Scientific Advances of Milk Enrichment with Conjugated Linoleic Acid to Produce Anti-Cancer Milk

AUTHORS

Mohammad-Hossein Biglu 1 , * , Hossein Janmohammadi 2 , Lila Mirzapour 3

1 Department of Basic Sciences, Paramedical Faculty, Tabriz University of Medical Sciences, Tabriz, IR Iran

2 Department of Animal Science, Faculty of Agriculture, Tabriz University, Tabriz, IR Iran

3 Tabriz Health Services Management Research Center, Tabriz University of Medical Sciences, Tabriz, IR Iran

How to Cite: Biglu M, Janmohammadi H, Mirzapour L. Scientific Advances of Milk Enrichment with Conjugated Linoleic Acid to Produce Anti-Cancer Milk, Int J Cancer Manag. 2017 ; 10(1):e5868. doi: 10.17795/ijcp-5868.

ARTICLE INFORMATION

Iranian Journal of Cancer Prevention: 10 (1); e5868
Published Online: January 30, 2017
Article Type: Research Article
Received: March 5, 2016
Revised: May 30, 2016
Accepted: January 14, 2017
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Abstract

Background: Conjugated linoleic acid (CLA) is an unsaturated fatty acid that after the discovery of its anti-cancerous properties numerous researches conducted to increase its concentration in the food products. The current study aimed to review researches in the field of milk enrichment with CLA to identify the main actors and highlight the achievements in the field; therefore, the effectiveness of these researches for producing anti-cancer milk was surveyed.

Methods: The journal of dairy science was selected for data extraction and related papers were identified via review of all published papers during years 2003 - 2013 and then were subjected into content analysis. Statistical analysis was done with the MEANS procedure of SAS software (Version 9.1).

Results: Among all collected papers, only 72 were eligible. Data analysis showed that the USA, Canada and France were the most productive countries in the field of milk enrichment with CLA. Surprisingly, Iran did not have any paper in this field in the journal of dairy science. The supplements that were added to the animal diets were enriched by linoleic or linolenic acids. Enhancing the milk cis-9, and trans-11 CLA content were the main objectives of researches. More than 70% of treatments led to the supply of the minimum amount of CLA to prevent cancer with serving milk twice a day.

Conclusions: Considerably, milk enrichment with CLA mostly has being attracted by developed countries. Enriching milk with CLA is a suitable scientific strategy to fight against cancer. It is important to increase researches in increasing trans- 10, and cis- 12 CLA.

Keywords

Milk Enrichment CLA Anti-Cancer

Copyright © 2017, Iranian Journal of Cancer Prevention. This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/) which permits copy and redistribute the material just in noncommercial usages, provided the original work is properly cited.

1. Introduction

Cancer is a major risk for human health in today’s world. As the international agency for research on cancer (IARC) has reported, in 2012, the global burden of cancer raised to 14 million new cases annually, a number expected to increase to 22 million per year in the next two decades. At the same time, cancer deaths were predicted to increase from 8.2 million cases to 13 million per year. In 2010, the total annual economic cost of cancer was estimated to rise about US$ 1.16 trillion. Improving the diet is an effective method for prevention and controlling the cancer. Conjugated linoleic acid (CLA) is an unsaturated fatty acid that naturally exists in ruminant milk and meat products. CLA is produced during the rumen bacterial biohydrogenation process when linolenic acid is converted to stearic acid (1, 2). After the discovery of the anti-cancer properties of CLA in 1987 (3), numerous researches have examined the effects of fatty acid on cancer in animal models and cell culture. Many of the these researches were focused on cis-9, trans-11 CLA and trans- 10-, cis-12 CLA isomers and used a mixture or single form of these isomers. Based on the results of researches, CLA affects different stages of cancer; CLA inhibit proliferation and can induce apoptosis in breast, prostate, colon, gastric, liver, endometrial and osteosarcomas cancer cells and also prevent metastasis (4-11). Furthermore, a study on mice showed that at least 0.5% CLA in the daily diet can reduce incidence of cancer (12). It can be concluded that CLA might be effective to control and prevent cancer.

