Fatty acid composition of the milk lipids of Fulani women and the serum phospholipids of their exclusively breast-fed infants

Fatty acid composition of the milk lipids of Fulani women and the serum phospholipids of their exclusively breast-fed infants

Early Human Development 60 (2000) 73–87 www.elsevier.com / locate / earlhumdev Fatty acid composition of the milk lipids of Fulani women and the seru...

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Early Human Development 60 (2000) 73–87 www.elsevier.com / locate / earlhumdev

Fatty acid composition of the milk lipids of Fulani women and the serum phospholipids of their exclusively breast-fed infants Dorothy J. VanderJagt PhD a , Christopher D. Arndt BS a , Seline N. Okolo MD b , Y.-S. Huang PhD c , Lu-Te Chuang MS c , a, Robert H. Glew PhD * a

Department of Biochemistry and Molecular Biology, School of Medicine, University of New Mexico Health Sciences Center, Room 249, BMSB, Albuquerque, NM 87131 -5221, USA b Department of Paediatrics, Jos University Teaching Hospital, Jos, Nigeria c Ross Products Division, Abbott Laboratories, Columbus, OH, USA Accepted 31 July 2000

Abstract We previously reported that, relative to milk of women elsewhere in the world, the lipid fraction of milk of Fulani women in northern Nigeria contained relatively low proportions of a-linolenic acid and docosahexaenoic acid (DHA). This led us to question the essential fatty acid status of Fulani infants and the relation between the proportion of critical n-3 and n-6 fatty acids in the serum phospholipids of the mothers, their milk, and the serum phospholipids of their exclusively breast-fed infants. We were also interested in the effect de novo intermediate chain length-fatty acids (C10–C14) had on the proportions of critical and non-essential fatty acids in milk. Capillary gas–liquid chromatography was used to analyze the fatty acid content of the total milk lipids of 34 Fulani women, as well as the fatty acid content of serum phospholipids of the women and their breast-fed infants during the first 6 months of life. The proportions of critical n-3 and n-6 fatty acids in the milk of the Fulani women were adequate, but the proportions of these same fatty acids were low in their exclusively breast-fed infants. The serum phospholipids of the infants contained 18.8% linoleic acid, 0.13% a-linolenic acid, 12.8% arachidonic acid, and 3.40% DHA, whereas, the mean percentages of linoleic, alinolenic, arachidonic and DHA in the serum phospholipids of the Fulani mothers’ were 21.4, 0.20, 9.79, and 1.97, respectively. There was a strong positive correlation between fatty acid *Corresponding author. Tel.: 1 1-505-272-2362; fax: 1 1-505-272-6587. E-mail address: [email protected] (R.H. Glew). 0378-3782 / 00 / $ – see front matter  2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S0378-3782( 00 )00111-0

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content of serum phospholipids of Fulani women and the fatty acid content of their milk lipids. As the proportion of C10–C14 fatty acids in the milk lipids increased, the proportions of critical n-3 and n-6 fatty acids in milk remained relatively constant; however, proportions of three non-essential fatty acids decreased dramatically. C10–C14 fatty acids do not appear to displace critical n-3 and n-6 fatty acids in milk.  2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: a-Linolenic acid; Linoleic acid; Intermediate chain-length fatty acids; Breast milk; Serum phospholipids; Fulani; Nigeria

