Sources of conjugated linoleic acid and estimated daily intake

12.2.1 Natural sources

In food, CLA is found in products from ruminants such as milk and meat, and products made from them such as cheese and cultured dairy products (Parodi, 2003). CLA has been shown to be produced in the rumen as a result of biohydrogenation from linoleic acid, a polyunsaturated fatty acid that is found in the pasture. Rumenic acid or 9c,11t-18:2 is the major CLA isomer produced. Biohydrogenation also produces 11t-18:1 fatty acid or vaccenic acid. CLA has also been shown to be produced in the mammary gland by A9 desaturation of vaccenic acid which passed via the circulatory system to the mammary gland and adipose tissue (Corl et al, 2001). The second most important CLA isomer, the 7t,9c-18:1, has been shown to be only produced in the mammary gland (Corl et al., 2002). Linolenic acid, a fatty acid present in the pasture, does not directly produce rumenic acid but contributes to its level by giving vaccenic acid which can then be converted to the 9c,11t isomer.

As previously discussed the major CLA isomer in dairy and meat products is rumenic acid or 9c,11t-18:2. This isomer represents more than 70% of the total CLA isomers (Table 12.1). It is accompanied by a mixture of other cis,cis, cis,trans, and trans,trans isomers. Some earlier studies reported CLA content of dairy products but it is only since the development of sample preparation and sophisticated chromatographic methods (Christie, 2003) that the problems of co-eluting peaks and artefact formation could be avoided. Direct gas-liquid chromatographic analysis does not allow separation of the different isomers. However, these can be resolved satisfactorily by using silver nitrate high-performance liquid chromatography (Sehat

Table 12.1 CLA content and content of 9c,11t (% of total isomers) in samples of butter, cheese and beef fat (adapted from Parodi, 2003)

AgNO3, HPLC

GLC on CPSil88, 100 m

9c,11t

7t,9c' Inm 11c,13t 9c,11c trans, trans

54 " " " " 55 56 57 58 59 60 61 62

11t,13t

12t,14t

9t,11t

12t,14c

12c,14t 11t,13c

11t,13t

12t,14t

9t,11t

12t,14c

12c,14t 11t,13c

10t,12t 8t,10t 10t,12c

ChromSpher5 lipids, 2 columns

Fig. 12.2 Silver nitrate high-performance liquid chromatographic analysis (AgNO3-HPLC) on two ChromSpher 5 lipids columns and gas liquid chromatographic analysis (GLC) on a 100-m CPSil88 column of fatty acid methyl esters of milk lipid (P. Juaneda and J. L. Sebedio, unpublished data, 2002).

9c,11t

10t,12t 8t,10t 10t,12c

ChromSpher5 lipids, 2 columns

Fig. 12.2 Silver nitrate high-performance liquid chromatographic analysis (AgNO3-HPLC) on two ChromSpher 5 lipids columns and gas liquid chromatographic analysis (GLC) on a 100-m CPSil88 column of fatty acid methyl esters of milk lipid (P. Juaneda and J. L. Sebedio, unpublished data, 2002).

et al, 1998), as reported in Fig. 12.2. As shown in Table 12.1 the CLA content in food products is low and it will be important to take this into account when discussing the data obtained from the human studies. Very similar CLA contents were reported for different countries, the quantities ranging from 1.0 to 28.7 mg/g of fat (Table 12.2). Of importance is the fact that the level of CLA in these products can be modulated by different factors such as the breed of the animal and the lactation number, and especially by the diet (Parodi, 2003). However, cows fed the same diet can exhibit large differences in milk CLA content (Parodi, 2003). The effect of the diet is

Table 12.2 CLA content (minimum and maximum, in mg/g fat), cows' milk from different countries (adapted from Parodi, 2003)

Country

(mg/g fat)

Maximum (mg/g fat)

Sample type

Australia

17

10.3

18.5

Cows (summer, fall)