The optimal intake of CLA for significant reduction of cancer risk in humans has not been established yet, but with the generalization of the studies on rats, a daily intake between 95 - 3500 mg of cis-9, trans-11 CLA is needed. According to the estimates, cis-9, trans-11 CLA intake among human population is about 15 to 1,500 mg per day, which is much less than the amount that is necessary to reduce the risk of cancer (13).

Producing foods with origin of ruminant that have high CLA content is a suitable way to increase the amount of CLA intake, especially cis-9, trans-11 CLA without any need to change eating habits. Milk and dairy products consist 66 to 80 percent of human’s CLA intake (14) and their consumption is more common in developed countries and seems to be increased in developing countries (15). Therefore, increasing the amount of CLA in milk can be a worthy way to control and prevent cancer. Nemours researches have been conducted to enhance milk CLA content. The majority of researches have changed animal diet composition to increase milk CLA content, because this method had a major effect on milk composition (16) and can be done by the farmers. Review of these researches could establish: the measures taken in milk enrichment with CLA, and the achievements that were accomplished in the field. Therefore, the present study aimed to do the content analysis on published papers in the field of enrichment of dairy cattle milk with CLA during years 2003 - 2013 in order to determine: 1- productive authors, organizations and countries; 2- the enriched levels, the amount and type of supplements that were added to dairy cattle rations for enrichment, and 3- the progresses of the researches to provide the optimal amount of needed CLA for controlling and preventing the cancer.

2. Methods

The current study is a meta-analysis, which used content analysis of papers. The Journal of dairy science was selected for data extraction. Based on journal citation reports, this journal has ranked second among agriculture, dairy and animal science journals in terms of impact factor (17). All volumes of the journal of dairy science, which were published during years 2003 - 2013 (11 volumes, 32 Numbers, 7 supplements) were reviewed. Only research papers went under content analysis. The results showed that the level of cis-9, trans-11 CLA, trans- 10-, cis-12 CLA isomers and total CLA were significantly (P < 0.05) increased in dairy cattle milk through the manipulation of the animal diet. Statistical analysis was done with the MEANS procedure of SAS software (Version 9.1). To determine the amount of cis-9, trans-11 CLA supplied by CLA-enriched milk in daily consumption of milk, the amount of cis-9, trans-11 CLA was calculated in 227 mL milk.

3. Results

A total of 72 papers met the inclusion criteria of the study. Figure 1 shows the number of papers in the field of milk enrichment with CLA during years 2003 - 2013. It illustrates that the number of papers published during the period of study increased and decreased narrowly; only in 2010 there was a significant decline so that the number of papers decreased from 7 papers in 2009 to 3 papers in 2010.

The Number of Papers in the Field of Milk Enrichment with CLA in Journal of Dairy Science (2003 - 2013)
Figure 1. The Number of Papers in the Field of Milk Enrichment with CLA in Journal of Dairy Science (2003 - 2013)

A total of 275 authors from 17 countries involved in publishing the papers in the field of milk enrichment with CLA. Table 1 shows 10 productive countries and their organizations list. The USA was the leading country by contribution of about 51.43% of papers, followed by Canada and France. Surprisingly, Iran did not have any papers in the field and China was the only Asian country which contributed a paper. 10 prolific authors and their countries of origin are shown in Table 2. AbuGhazaleh A. A. was the most prolific author by participating in the production of 11.43% of papers, followed by D. E. Bauman and K. J. Shingfield (each with 10% of papers). As shown in Table 2, six prolific authors are from the USA. The number of participant organizations in publishing papers in this field was 78, of which 13 organizations involved in at least 3 papers (Table 3). Cornell University and National Institute for agricultural research, each sharing 11.43% of papers were leading organizations. The following was agriculture and agri-food Canada.