1. Introduction The Fulani are semi-pastoral nomads of Africa’s western Sahel who spend much of the year migrating in search of pasture for their cattle. In 1999, we reported on the fatty acid composition and content of vitamins A and E, b-carotene, and antioxidants in the milk of Fulani women in northern Nigeria [1,2]. We found that, relative to the milk of women elsewhere in the world, the lipid fraction of the milk of the Fulani women contained relatively low proportions of a-linolenic acid and docosahexaenoic acid (DHA). The two essential fatty acids (linoleic acid and a-linolenic acid) and the critical n-6 and n-3 fatty acids, arachidonic acid and DHA, respectively, are important for the growth and development of the infant’s retina and central nervous system, especially during the first 6 months of life [3–8]. We were therefore interested in the essential fatty acid status of infants who were being exclusively breast-fed by these women. It is widely accepted that the proportions of fatty acids in the serum phospholipids of an individual accurately reflect the fatty acid composition of the complex lipids in their tissues [9]. In the present study, which was conducted in the hamlet of Vwang in the Jos Plateau of northern Nigeria, we collected blood serum from 34 Fulani women and their exclusively breast-fed infants and, simultaneously, milk from the mothers. Herein, we report on the fatty acid status of the breast-fed Fulani infants and describe the relations we observed between the proportions of critical n-3 and n-6 fatty acids in the serum phospholipids of the mothers, their milk, and the serum phospholipids of the exclusively breast-fed infants they were nursing. In addition, we investigated the effect increasing the proportion of de novo intermediate chain length fatty acids (C10–C14) had on the essential fatty acid content of human milk. In a previous study that involved women from the Yoruba, Hausa, and urban Fulani we found that intermediate chain-length fatty acids spared n-3 and n-6 polyunsaturated fatty acids in the lipid fraction of human milk [10]. High proportions of de novo fatty acids in milk appear to be teleologically important for infant survival by providing a high-energy source that is easily digested and readily oxidized by the neonate. In the present study we sought to extend this finding to a homogenous population of rural Fulani.

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2. Materials and methods

2.1. Study population The study population was recruited from the Fulani hamlet of Vwang, which is located about 50 km from Jos, Nigeria. Thirty-four mother / infant pairs of pure Fulani ethnicity were selected for this study. This study was approved by the Ethics Review Board of the Jos University Teaching Hospital.

2.2. Study procedure The weights and heights were obtained for the mothers, and the ages, weights, and recumbent lengths of the infants were obtained. Milk was collected from mothers who were between 11 days and 6 months postpartum. Approximately 10 ml of milk was obtained from each mother (between 07:00 and 09:00 h) by manual expression into sterile, dry plastic containers midway through nursing. Aliquots were transferred to cryovials and stored at 2 208C until they were transported on dry ice to the United States for analysis. Blood from mothers and infants was drawn into 10 ml vacutainers devoid of anticoagulant and allowed to stand at room temperature for 45 min. After centrifugation the serum fraction was aliquoted into 2 ml cryovials and stored at 2 208C.

2.3. Fatty acid analysis Milk samples were thawed, warmed to 378C and vortexed vigorously before analysis. Prior to extraction and methylation, the samples were flushed with nitrogen to minimize oxidation. Lipids were extracted using the method described by Folch et al. [9]. The phospholipid fraction of serum was separated by thin layer chromatography using hexane / diethyl ether / acetic acid (80:20:1, v / v). Boron trifluoride– methanol (14%, w / v) was used for transesterification of the total lipid fractions and the fatty acid methyl esters were extracted using hexane [11]. Fatty acids were quantified using a Hewlett-Packard gas chromatograph (5890 Series II; Mississauga, Ontario) equipped with a flame-ionization detector and a 50-m fused-silica capillary column coated with 0.25 mm Supelcowax 10 (Supelco Inc., Bellefonte, PA) [12]. The injector and detector temperatures were 2308C. The detector response factors for different fatty acids were calculated by analyzing known quantities of authentic standards (Nu-Chek-Prep, Inc., Elysian, MN, USA and Supelco, Inc., Bellefonte, PA). The fatty acid areas in each chromatogram were integrated automatically by computer. Results are expressed as g / 100 g total fatty acid.

2.4. Statistical analysis Group comparisons were made using a two sample t-test. The t-tests and regression

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analyses were performed using NCSS 6.0 (Kaysville, Utah). A P value of 0.05 was considered significant.

3. Results

3.1. Comments on the study population Table 1 contains a summary of the anthropometric characteristics of the 34 Fulani mothers and infants who participated in this study. The mothers ranged in age from 15 to 45 years and their parity ranged from 1 to 10. Body mass index (BMI) varied from 16 to 24. Infant age varied from 11 days to 6 months. There was no correlation between BMI of the mothers and their age or parity.