Austria

13

5.2

14.4

Butter

France

198

2.1

15.6

Butter

Germany

909

1.0

10.5

Butter (barn-fed cows)

593

4.9

18.9

Butter (pasture-fed cows)

Italy

12

6.3

16.7

Butter

Ireland

23

5.6

18.2

Butter

Netherlands

63

3.6

13.3

Butter

Switzerland

11

7.1

10.8

Milk(June-Sept lowland

grazing)

21

19.2

28.7

Milk(June-Sept highland

grazing)

United Kingdom

21

6.0

14.4

Butter

illustrated in Table 12.2 for Germany and Switzerland. For the study carried out in Germany, preparing a butter from milk from pasture-fed cows resulted in almost triple the mean CLA content compared with butter prepared using milk from barn-fed cows. Similar results were reported for the study carried out in Switzerland when comparing milk from highland and lowland grazing cows. Milk from goats and sheep also contains similar quantities of CLA but their contribution to the daily intake would only be of importance for a number of countries from the Mediterranean region.

Feeding strategies to enrich CLA content of milk have also been studied. Basically, two methods have been used to modify the CLA content of milk fat. The first one is to use techniques that increase the level of substrates for CLA and vaccenic acid synthesis in the rumen and the second is to modify the rumen microbial activity. Plant oils - primarily from cereals and oilseeds - fish oils, and pasture feeding were used to elevate milk fat CLA concentration. For more details, readers are referred to the comprehensive review of Stanton et al. (2003). As an example, Corl et al. (2003) have obtained a butter containing about 16% vaccenic acid (11t-18:1) and around 4% of 9c,11t-18:2 compared with 1.3% of 11t-18:1 and 0.5% 9c,11t-18:2 in the control butter by feeding cows with a corn-based total mix ration (control butter) while the cows producing the high-trans milk were fed with the same ration supplemented with 2 g/100 g sunflower oil and 1 g/100 g fish oil.

Daily intakes of CLA isomers have been estimated in different countries, as reported in Table 12.3, using different techniques such as utilisation of national dietary survey data, food frequency questionnaires, dietary assessment, and 7-day dietary records. The daily intake is quite low and interestingly two studies that have compared men and women found a lower CLA intake for women; this may result from a lower intake of meat and dairy

Table 12.3 Estimated daily intake of CLA in different countries (adapted from Parodi, 2003)

Country

CLA intake (mg)

Australia

500-1500

Germany

Men, 430

Women, 350

Women, 246

Sweden

(323)*

(160)

USA

Men, 3-486 (176)

Women, 1-399 (104)

Men, 0-454 (212)

Women, 0-520 (150)

* Values in parentheses represent means.

* Values in parentheses represent means.

products in the latter. Daily intakes range from 0 to about 400-500 mg in most of the countries studied, except in Australia where larger intakes (up to 1.5 g/day) have been observed (Table 12.3).

12.2.2 Industrial sources

CLA supplements have mainly been sold as soft-gel capsules since 1995 in the United States and, more recently, in different European countries and Japan. Recently, Saebo (2003) reported the composition of 17 commercial CLA capsules sampled in January-March 2002 in different countries. Conjugated acid mixtures are usually obtained by alkali isomerisation of linoleic acid followed by a purification process in order to remove contaminants - such as polymers, sterols, and sometimes heavy metals - that could arise from the isomerisation process if mineral acids are used in a stainless steel reactor. Sunflower and safflower oils are the two types of starting material used for the isomerisation process. Products may be classified as four- or two-isomer mixtures. The two-isomer product is a mixture of the 9c,11t-18:2 and the 10t,12c-18:2 in about equal quantities while the four-isomer product is a mixture of 9c,11t-, 10t,12c-, 8t,10c- and 11c,13t-18:2 isomers. Furthermore, most of the products contain between 60 and 80% CLA as free fatty acids (Table 12.4). Further work is being carried out in order to produce triacylglycerols (Saebo, 2003).

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