Table 1. Ten Top Productive Countries in CLA and the Number of Their Involved Organizations in Publishing Papers in the Field of Milk Enrichment with in the Journal of Dairy Science (2003 - 2013)
RankCountryPapersPercentageOrganization No
1USA3651.4328
2Canada142013
3France912.862
4Finland7102
5Belgium34.296
6Brazil34.294
7China34.294
8Germany34.291
9England22.862
10Netherlands22.865
Table 2. Ten Top Prolific Authors in Field of Milk Enrichment with CLA and Their Country and Organization in the Journal of Dairy Science (2003 - 2013)
RankAuthorsPapersPercentageCountryOrganization
1A. A. AbuGhazaleh811.43USASouth Dakota State University; Southern Illinois University
2D. E. Bauman710USACornell University
3K. J. Shingfield710England; FinlandThe University of Reading; MTT Agrifood Research Finland
4A. R. Hippen68.57USASouth Dakota State University
5D. J. Schingoethe68.57USASouth Dakota State University
6H. V. Petit57.14CanadaAgriculture and Agri-Food Canada
7K. F. Kalscheur57.14USASouth Dakota State University
8Y. Chilliard57.14FranceNational Institute for Agricultural Research
9A. Ferlay45.71FranceNational Institute for Agricultural Research
10A. L. Lock45.71USACornell University; University of Vermont
Table 3. The Most Productive Organizations in the Field of Milk Enrichment with CLA and Their Country in the Journal of Dairy Science (2003 - 2013)
RankOrganizationPapersPercentageCountry
1Cornell University811.43USA
2INRA811.43France
3Agriculture and Agri-Food Canada710Canada
4South Dakota State University68.57USA
5University of Idaho57.14USA
6MTT Agrifood Research Finland57.14Finland
7University of Helsinki45.71Finland
8Southern Illinois University45.71USA
9University of Alberta45.71Canada
10Bioproducts Inc.34.28USA
11Ghent University34.28Belgium
12North Carolina A&T State University34.28USA
13US Food and Drug Administration34.28USA

Based on extracted data, the total number of experiments was 75 which consisted of 171 dietary treatments that significantly increased the content of CLA in the dairy cattle milk. The Milk contain cis-9, trans-11 CLA was significantly enriched via 147 treatments and 61 treatments had significantly increased the milk trans- 10-, cis-12 CLA content and the milk total CLA content was considerably increased by 71 treatments. Dietary supplements which were added to dairy cattle rations in order to raise the level of CLA in milk were included as oilseeds, oils (plant and animal oils), marine oils and feeds, forages, mixed dietary and other feed. Based on the result of the experiments the amount of total fatty acids of CLA-enriched milk was between 1.44% - 5.21%. CLA-enriched milk cis-9, trans-11 CLA, trans- 10-, cis-12 CLA and total CLA content were reported as 0.06% - 6.05% of FA, 0.0097 - 1.82% of FA and 0.35% - 5.22% of FA, respectively.

Oil seeds: Table 4 shows oil seeds which were used to proliferate the milk content CLA and their amount in dairy cattle rations. Oilseeds include wild flaxseed, sunflower, cotton seeds, linseed, Solin (high linoleic flaxseed), corn, canola and rapeseed, which are rich in linoleic or linolenic acids. The total number of oilseed treatments was 43. Flaxseed was used in 58.14% of treatments. Oilseeds proliferated the milk cis-9, trans-11 CLA content, about 0.33% - 1.484% of FA, the content of trans- 10-, cis-12 CLA about 0.01% - 0.2% of FA and the content of total CLA about 0.57% - 1.9% of FA. The maximum level of cis-9, trans-11 CLA in milk was proliferated by 5.9% flaxseed; the maximum level of trans- 10-, cis-12 CLA and total CLA were proliferated by 21% corn and 12.6% linseed, respectively.