3.2. The fatty acid composition of total milk lipids of Fulani women The mean proportions of linoleic, a-linolenic, arachidonic acid and DHA in the milk were 13.5, 0.66, 0.51 and 0.20 g / 100 g total fatty acid, respectively (Table 2). These values are comparable to those for milk lipids of women from various countries around the globe (Table 3), and all four values are higher (20–35%) than the values we reported previously for lactating Fulani women in Nigeria [1]. The contribution de novo fatty acids made to the fatty acid total in the milk of the Fulani women was 22%, a value that is considerably lower than that seen in our previous studies of Fulani (36%) and Yoruba (43%) populations in Nigeria [1,14]. Overall, these data indicate that the milk of the women in the present study was not deficient in either of the essential fatty acids or the critical n-3 (e.g. DHA) or n-6 (e.g. arachidonic acid) fatty acids derived from the essential fatty acids. No significant correlations were observed between the proportions of linoleic acid, a-linolenic acid, arachidonic acid, DHA in milk and the age, or parity of the mother. There were no correlations between the proportions of these four fatty acids in milk and the BMI of all mothers in the study; however, a correlation was observed for both arachidonic acid and DHA in milk versus BMI for those whose BMI was less than or equal to 22. The proportion of arachidonic acid and DHA increased linearly with BMI within this subgroup (P 5 0.036 and 0.042, respectively). The percentage of linoleic acid, a-linoleic acid, arachidonic acid, and de novo fatty acids did not Table 1 Summary of the anthropomorphic characteristics of the Fulani women and their infants (x¯ 6S.D.) Parameter

Mothers (n 5 34)

Infants (n 5 32)

Age (years or weeks) Weight (kg) Height (m or cm) BMI (kg / m 2 ) Parity

26.067.5 49.367.0 1.6060.07 19.462.4 4.362.8

11.467.6 4.761.6 56.965.5 14.063.7 –

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Table 2 The fatty acid composition (wt%) of total milk lipids of Fulani women (x¯ 6S.D.) Fatty acid

All subjects (n 5 34)

Saturated C10:0 C12:0 C14:0

0.2860.10 9.1063.34 12.565.51

Total de novo C15:0 C16:0 C18:0 C20:0 C22:0 C24:0

21.868.44 0.3360.12 24.263.09 4.4561.09 0.1660.07 0.1060.06 0.1060.06

Monounsaturated C16:1 C18:1n9 C20:1 C22:1 C24:1n9

2.2361.05 29.165.01 0.4460.17 0.0860.02 0.0760.02

Polyunsaturated (n-6) C18:2 C18:3 C20:2 C20:3 C20:4 C22:4 C22:5 (n-3) C18:3 C20:5 C22:5 C22:6

13.563.16 0.1460.04 0.3660.07 0.4960.11 0.5160.08 0.1760.06 0.0860.03 0.6660.27 0.0660.03 0.1160.03 0.2060.08

correlate with the length of lactation. However, the proportion of DHA was significantly correlated with length of lactation and decreased from 0.26 to 0.13% between 11 days and 25 weeks postpartum.

3.3. The fatty acid composition of the serum phospholipids of the Fulani mothers Lipoproteins in the mother’s blood represent a source of essential fatty acids that can be passed on to the baby via breast milk. The mean percentages of linoleic, arachidonic, a-linolenic and DHA in the serum phospholipids of the Fulani mothers’ were 21.4, 9.79, 0.20, and 1.97, respectively (Table 4). The fatty acid compositions of the serum phospholipids of Fulani and the Hausa mothers we previously studied were similar with the exception of DHA. The mean

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Table 3 Comparison of fatty acid compositions of breast milk from different parts of the world Fatty acid

Jos [13] Nigeria (n 5 43)

Nigeria [14] (Yoruba) (n 5 13)

Decanoic C10:0

0.28 (0.10)

1.70 (0.64)

2.1

Lauric C12:0

9.10 (3.34)

10.11 (4.12)

31.4 (4.2)

Nigeria [1] (Fulani) (n 5 18) 3.92 (1.62)

