The content of conjugated linoleic acid and vaccenic acid in the breast milk of women from Gdansk and the surrounding district, as well as in, infant formulas and follow-up formulas. nutritional recommendation for nursing women

The positional and geometrical cis and trans isomers of octadecadienoic acid (C18:2) with conjugated double bonds are collectively referred to as conjugated linoleic acids (CLAs). CLAs are the indirect hydrogenation product of polyunsaturated fatty acids (PUFA) – linoleic acid (LA, C18:2 9c, 12c) and alpha-linolenic acid (ALA, C18:3 9, 12, 15 all cis) – in the digestive tract of ruminants. The biohydrogenation process begins with LA isomerization by the enzymes produced by Butyrivibrio fibrisolvens. The produced molecule of conjugated diene of LA is converted to vaccenic acid (VA, C18:1 11-trans) and elaidic acid (EA, C18:1 9-trans). However, not all CLA molecules are converted to monoenoic FAs. Some CLAs are absorbed and transported to milk fat, intramuscular fat and adipose tissue [1]. The most prevalent CLA in ruminant meat and milk is rumenic acid (RA), a 9-cis 11-trans isomer of CLA. RA represents from 73% to 93% of total CLAs present in fat from the tissue and milk of ruminants [1, 2]. The best sources of CLA is beef meat (1.2-10.0 mg CLA/g fat), lamb meat (4.3-11.0 mg CLA/g fat) and cow’s milk fat which is characterized by the highest content of CLA, from 2 to 37 mg/g [3].

This group of fatty acids (FA) has attracted considerable interest when research studies demonstrated that CLA is one of the few chemical compounds of animal origin whose biological activity delivers health benefits. CLA has strong anticarcinogenic properties against various types of cancer in the different stages of disease progression, including breast cancer [2, 4, 5, 6, 7, 8]. Rumenic acid (RA) is the most potent anticarcinogenic compound [7, 9]. In experimental animals, CLA reduced atherosclerosis and total cholesterol levels by contributing to a healthy LDL/HDL ratio [10]. Previous studies of animals demonstrated that CLA can normalize glucose metabolism. However, the results of a recent meta-analysis evaluating the combined influence of 9c, 11t CLA and 12t, 10c CLA isomers on fasting blood sugar levels did not confirm these effects in humans [11]. CLA stimulates metabolism, increases muscle mass, reduces fat tissue and influences fat metabolism [12, 13, 14].

In the human body, CLA is derived mainly from dietary sources. 9-cis, 11-trans and 9-trans, 11-trans CLA isomers were found in human adipose tissue, which also contained trace amounts of 9-trans, 11-cis and 9-cis, 11-cis isomers [15]. Dietary supplementation with LA does not increase CLA levels in the blood plasma, which indicates that gut bacteria are unable to isomerize LA to CLA [16]. However, VA from dietary sources increases CLA levels in the blood serum, which implies that VA is desaturated to RA in the human body [17, 18]. Δ9-desaturase transforms stearic acid C18:0 and uses VA as a substrate for enzymatic conversion, probably due to similarities in the linear structure of VA molecules. Therefore, a diet that is abundant in CLA as well as VA is a good source of conjugated octadecadienoic acid [18]. CLA isomers are also present in human breast milk, from 0.3 to 1.2 g/100g fatty acids [19].

In view of the potential health benefits of CLA, the aim of this study was to determine the CLA and VA content of human breast milk from mothers consuming different diets: i.e. a conventional diet, without limiting the intake of dairy products and a diet low in dairy products, and to compare the results with CLA and VA levels in infant formulas and follow-up formulas for children that for various reasons are not fed breast milk.

Mean intakes of total energy, protein, carbohydrate, total fat, saturated, monounsaturated and polyunsaturated fatty acids were not significantly different between the two diets (Table I). Only the consumption of dairy fat was greater in the group of women eating a standard diet, without limiting dairy product intake. The average daily milk fat intake estimated in this study group was < 12.7 g.

