleic acid on lipid levels in white adipose tissue of Sprague-Dawley rats. Biosci Biotechnol Biochem 1999;63:1104-6. [PUBMED]
22.	Gebauer SK, Psota TL, Kris-Etherton PM. The diversity of health effects of individual trans fatty acid isomers. Lipids 2007;42:787-99. [PUBMED]
23.	Glew RH, Hauwa MSO, Huang YS, Chuang LT, VanderJagt DJ. Fatty acid content of the smoked, fresh-water fish Clairas gariepinus (Wanka Harwada, Hausa) in Northern Nigeria. Highland Med Res J 2004;2:8-13.
24.	Simopoulos AP, Leaf A, Salem N Jr. Workshop on the essentiality of and recommended dietary intakes for omega-6 and omega-3 fatty acids. J Am Coll Nutr 1999;18:487-9. [PUBMED]
25.	Bergmann RL, Haschke-Becher E, Klassen-Wigger P, Bergmann KE, Richter R, Dudenhausen JW, et al. Supplementation with 200 mg/day docosahexaenoic acid from mid-pregnancy through lactation improves the docosahexaenoic acid status of mothers with a habitually low fish intake and of their infants. Ann Nutr Metab 2008;52:157-66. [PUBMED]
26.	Makrides M, Hawkes JS, Neumann MA, Gibson RA. Nutritional effect of including egg yolk in the weaning diet of breast-fed and formula-fed infants: A randomized controlled trial. Am J Clin Nutr 2002;75:1084-92. [PUBMED]
27.	Hoffman DR, Theuer RC, Castañeda YS, Wheaton DH, Bosworth RG, O›Connor AR, et al. Maturation of visual acuity is accelerated in breast-fed term infants fed baby food containing DHA-enriched egg yolk. J Nutr 2004;134:2307-13.
28.	Hwangbo J, Kim JH, Lee BS, Kang SW, Chang J, Bae HD, et al. Increasing content of healthy fatty acids by cheese byproduct. Asian-Aust J Anim Sci 2006;19:444-9.
29.	Suksombat W, Samitayotin S, Lounglawan P. Effects of conjugated linoleic acid supplementation in layer diet on fatty acid compositions of egg yolk and layer performances. Poult Sci 2006;85:1603-9. [PUBMED]
30.	Hendrickse RG. Some observations on the social background to malnutrition in Tropical Africa. Afr Aff 1966;65:342-9.
31.	Konczacki Z. Infant malnutrition in Sub-Saharan Africa: A problem in socioeconomic development. Can J Afr Stud 1972;6:433-49.
32.	Glew RH, Wold RS, Herbein JH, Wark WA, Martinez MA, Vanderjagt DJ. Low docosahexaenoic acid in the diet and milk of women in New Mexico. J Am Diet Assoc 2008;108:1693-9.