Malaysia [17] (Indian) (n 5 10)

Khon-kaen [18] Thailand (n 5 57)

18.4 (7.82)

6.99 (0.59)

7.4 (6.7)

8.44 (4.1)

4.4 (1.5)

4.92 (1.47)

5.18

5.68 (2.01)

8.80 (0.85)

15.9 (7.6)

8.90 (4.05)

5.9 (1.8)

5.25 (1.64)

6.49

6.10 (1.73)

24.2 (3.09)

21.68 (3.87)

20.1 (2.2)

21.5 (3.75)

5.65 (2.29)

3.4 (0.4)

3.80 (1.90)

14.1 (0.5)

3.94 (0.11)

21.8 (4.9)

6.8 (2.8)

25.8 (3.28)

4.96 (0.71)

24.3 (2.5)

3.53 (0.71)

Oleic C18:1n-9

29.1 (5.01)

28.82 (5.58)

20.8 (1.7)

21.5 (5.25)

47.0 (1.5)

23.9 (6.0)

30.7 (5.14)

Linoleic C18:2n-6

13.5 (3.16)

10.58 (2.35)

8.2 (0.6)

10.0 (2.13)

13.01 (0.3)

15.7 (3.9)

10.71 (4.66)

6.55 (2.77)

Arachidonic C20:4n-6

0.51 (0.08)

0.52 (0.12)

Docosapentaenoic, C22:5n-3 Docosahexaenoic, C22:6n-3

0.11 (0.03)

0.13 (0.06)

0.20 (0.08)

0.32 (0.17)

a

0.6 (0.4)

, 0.1 0.3 (0.1)

(n 5 198)

0.95

Palmitic C16:0

, 0.1

(n 5 38)

1.19 (0.37)

13.9 (4.77)

0.41 (0.16)

Canada [21]

1.1 (0.3)

9.0 (1.5)

0.66 (0.27)

Germany [20]

1.22 (0.68)

12.46 (5.45)

a-Linolenic C18:3n-3

China [19] (Rural) (n 5 16)

1.30 (1.1)

12.5 (5.51)

4.45 (1.09)

(n 5 23)

South [16] Africa (n 5 18)

0.92 (0.10)

Myristic C14:0

Stearic C18:0

Gambia [15]

28.5 (5.45)

18.9 (3.24)

5.38 (1.01)

22.6

7.42

NR

18.30 (2.25)

6.15 (0.97)

30.6 (4.21)

32.1

30.6 (2.66)

18.4 (5.84)

10.3

10.5 (2.62)

0.50 (0.16)

0.84 (0.20)

0.1 (0.1)

0.33 (0.20)

0.42 (0.24)

2.71 (0.60)

0.81

1.16 (0.37)

0.42 (0.22)

0.08 (0.01)

1.0 (1.4)

0.13 (0.06)

0.26 (0.12)

0.80 (0.14)

0.59

0.35 (0.11)

0.07 (0.04)

0.20 (0.05)

0.1 (0.2)

0.19 (0.12)

NR

0.15 (0.08)

0.39 (0.28)

0.1 (0.2)

0.90 (0.36)

NR

This report, Table 2, left-hand column. The numbers in parentheses indicate 1 S.D. NR, not reported.

NR 0.68 (0.29)

0.18 0.39

NR 0.14 (0.10)

D. J. VanderJagt et al. / Early Human Development 60 (2000) 73 – 87

Jos [this study] Nigeria (Fulani) (n 5 34)a

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Table 4 Comparison of fatty acid composition (wt%) of serum phospholipids of the Fulani mothers and Hausa mothers of Jos, Nigeria (x¯ 6S.D.) Fatty acid

Fulani (this study) (n 5 34)

Hausa a (n 5 15)