All the samples of fat from mature breast milk contained CLA isomers. In the group of mothers who consumed a standard diet, the CLA content of milk fat ranged from 0.35% to 0.64% of total FAs (0.49 ± 0.08% on average). In the group of women who limited their consumption of dairy products on account of their children’s allergy to cow’s milk, the CLA content of milk fat ranged from 0.14% to 0.41% (0.27% on average) (Table II). In both groups, RA was the predominant CLA which accounted for 72-99% of all CLA isomers in the milk fat samples examined. The second most abundant CLA was isomer 10t, 12c which accounted for 10-19% of total CLAs.

The remaining CLA isomers (9t,11c; 9t,11t; 10c,12c) represented 6-8% of the CLAs identified in milk samples. The content of RA (MD 0.18; 95% CI: 0.1359– 0.2241, p<0.001) and total CLA isomers (MD 0.22; 95% CI: 0.1759-0.2641, p<0.001) was statistically higher in the milk of women eating a standard diet than in the milk of women consuming a diet deficient in cow’s milk fat.

The content of VA in breast milk fat was also highly correlated with the mothers’ intake of products rich in cow’s milk fat. The concentration of VA was significantly higher in the milk of women with a standard diet than in the milk of mothers who limited their intake of dairy products (0.69% vs. 0.36% of total FAs respectively, p<0,001).

The relatively strong positive correlations between the RA content of human milk fat and the content of other FAs typically found in milk fat, i.e. VA (r=0.662, p<0.05) and pentadecanoic acid PA C15:0 (r=0.754, p<0.05), provide evidence that CLA isomers in breast milk originate from diets rich in dairy products (Figure I). Similarly to CLA and VA, PA is synthesized by the ruminal microbiota in cattle. PA is not synthesized in the human body, and it is not effectively metabolized, because a PA molecule has an odd number of carbon atoms [22].

Fig. 1

Only trace amounts of two CLA isomers: RA and 10t,12c isomer, were detected in the infant formulas evaluated. RA accounted for up to 0.04% of total FAs. The analyzed formulas also contained VA (up to 0.05% of total FAs) (Table III). In 3 out of the 11 tested products, the content of CLA isomers was below the detection limit (0.01% of total FAs). The presence of CLA in IF and FF can be attributed to the fact that the formulas examined contain skimmed cow’s milk which may contain trace amounts of the analyzed FAs.

The results of this study confirm that the CLA content of human breast milk is highly correlated with the mother’s diet. In the group of women whose diets were deficient in cow’s milk and dairy products, the CLA content of breast milk accounted for less than 55% of the CLA content determined in the breast milk samples collected from women consuming dairy products.

CLA concentration in cow’s milk ranges from 2.3 to 7.2 mg/g of fat, depending on the cattle’s diet [23]. High-fat dairy products are more abundant in CLA whose content reaches 8.1-9.5 mg/g of fat in butter and 12.4-14.0 mg/g of fat in ripened cheese [3]. In the groups of respondents evaluated, the consumption of CLA isomers was determined during nutritional interviews and based on average CLA levels in various food products. Daily CLA intake was estimated at 38 mg, but not more than 60 mg, in the group of women consuming a diet deficient in dairy products, and at 115 mg, but not less than 92 mg, in the group of women eating a standard diet. According to research, breast milk is enriched with CLA around 28 hours after the consumption of foods abundant in these fatty acids [19].

For millennia, human infants have been fed mother’s milk or the milk of domesticated ruminants. For this reason, CLAs, mainly RA, as well as other milk fat components, such as cholesterol, VA and sphingomyelins, are natural ingredients in infant diets. Unfortunately, cow’s milk proteins have allergizing properties. After diagnosing a child’s allergy to cow’s milk protein, many women simply stop breast-feeding and start using a soy protein-based infant formula. Many of them just do not know that they can limit the presence of an allergy factor in breast milk by changing their diet. Women who do not consume dairy products should supplement their diets with CLA from other sources, such as butter, ruminant meat and the milk of other mammals, including goat and sheep milk, which do not cause allergic reactions. However, the milk of other ruminants contains antigens similar to those found in cow’s milk, therefore, cross-reactivity with proteins from other types of milk may occur over time.