Table 4. The Oilseeds Used in Milk Enrichment with CLA, Milk CLA Content (% of Total Fatty Acids) and Content of cis-9, trans-11 CLA (mg) in 227 mL CLA-Enriched Milk
Dietary Supplement% of DMCis-9, trans-11 CLATrans- 10-, cis-12 CLATotal CLATotal Fatty AcidCis-9, trans-11 CLA, mg/227 mL
Camelina2.90.927-1.0262.5153
5.91.484-1.7371.4450
2.91.02-1.333.6785
Cottonseed9.440.66--3.6755
Sunflower seed8.71.43--3.37110
10.550.97--366
Flaxseed8.30.73-0.973.4457
8.31.46-1.832.7291
12.6--1.4--
12.6--1.4--
12.6--1.9--
11.80.77--4.4580
90.98--3.6980
6.50.870.1213.9979
9.31.24--4.24120
9.320.68--3.351
6.50.56-0.574.355
9.90.35-0.354.7538
50.915-1.083.573
101.64-1.893.5130
151.122-1.282.9174
50.710.0150.813.0750
100.770.020.962.7147
150.920.011.053.1265
8.30.91--4.3189
11.831.19--4.27115
8.30.9--4.388
7.90.43--4.2842
7.90.53--4.3652
90.8--3.767
90.9--2.8959
-*0.91--3.3570
-*0.90.0171.23.3564
Solin101.3--3.86114
Corn30.80.39-0.413.7934
20.90.33-0.354.0730
70.860.12-3.7270
140.970.13-3.7880
211.130.2-3.385
Canola seed9.320.61--3.143
8.130.61-0.843.3445
8.130.56-0.792.9423
Rapeseed6.91--4.1895
-*0.84---
-*0.9---
-*0.744---
-*0.89---

Oils: At 50 dietary treatments, oils were added to the dairy cattle rations. The oils included synthetic CLA, unsaturated fatty acids, animal fat and plant oil seed (sunflower oil, flaxseed oil, cottonseed oil, corn oil, soybean oil, safflower oil, rapeseed oil). In the CLA-enriched milk by oils, range of cis-9, trans-11 CLA content were between 0.43% - 4.12% of FA, range of trans- 10-, cis-12 CLA and total CLA contents were 0.0038% - 1.82% of FA and 0.6% - 5.44% of FA, respectively. The maximum enriched level of cis-9, trans-11 CLA was resulted with 6% safflower. Using 3% linseed oil and rumen injection of 80g/d synthetic CLA, respectively, caused the maximum level of trans- 10-, cis-12 CLA and total CLA in milk (Table 5).

Table 5. The Oils Used in Milk Enrichment with CLA, Milk CLA Content (% of Total Fatty Acids) and Content of cis-9, trans-11 CLA (mg) in 227 mL CLA-Enriched Milk
Dietary Supplement % of DMCis-9, trans-11 CLATrans- 10-, cis-12 CLATotal CLATotal Fatty AcidCis-9, trans-11 CLA), mg/227 mL
CLA synthetic1.13-0.090.6--
1.22-0.081.08--
2.11-0.161.7--
3.75-0.252.69--
0.43-0.0280.83--
10.480.050.722.6230
10.440.170.92.6230
0.751.720.082.282.1484
2.51.070.28-3.8683
CLA synthetica20--2.56--
40--2.98--
80--5.44--
1501.770.85-1.7872
2-0.02---
4-0.06---
6-0.1---
40.660.19-1.9530
100.830.08-2.3745
100.80.09-2.3440
5-0.18---
1000.830.022.592.1640
DHA0.51.040.51-3.2476
LA2.71.43--3.13100
LA+DHA-2.67--3.01180
tallow1.670.71--3.5660
Sunflower oil1.51.46-1.9--
31.85-2.363.63150
4.53.28-3.873.39250
21.610.0069-3.27120
2.90.64-0.833.64-
1.99--1.11--
2.94--0.976--
4.18-1.95--
Flaxseed oil31.341.82-3.45100
32.54--2.22130
41.6--3.21120
2.571.18--3.4492
5.121.39--3.35100
7.671.65--3.27120
21.54--3.59130
1.90.67--1.8328
52.670.08-3.07190
1.880.876-0.955.21104
2.961.73-1.824.52177
4.442.09-2.224.6822
3.30.43-0.454.6746
Camelina oil2.90.5670.0040.793.9351
Ca salt of palm fatty acids1.60.44--3.3430
Cottonseed oil20.6--3.3446
Corn oil20.69--3.1850
1.50.89--3.2866
53.680.09-3.05250
Soybean oil21.02--3.0570
42.39--3.26180
1.70.650.0090.753.5953
3.40.960.0161.093.1470
50.9840.111.232.7360
PUFA2.50.630.03-2.4335
Safflower oil63.360.05.2.83220
64.120.042.97280
52.160.093.06150
54.090.122.83260
Rapeseed oil2.90.560.00380.773.8650