Saturated C14:0 C15:0 C16:0 C18:0 C20:0 C22:0 C24:0

0.2460.06 0.2160.06 30.061.44 14.861.34 0.1360.03 0.2060.05 0.1960.05

0.2560.08 0.1560.06 27.261.29 15.661.09 0.1460.03 0.1460.07 0.1360.11

Monounsaturated C16:1 C18:1n9 C20:1 C22:1 C24:1n9

0.6960.32 13.061.96 0.2060.06 0.1960.08 0.2460.06

0.4660.18 12.661.39 0.2560.09 , 0.03 , 0.03

21.462.64 0.1260.04 0.4660.19 3.0960.71 9.7961.40 0.7260.17 0.8360.28

18.362.19 0.0860.06 0.5060.16 3.7960.59 11.061.56 0.4860.10 0.4760.14

0.2060.05 0.4060.17 0.8460.17 1.9760.74

0.1360.06 0.8060.43 1.0360.31 4.4261.96

Polyunsaturated (n-6) C18:2 C18:3 C20:2 C20:3 C20:4 C22:4 C22:5 (n-3) C18:3 C20:5 C22:5 C22:6 a

The participants in this study (see Ref. [12]) were members of the Hausa ethnic group.

DHA proportion in the serum phospholipids of the Fulani women in this study was less than half the values previously reported for Hausa women (1.97 vs. 4.42%). There was no statistically significant correlation between the proportions of the essential or critical n-3 and n-6 fatty acids in the maternal serum phospholipid fraction and BMI. One of the central aims of this study was to investigate the relation between the fatty acid status of Fulani mothers and that of their infants whose sole dietary intake was breast milk. In considering the issue of the efficiency of transfer of essential fatty acids from mother to baby, we deemed it useful to test the correlation between the proportions of particular fatty acids in the mothers’ serum phospholipids and the proportions of these critical n-3 and n-6 fatty acid fractions in the milk they were producing. As shown in Table 5, there were strong positive correlations between the

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Table 5 Correlation parameters for relationships between the fatty acid proportions in maternal serum phospholipids and milk lipids, and de novo fatty acids (C10–C14) and specific fatty acids in milk Variables Maternal serum phospholipids

Milk lipids

Linoleic acid a-Linolenic acid Arachidonic acid DHA

Linoleic acid a-Linolenic acid Arachidonic acid DHA

Milk lipids

Milk lipids

Total Total Total Total Total Total a

de de de de de de

novo novo novo novo novo novo

fatty fatty fatty fatty fatty fatty

acids acids acids acids acids acids

Palmitic acid Stearic acid Linoleic acid a-Linolenic acid Arachidonic acid DHA

r

P

0.64 0.51 0.75 0.93

, 0.001 0.002 , 0.001 , 0.001

0.58 0.77 0.25 0.06 0.29 0.28

, 0.001 , 0.001 NS a NS NS NS

NS, P . 0.05.

proportions of linoleic acid, a-linolenic acid, arachidonic acid, and DHA in mothers’ serum phospholipids and the proportion of these same fatty acids in their breast milk.

3.4. The fatty acid composition of the serum phospholipids of the exclusively breast-fed Fulani infants In an effort to inquire into the relation between the fatty acid composition of the milk lipids of the Fulani mothers and the fatty acid status of their exclusively breast-fed infants, we determined the fatty acid composition of the phospholipid fraction of the sera of the nursing infants. Among the infants in our study, the mean percentages of linoleic, arachidonic, a-linolenic and DHA in their serum phospholipids were 18.8, 12.8, 0.13, and 3.40, respectively (Table 6). For comparison purposes, the fatty acid composition of the serum phospholipids of infants in other populations are also presented in Table 6. The mean proportions of linoleic acid and a-linolenic acid were lower than those reported for 3-month-old breast-fed infants in Japan [22], but higher than those of Hausa infants in Jos, Nigeria [13]. In contrast, the proportion of arachidonic acid in the phospholipid fraction of the Fulani infants’ blood (12.8%) was more than twice that of the Japanese comparison group (5.75%), and about the same as that of infants in the United States (12.5%) [23]. The proportion of DHA in the serum phospholipids of the Fulani infants (3.40%) was comparable to that of Japanese infants (3.49%) and higher than that of infants in the United States (1.86%). Proportions of linoleic acid and arachidonic acid in the serum phospholipids of the Fulani infants were comparable to those of the Hausa infants we studied previously in Jos, Nigeria [13]. The proportion of three of the four fatty acids (linoleic, a-linolenic and arachidonic acid) in the phospholipids of the mothers’ and infants’ blood serum were similar to

