From dietary fats to vitamins and minerals, researchers mapped how a mother’s nutritional intake relates to the complex fatty acid composition of her milk.
Maternal diet and fatty acids in human milk: results of the MELK study. Image Credit: Pixel-Shot / Shutterstock
A recent cross-sectional study published in the European Journal of Nutrition found that higher maternal intakes of omega-3 and omega-6 polyunsaturated fatty acids (PUFAs) and trans fatty acids were associated with higher proportions of the corresponding fatty acids in breast milk.
Background
Breastfeeding is widely promoted worldwide as safe, providing complete nutrition, being available on demand, and inexpensive. Exclusive breastfeeding is recommended for the first six months of life, with breastfeeding continuing alongside complementary foods for up to two years.
Prior research has shown that the fatty acid composition of human milk is associated with infant health and development. Saturated fatty acids (SFAs) constitute a significant portion of the fat in human milk, providing dietary energy for the infant. These include long-chain SFAs, such as palmitic acid esterified at the sn-2 position, which promote fat and calcium absorption in the form found in breast milk.
It also contains short-chain fatty acids (SCFAs) like butyrate, formic acid, and acetate, which are negatively associated with excessive fat deposition in infancy.
Unsaturated fatty acids in breast milk include monounsaturated fatty acids (MUFAs), of which oleic acid is the most abundant. They constitute a major component of fatty acids in human milk, serving as a major energy source.
PUFAs have important developmental roles, with DHA linked to visual and neurocognitive development, ARA to growth and brain development, and PUFAs more broadly to immune function.
Human milk composition
Human milk contains about 3.8-3.9 g/100 mL fat, providing about half of the infant’s daily energy requirement. Carbohydrates and proteins account for the rest.
Fatty acids in milk are both produced in situ by mammary gland cells and absorbed from the diet. De novo mammary synthesis contributes a large part of SFAs, especially short- and medium-chain SFAs, from non-fats. Long-chain fatty acids are mostly absorbed from the diet or transferred from the mother’s fat stores.
Sources of fatty acids
Some PUFAs like linoleic acid (LA) and alpha-linoleic acid (ALA) are never synthesized in the body, while ARA can be synthesized from LA, and EPA and DHA from ALA, although only in small amounts. Most available EPA and DHA come from fish and seafood, with poultry and eggs also contributing in smaller amounts.
Thus, essential fatty acids primarily originate from the maternal diet or maternal fat reserves.
ALA comes mostly from soybean and canola oils, flaxseed oil, and certain nuts.
Maternal diet and milk fatty acids
Previous studies suggest that maternal fish intake is associated with higher levels of ALA, DHA, and EPA in human milk. Increased maternal PUFA consumption is associated with PUFA levels in human milk, while higher maternal SFA intake has been negatively correlated with milk PUFAs.
Fats in human milk depend on multiple sources, including de novo synthesis, uptake of fatty acids from the plasma, and triglyceride assembly and secretion. These processes are supported by micronutrients involved in cellular metabolism, enzymatic reactions, gene regulation, and protection against oxidative damage.
Maternal nutritional status may therefore influence the quantity and composition of milk lipids. However, apart from PUFAs, there is little data on how maternal dietary intake is linked to milk fatty acid composition.
The current cross-sectional Dutch study thus examines human milk composition in relation to maternal dietary intake, using strict sampling protocols to account for the dynamic fatty acid content of human milk.
Study characteristics
The study included 109 mother-infant pairs living in the Netherlands. Mothers were enrolled at approximately 6-8 weeks postpartum, with healthy term infants weighing at least 2.5 kg and mothers with a pre-pregnancy body mass index (BMI) of 18.5-24.9 kg/m2 who were exclusively breastfeeding.
The mean maternal age was 32 years, and the mean maternal body mass index was approximately 22 kg/m2. All were highly educated women.
The mean infant birthweight was about 3.6 kg, the average birth length was 50.3 cm, and the mean gestational age at birth was 40 weeks.
Over the four-week study period, two milk samples were collected between 6:00 and 8:00 am, with a 20 mL aliquot taken from each full expression, and one 24-hour urine sample was collected.