Natural products should be the main source of CLA in the diet of nursing women, in particular products where healthy RA accounts for 73-98% of total CLAs. Another CLA isomer, fatty acid C18:2 10t,12c, which is found in substantial amounts in CLA dietary supplements, decreases the fat content of human breast milk [24,25]. There is evidence to indicate that CLA isomer 10t,12c can increase the risk of lipodystrophy and insulin resistance in humans [25]. For this reason, CLA supplements are not recommended, in particular for nursing women, until the health implications of this CLA isomer have been comprehensively researched [26].

In ruminant milk, CLA 10t,12c accounts for less than 10% of all CLA isomers. In this study, the daily consumption of CLA 10t,12c did not exceed 6 mg in women with a milk-free diet, and it was estimated at 11 mg in the group of women consuming a standard diet. CLA supplements are much more abundant in the above isomer whose content can exceed 500 mg in a daily dose. The intake of dairy fat would have to be increased to at least 700 g to supply an equivalent dose of CLA 10t,12c.

VA is also an important component of human breast milk. According to Adlof et al. [17] and Turpeinen et al. [18], VA, a trans FA that occurs naturally in cow’s milk, can be converted to RA in the human body. Naturally occurring trans FAs (VA) can be converted to CLA in the human body, which clearly differentiates them from synthetic trans FAs. The latter are found in hydrogenated fats and are harmful for health. The development of production technology has contributed to a significant reduction in trans FA levels in non-dairy spreads. However, hydrogenated fats, such as bakery shortening, still contain substantial amounts of trans FAs. Hidden fats in foods, including confectionary products, bakery shortening, snacks, pastry and fast foods, are the main source of synthetic trans FAs in the human diet [27].

Infant formulas have been used in human nutrition for a relatively short time (3 to 4 generations). The composition of IFs should resemble breast milk as closely as possible, and all the differences should be minimized. Despite the above, recent industrial practices seem to deliver the opposite effect. Milk fat in infant formulas is being completely replaced with vegetable oil. In this study, cow’s milk fat was not found in any of the analyzed IF and FF, which therefore contained only trace amounts of CLA. Vegetable oils, including palm, rapeseed, coconut, sunflower and single cell (algal) oils, are the main sources of fat in IF and FF. Nutritional guidelines relating to daily CLA intake have not been developed to date, and optimal levels of CLA isomers in the human diet have not been determined. According to research, natural food products should be the main source of CLAs [3, 19, 24, 25].

Further research is needed to investigate the role of CLAs in the human diet. The results of studies analyzing the biological activity of RA are highly promising, therefore the elimination of CLA isomers from infant formulas should be discouraged, because the resulting products do not cater to the nutritional needs of growing infants. The aim of recent industrial practices is to simplify the production process rather than develop products with nutritional benefits. Infant formulas are the only source of food for children allergic to milk. Formulas deprived of milk fat do not contain CLAs or VA, which is a substrate for the synthesis of CLA isomers.

The RA content of human breast milk can be increased by consuming foods that are abundant in these isomers. Natural products that are rich in FAs, including butter, ripened cheese and the meat of ruminants, are the recommended sources of CLA in the diets of nursing mothers. High-fat dairy products and ruminant meat are rich sources of CLAs, in particular RA, for children who are strongly allergic to cow’s milk proteins. Dietary supplements are not recommended due to high levels of C18:2 10t,12c, one of the CLA isomers, whose health benefits have not been confirmed. CLA supplements contain significantly higher levels of 10t,12c CLA isomers than natural foods.

Unlike breast milk fat, the fat isolated from commercially available IFs does not contain CLAs. The results of the study indicate that the majority of commercially available IF and FF do not contain sufficient amounts of RA and VA, and that their FA composition is deficient in comparison with breast milk. The replacement of cow’s milk fat with coconut oil or palm oil in IF and FF should be discouraged, because it does not cater to the nutritional needs of growing infants.