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Table 6 Fatty acid composition (wt%) of serum phospholipids of the exclusively breast-fed Fulani infants compared to infants of Japan, United States and Jos, Nigeria (x¯ 6S.D.) Fatty acid

Fulani (this study) (n 5 32)a

Japan [21] (n 5 13)

United States [22] (NR)

Jos, Nigeria [12] (Hausa) (n 5 15)

Saturated C14:0 C15:0 C16:0 C18:0 C20:0 C22:0 C24:0

0.4360.14 0.1560.05 26.861.41 16.561.20 0.2760.05 0.4660.08 0.3260.07

NR NR 26.0360.56 10.8060.41 NR NR NR

NR NR NR NR NR NR NR

0.1060.27 , 0.03 29.863.04 19.462.53 , 0.03 , 0.03 , 0.03

Monounsaturated C16:1 C18:1n9 C20:1 C22:1 C24:1n9

0.3860.10 11.061.84 0.2860.08 0.0560.02 0.8760.18

NR 20.0162.26 NR NR NR

NR 11.663.4 NR NR NR

0.0860.22 11.761.83 0.0360.13 , 0.03 , 0.03

18.863.06 0.0560.02 0.7260.20 3.5760.72 12.862.30 0.7260.15 0.9760.36

25.1261.86 NR NR NR 5.7360.20 NR NR

21.666.7 0.2160.27 0.5461.14 3.761.4 12.563.8 1.8761.01 0.7360.79

18.161.78 , 0.03 0.3260.36 2.8961.50 10.663.33 0.1060.28 0.1660.34

0.1360.05 0.2660.13 0.8160.21 3.4060.77

0.4060.09 1.0560.34 NR 3.4960.40

0.2160.19 0.9660.95 0.6260.56 1.8661.4

, 0.03 0.0660.22 0.5260.58 5.6864.75

Polyunsaturated (n-6) C18:2 C18:3 C20:2 C20:3 C20:4 C22:4 C22:5 (n-3) C18:3 C20:5 C22:5 C22:6 a

There were insufficient volume of serum available from two of the infants to support fatty acid analysis. NR, not reported.

one another; however, the mean DHA percentage of the infants’ serum phospholipids (Table 6) was 73% higher than that of their mother’s serum phospholipid fraction (Table 3).

3.5. Relation between fatty acid composition of total milk lipids and fatty acid composition of infant serum phospholipids We next examined the relation between the proportion of selected fatty acids in the mothers’ milk and the proportion of the same fatty acids in the serum phospholipids of the infants. A statistically significant correlation was noted between the proportions of linoleic acid (P 5 0.02) and DHA (P 5 0.04) in the milk lipids of the mothers and the proportions of these same fatty acids in the serum phospholipids of their nursing

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Fig. 1. The relation between the proportion of total de novo fatty acid in milk (wt%) and the proportion of oleic acid and a-linolenic acid in milk (wt%). Panel A: y 5 2 0.543x 1 41.0; r50.91, P,0.001; panel B: y 5 (2.05E 2 03)x 1 0.613; r50.063, P50.723.

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infants. However, no such statistically significant correlation was found for either a-linolenic acid or arachidonic acid.

3.6. The effect of the proportion of total de novo fatty acids milk on essential fatty acid percentages in milk The effect of increasing proportion of de novo fatty acids on the proportion of a non-essential fatty acids (oleic acid, P , 0.001) and an essential fatty acid (alinolenic acid, P 5 0.72) in the milk lipids of the Fulani women is shown in Fig. 1. The weight percentages of the three selected non-essential fatty acids (palmitic acid, stearic acid and oleic acid) decreased, on average, 40% as the weight percent of de novo fatty acids in the milk lipids increased from 10 to 40% (Table 5). The proportion of linoleic acid, a-linolenic acid, arachidonic acid, and DHA in the milk lipid fraction were not correlated with the proportion of C10–C14 de novo fatty acids in the milk lipids of the Fulani mothers (Table 5).