aHas been injected into the rumen.

Forages: The numbers of forage treatments for the milk enrichment with CLA were 11 including, silages of corn, red clover and grass, alfalfa hay and fresh forage. The treatments only significantly increased the milk cis-9, trans-11 CLA content. Enriched level of cis-9, trans-11 CLA was between 0.4% - 109% of FA and 80% fresh grass had resulted maximum level (Table 6).

Table 6. The Forages Used in Milk Enrichment with CLA, milk CLA Content (% of Total Fatty Acids) and Content of cis-9, trans-11 CLA (mg) in 227 mL CLA-Enriched Milk
Dietary Supplement% of DMCis-9, trans-11 CLATrans- 10-, cis-12 CLATotal CLATotal Fatty AcidCis-9, trans-11 CLA, mg/227 mL
Corn silage50.11.37--2.4520
77.51.61--4.47160
red clover silage500.4--3.7834
Grass silage500.45--3.6838
Alfalfa hay49.20.69--3.1350
Fresh forage501.57--3.42120
651.61--3.91143
801.9--3.71160
300.54--4.3950
601.21--4.09110
1001.65--4.01150

Agricultural byproducts: Distillers grains plus soluble, corn germ and mustard bran were agricultural byproducts which were used in 6 treatments for milk enrichment with CLA. CLA-enriched milk cis-9, trans-11 CLA content was about 0.6% - 1.11% of FA. Also 10% Distillers grain plus soluble led to the maximum level (Table 7).

Table 7. Agricultural Byproducts Used in Milk Enrichment with CLA, Milk CLA Content (% of Total Fatty Acids) and Content of cis-9, trans-11 CLA (mg) in 227 mL CLA-Enriched Milk
Dietary Supplement% of DMCis-9, trans-11 CLATrans- 10-, cis-12 CLATotal CLATotal Fatty AcidCis-9, trans-11 CLA (mg/227 ml)
Distillers grain plus solubles50.6--3.3564
100.77--3.3360
150.83--3.2460
101.110.131.243.3310
Corn germ3.640.92--3.5570
Mustard bran7.9-0.009---

Marine oils and feeds: Treatments of marine oils and feeds included 14 treatments of fish oil and algae. CLA-enriched milk cis-9, trans-11 CLA content by marine was about 0.14% - 6.05% of FA and content of trans- 10-, cis-12 CLA and total CLA were about 0.0097% - 1.12% of FA and 0.6% - 4.18% of FA, respectively. Injection of 16g/d fish oil into the rumen increased cis-9, trans-11 CLA content at maximum level. The highest levels of trans- 10-, cis-12 CLA and total CLA were achieved as a result of adding 0.97% fish oil and adding 1% algae to dairy cattle rations (Table 8).

Table 8. The Marine Oils and Feeds Used in Milk Enrichment with CLA, Milk CLA Content (% of Total Fatty Acids) and Content of cis-9, trans-11 CLA (mg) in 227 mL CLA-Enriched Milk
Dietary Supplement% of DMCis-9, trans-11 CLATrans- 10-, cis-12 CLATotal CLATotal Fatty AcidCis-9, trans-11 CLA, mg/227 mL
Fish oil16, g/da6.05--2.76380
16, g/da0.49--3.5640
0.50.141.04-3.410
2.143.41--3.17230
1.10.450.1540.63.5636
0.97-1.12---
algae8.51--3.2210
2.144.21--3.43330
111.360.0097-3.1698
1--3.59--
1--4.18--

aHas been injected into the rumen.