Demographic and lifestyle data were collected, along with four 24-hour food records. The researchers also collected anthropometric data for both mothers and infants, as well as infant growth and development data. Analyses were adjusted for maternal age, pre-pregnancy BMI, and parity, with correction for multiple statistical comparisons.
Milk fatty acid characteristics
On average, the milk contained about 4 g of total fat per 100 mL, with MUFAs accounting for 43.6%. In particular, oleic acid accounted for 38.6% of total fatty acids.
Omega-3 fatty acids accounted for 1.85% (with ALA predominant at 1.25%), compared to 14.2% for omega-6 fatty acids (mostly LA at 13% of total fatty acids).
SFAs accounted for almost 40% of total fatty acids, palmitic acid being the single largest component at 20.4%.
The women had a mean daily energy intake of 2277 kcal, with a mean fat intake of 97 g/day. SFAs, PUFAs, and MUFAs made up 13.2%, 7.7%, and 14.2% of the energy intake, respectively. Of PUFAs, omega-3 and omega-6 fatty acids contributed 0.9% and 6.6% of total energy, respectively.
Fish intake, assessed by the food frequency questionnaire (FFQ), and DHD-P scores were low, with a mean DHD-P score of 93.5/190.
Association with fatty acids in milk
Higher maternal intakes of omega-3 and omega-6 PUFAs were associated with higher proportions of the corresponding fatty acid classes and total PUFAs in milk. Conversely, both omega-3 and omega-6 dietary intake were associated with lower proportions of SFAs and trans fatty acids in milk.
Higher omega-3 PUFA intake was associated with greater proportions of the omega-3 PUFAs ALA, DHA, and EPA, and the omega-6 PUFA LA, in milk. Higher omega-6 PUFAs in the diet were associated with increased LA and ALA proportions. Moreover, DHA and EPA intake was reflected in their proportions in milk.
MUFAs in the diet were not associated with MUFA proportions in milk.
SFA intake was not associated with total SFAs in milk, but it did show a positive association with individual SFAs such as stearic and arachidic acids. Trans fat intake was associated with a higher proportion of trans fatty acids in milk.
Dietary fiber was also associated with higher levels of total and omega-6 PUFAs in milk, but with lower levels of total SFAs. Increased fiber was linked to higher levels of LA and several medium-chain fatty acids, but inversely associated with palmitic and stearic acids.
Higher carbohydrate intake was linked to higher lauric acid but lower proportions of trans-vaccenic and elaidic acids.
Higher diet quality showed a weak association with milk fatty acids, including omega-3, omega-6, and total PUFAs, but lower trans fatty acids.
Beyond dietary fats and other macronutrients, several micronutrients were also associated with milk fatty acid composition. Higher intakes of some micronutrients were associated with greater PUFA proportions, while vitamins B1 and E and copper were associated with lower SFA and trans fatty acid proportions.
These associations suggest that micronutrient intake may be linked to milk fatty acid composition, although whether the relationships are direct remains uncertain.
Limitations
The authors suggest that a larger sample may have been necessary to detect weaker associations with some dietary components. The sample comprised highly educated women with healthy lifestyles, limiting the generalizability. The effect of a healthy diet may be especially difficult to distinguish from that of isolated nutrients.
Both milk samples were collected during the same morning time window rather than across a 24-hour period, though milk composition is dynamic. Differences in sample handling and homogenization might have led to variability in the samples after collection. Maternal blood fatty acid concentrations were not available.
The fatty acid findings also report proportions of total milk fatty acids rather than absolute concentrations. The DHD-P index was developed for pregnancy rather than lactation, and dietary biomarkers indicated some over-reporting of nutrient intake.
The study was funded by Ausnutria B.V. Two authors were employed by the company, while two others reported receiving research support from Ausnutria B.V. and Regiodeal FoodValley.
Conclusions
The study supports prior research indicating associations between maternal dietary intake and fatty acids in human milk. However, the nature of the relationship between micronutrient intake and milk fatty acid proportions remains unclear. Alternatively, micronutrient intake might be a marker of dietary pattern that itself affects the fatty acid composition of milk.
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