3.7. The effect of total de novo fatty acid proportion in total milk lipids on fatty acid composition of infant serum phospholipids Finally, after investigating the effect of total de novo fatty acid proportion on non-essential and critical n-3 and n-6 fatty acid fractions in milk, we sought to determine if there was any relation between total de novo fatty acid proportion in milk lipids and the fatty acid composition of a nursing infant’s serum phospholipids. We reason that a high proportion of de novo fatty acid could only be beneficial for infant health if it did not have a negative impact on the proportion of essential fatty acids in the serum phospholipid fraction of the infant. As hypothesized, there was no correlation between the proportions of de novo fatty acid in milk and the proportions of linoleic acid, a-linolenic acid, arachidonic acid or DHA in the serum phospholipids of the infants.

4. Discussion

4.1. Goals of the study Our motivation for undertaking this study had two components: first, there was the practical issue of the essential fatty acid status of exclusively breast-fed Fulani infants, a concern that was raised by our previous finding of relatively low proportions of linoleic acid and DHA in the milk of Fulani women [1]; second, we were interested in the relations between the proportions of critical n-3 and n-6 fatty acids in the serum phospholipids of the mothers and the milk they were producing, and between the fatty acid composition of the milk lipids and the serum phospholipids of their exclusively breast-fed infants.

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4.2. Infant fatty acid status With regard to the practical question of the fatty acid nutrition of Fulani infants, our main finding was that the serum phospholipids of these exclusively breast-fed, 2-week to 6-month-old infants contained relatively low proportions of the two essential fatty acids, namely linoleic acid and a-linolenic acid: the proportion of linoleic acid in the serum phospholipid fraction of the Fulani infants (Table 6) was about 25% less than that reported for Japanese infants [22] and about 15% less than that of infants in the United States [23]. As for a-linolenic acid, the other essential fatty acid, it comprised only 0.13% of the fatty acids in the phospholipid fraction of the sera of the Fulani infants, a value that is 30–65% below the corresponding value reported for infants in the United States (0.21%) and Japan (0.40%) (Table 6). Because the milk of the Fulani mothers who were nursing these infants contained proportions of linoleic acid and a-linolenic acid that are well within the range of values reported for women in other countries around the world (Table 3), the relatively low representation of these same two essential fatty acids in the serum phospholipids of the Fulani infants in this study cannot be attributed to deficiencies of these fatty acids in their mothers’ milk. However, in light of our finding of relatively high proportion of arachidonic acid and DHA in the serum phospholipids of the Fulani infants (Table 6) (two critical n-6 and n-3 fatty acids that can be synthesized in humans from linoleic acid and a-linolenic acid, respectively), a more likely explanation is that the relatively low percentages of the two essential fatty acids in the infants’ phospholipids was the result of their extensive metabolic conversion to arachidonic acid and DHA by means of the desaturase / elongase pathway [24]. A second explanation is that, although the milk of the Fulani mothers contained adequate amounts of linoleic acid and a-linolenic acid, inefficient digestion and absorption of fat in the infants’ intestines resulted in a relatively low representation of these essential fatty acids in their serum phospholipids [25].

4.3. Fatty acid composition of milk lipids It is noteworthy that the overall fatty acid composition of the milk lipids of the Fulani women who participated in this study differed from that which we reported in our previous study of lactating Fulani women [1]. It is possible that differences in the overall nutritional status or body fat content of the women in this study and our previous one [1] account for the differences we observed in the fatty acid composition of the milk of the two populations of subjects. Evidence in support of this hypothesis is our observation that the proportions of arachidonic acid and DHA did correlate with BMI in those women whose BMI was less than or equal to 22.0. Since milk was collected at different times of the year in our two studies, it is more likely that significant quantitative and qualitative differences in the diets of the lactating women between the cold / dry season (November–March) and the rainy season (June– September) when our two studies were conducted account for the differences in the fatty acid composition of the milks. The milk lipid of the Fulani women in our previous study, which was conducted in February 1998, contained 36% intermediate