Mixed diets: Based on the data analysis, in 28 treatments combinations of oilseeds, oils, oil seeds and oil were examined. CLA-enriched milk cis-9, trans-11 CLA content ranged from 0.52% - 4.47% of FA and the content of trans- 10-, cis-12 CLA was about 0.2% - 0.28% of FA. Mixed diets increased the content of total CLA for levels 1.12% - 3.47% of FA. Adding mixture of fish oil and algae, soybean oil and synthetic CLA, fish oil and sunflower oil to dairy cattle rations respectively, led to the highest levels of cis-9, trans-11 CLA, trans- 10-, cis-12 CLA and total CLA in milk (Table 9).

Table 9. The Mixed Diets Used in Milk Enrichment with CLA, Milk CLA Content (% of total Fatty Acids) and Content of cis-9, trans-11 CLA (mg) in 227 mL CLA-Enriched Milk
Dietary Supplement% of DMCis-9, trans-11 CLATrans- 10-, cis-12 CLATotal CLATotal Fatty AcidCis-9, trans-11 CLA, g/227 mL
Flaxseed + flax oil-0.53--2.328
Flaxseed + buck wheat-1.27-1.333.53100
Corn + pasture6.66--1.12--
PUFA + pasture15.83--1.91-
PUFA+ Ca salt of palm fatty acids1.60.52--3.2240
Flax oil + DHA-1.45-1.453.27110
Flax oil + soybean oil-1.81--3.3140
Soybean oil + CLA-0.980.11.272.1750
-0.790.281.412.2240
Fish oil + sunflower oil-1.040.07-2.8170
-1.70.12-2.66100
-4.46--3.54360
-3.040.0753.472.9200
-1.64--3.5130
-0.84--2.5148
-1.53--2.95100
---
Fish oil + soy beans-1.170.03-3.81100
-1.030.03-3.890
-1.190.03-4.03110
Fish oil + soybean oil-1.160.04-3.1780
Fish oil + stearic acid-0.70.02-3.1450
Fish oil + algae-3.69--3.49290
4.47--3.74360
Fish oil + flaxseed-1.060.03-3.0874
Fish oil + Ca salt of palm fatty acids-1.20.071.273.3290
-0.7--3.7660
-1.04--3.6185
Fish oil + Ca salt of palm fatty acids + soybeans-1.360.081.443.33100
Fish oil + Ca salt of palm fatty acids + soybean oil-1.740.081.822.89110

Canola meal and concentrate were other dietary supplements which were added to dairy cattle rations for enriching milk with CLA. Canola meal increased total CLA levels to 1.17% of FA. However, concentrate increased level of cis-9, trans-11 CLA to 1.13% of FA (Table 10).

Table 10. Other Dietary Supplements Used in Milk Enrichment with CLA, Milk CLA Content (% of Total Fatty Acids) and Content of cis-9, trans-11 CLA (mg) in 227 mL CLA-Enriched Milk
Dietary Supplement% of DMCis-9, trans-11 CLATrans- 10-, cis-12 CLATotal CLATotal Fatty AcidCis-9, trans-11 CLA, mg/227 mL
Canola meal13--1.17--
Concentrate2.551.13--3.81100

3.1. Amount of CLA in One Serving CLA-Enriched Dairy Cattle Milk

Data analysis showed 227 mL (one serving) of CLA-enriched milk can provide 10 - 380 mg cis-9, trans-11 CLA. Among all treatments, 52 treatments led to supply of 95 - 380 mg, 58 treatments let to supply of 48 - 92 mg and 30 treatments led to supply of 10 - 48 mg cis-9, trans-11 CLA in 227 mL CLA-enriched milk. (Tables 4 - 10)

4. Discussion

According to the findings, papers production had no clear trend during the period of study. The number of papers in 2010 was lower than other years which increased again in 2011. The majority of researches in the field of dairy cattle milk enrichment with the CLA were done by developed countries especially the USA during the period of study. Reviewing selected papers showed that 47.46% of papers which were published in years 2003 - 2009 were those of the USA. By the end of 2009, the USA had an annual average of 4.12 papers and the average number of papers from 2010 to 2013 were 2.28 annually. Most productive countries like Finland and Belgium in term of paper publication were mostly active in 2011. Hence it can be concluded that the performance of the USA and other chief productive countries could be the main reason of the falling number of papers in 2010 and rising number of papers in 2011.