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chain-length (i.e. C10–C14, de novo) fatty acids. In sharp contrast, de novo fatty acids accounted for only 22% of the fatty acids in the lipid fraction of milk that was obtained from Fulani women in July 1998. The proportion of de novo fatty acids in human milk increases as the consumption of carbohydrates by the lactating woman increases [26]. It may be that the Fulani women who participated in our July 1988 study were consuming less carbohydrate than those who participated in the one we conducted in February 1998. Stocks of millet and sorghum, the staples of the Fulani, are usually more abundant in February than they are in the middle of the rainy season, and these cereals are rich in carbohydrate. It would be useful to compare the diets of Fulani women at various times during the year.

4.4. Relation between mother and milk fatty acid status The present study also yielded some interesting observations about the relation between the fatty acid composition of the mothers’ serum phospholipids and the milk lipids they were producing, and between that of their milk and serum phospholipids of their exclusively breast-fed infants. First, we found that the proportions of linoleic acid, a-linolenic acid, arachidonic acid and DHA in the serum phospholipids of the lactating women correlated strongly and positively with the percentages of these fatty acids in their milk lipids (Table 5). Such proportionality between the n-3 and n-6 fatty acid content of the mothers’ serum phospholipids and her milk lipids suggests that transfer of these critical fatty acids from blood to milk is a relatively efficient process.

4.5. Relation between milk and infant fatty acid status However, there were fewer correlations between the fatty acid composition of the milk lipids of the Fulani mothers and that of the serum phospholipids of the infants they were nursing; the correlations that were statistically significant were not as strong as they were in the case of mothers’ serum phospholipids and milk lipids. We did find, however, that the proportions of linoleic acid and DHA in the mothers’ milk correlated with the proportions of these two fatty acids in the serum phospholipids of their exclusively breast-fed infants. The correlation we found between proportions of DHA in milk lipid and infant serum phospholipid confirms data previously reported by Australian researchers [27]. On the other hand, we found no such correlations for a-linolenic acid or arachidonic acid. Since we did not determine the volume of breast milk consumed by individual infants or the fat content of the milk, we cannot rule out the possibility that the lack of correlation between the proportion of some fatty acids in milk and that of the infant’s serum phospholipids may have been due to differences in the amount of fat they consumed.

4.6. Influence of intermediate chain-length fatty acid in milk Finally, in the present study we obtained additional evidence that intermediate chain-length fatty acids may modulate the relative proportions of essential and

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non-essential fatty acids in human milk [10]. Although C10–14 fatty acids arise primarily by de novo synthesis in mammary tissue, they can also appear in human milk as a result of maternal diet. As de novo fatty acids contributed more to the lipid fraction of the milk of the Fulani women, in general, there was a larger proportional decrease in the percentages of the non-essential fatty acids, relative to the essential fatty acids, in the lipid fraction (Fig. 1 and Table 5). Although the mechanism responsible for this apparent selective incorporation of essential fatty acids into the triacylglycerols of human milk is unclear, the significance of this phenomenon is not. The preferential incorporation of essential fatty acids, arachidonic acid and DHA into milk triacylglycerol ensures that the exclusively breast-fed infant will receive an adequate supply of these critical n-3 and n-6 fatty acids during an especially vulnerable period, namely the first 6 months of life.

5. Conclusion In conclusion, this study draws attention to the relatively low proportions of certain critical n-3 and n-6 fatty acids in exclusively breast-fed Fulani infants, and raises questions about the relation between the fatty acid status of these infants and their growth and development in a region of the world that has one of the highest rates of mortality before the age of five years. Our data also raise new questions about how mammary tissue regulates the fatty acid composition of milk lipids in general and, in particular, how intermediate chain-length fatty acids influence the relative proportions of essential and non-essential fatty acids in human milk lipids.

Acknowledgements This study was supported by a grant from the Fogarty International Center of the National Institutes of Health.

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