Production of animal products with high nutritional value is economically very expensive and depends on local and seasonal conditions, and the availability of resources. Usage of dietary supplements like plant oils or fish oil for producing high nutritional value foods, due to its cost may discourage farmers from this action (15). The findings of this study showed that a large number of productive countries were the developed countries, which have high incomes (18). According to statistics which was provided by FAO, the USA ranks second in terms of milk production and per capita milk consumption in the USA is higher than usual (150 kg/capita/year), also rank of India and Pakistan in terms of milk production are first and fifth, respectively. Milk consumption per capita is higher than usual in Pakistan (19, 20). However, India and Pakistan did not have any paper in the field of milk enrichment with CLA, which confirms that the economical power of developing countries as well as China (18) is an important factor to carry out such researches in these countries. On the other hand, findings of this current study indicated that 28 from 78 organizations, and 50% of main productive organizations are located in the USA, also 6 prolific authors are from this country; hence 52.77% of papers belonged to the USA. No paper was published from Iran. It could be because milk production by Iran is less than the amount of population’s need and the per capita consumption of milk is medium or less than normal amount (19); therefore, the amount of milk production is considered more important than the quality of milk in Iran.

Based on the review study by TILAK R. et al. pasture grass, plant oil seeds and oils, marine oils and feed and animal fat that are rich in linoleic or linolenic acids, or are able to change the rumen biohydrogenation process, are effective substances to enhance the content of CLA in milk (21). The results of this study are in accordance with their findings and indicated that dietary supplements such as sunflower, soybean, flaxseed and their oils which are rich in linoleic or linolenic acids, and marine oils which are rich in long-chain PUFA could proliferate the milk CLA content. Increasing vaccenic acid production in the rumen is a key way to raise the milk CLA content, so supplementation of dairy cattle rations with diets which are rich in linoleic, linolenic acids or diets containing marine oil-rich in long-chain PUFA and by changing rumen biohydrogenation process could inhibit vaccenic acid reduction to stearic acid- increase the content of CLA in milk (22).

Given that cis-9, trans-11 CLA is a predominant CLA isomer in ruminant milk and meat that has health benefits in humans, increasing the content of this isomer in meat and milk is the main objective in the field of enrichment of ruminant products with CLA (23, 24). The results of this study indicated that 93.96% of treatments had significantly increased the milk cis-9, trans-11 CLA content. This could represent that increasing this isomer is the purpose of the most researches conducted in this field.

The recommended amount of milk for daily consumption is two glasses. Based on the results of this study, more than 70% of treatments provided the minimum amount of cis-9, trans-11 CLA needed for preventing and reducing the cancer risk in humans with two glasses of milk a day.

Based on the report of Larsson et al. long-term consumption of high-fat dairy products and a high CLA intake could reduce the risk of colon cancer (25). Although the research on milk enrichment with CLA could not provide the maximum amount of CLA which is necessary for reducing cancer risk, but according to success of researches on providing the minimum amount, it can be concluded that the milk enrichment with CLA is an effective action against cancer.

Although milk enrichment with CLA is expensive, but due to the high rate of cancer in Iran (26, 27), it would be more reasonable and cost-beneficial to produce the enriched milk with CLA acceptably in the countries like Iran.

Results of our study revealed that studies related to the milk trans- 10-, cis-12 CLA content were not considered appropriately. Since this isomer, either directly or indirectly through reducing obesity and as a result, obesity related cancers is effective in reducing cancer risks (7, 28, 29). further research to consider this isomer in milk enrichment is recommended.

Footnotes

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