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DAIRY PRODUCTS

CLA-enriched designer milk production

Tuesday, 17 March, 2015, 08 : 00 AM [IST]

C Kathirvelan, S Banupriya and A K Tyagi

Introduction
Recent evidences of epidemiological linkages between diet and chronic diseases have prompted the search for new clinical insights into the relationship between food and the onset (or prevention) of disease.

In this context, the extra nutritional therapeutic attributes of milk and milk products are no longer debated. Advances in biotechnology and genetic engineering have hinted at possibilities that were hitherto not fathomed in the field of dairying. It is now firmly established that a new generation of value-added products can be harvested from milk and milk products. While until recently, emphasis has been on breeding large animals to produce more milk, the attention is now tuned to adding more value to milk and studying its health implications. Milk composition can be altered by nutritional management or through the exploitation of naturally occurring genetic variation among cattle. By a thorough understanding of the biochemistry, genetic traits and changes in the cows diet that affect milk synthesis and composition, ways and means to manipulate milk composition to suit specific needs can be found. By combining the two approaches of nutritional and genetic interventions, researchers are now hoping to develop ‘designer milk’ tailored to consumer preferences or rich in specific milk components that have implications in health.

Conjugated linoleic acid enriched milk
Conjugated linoleic acid (CLA) refers to a mixture of positional and geometric isomers of linoleic acid (cis-9, cis-12, C18:2) with two conjugated double bonds at various carbon positions in the fatty acid chain. It is formed as an intermediate during the biohydrogenation of linoleic acid by linoleic acid isomerase from the rumen bacteria Butyrivibrio fibrisolvens (Kritchevsky, 2000) or from the endogenous conversion of trans-11, C18:1 (Transvaccenic Acid) another intermediate of linoleic or linolenic acid biohydrogenation by ?9-desaturase in the mammary gland.

Milk fat is the richest natural dietary source of CLA. Milk contains an average 4.5mg CLA/g of fat (Kelly et al., 1998). Recent studies have shown that the CLA content of milk fat can be markedly enhanced by dietary manipulation especially those involving dietary addition of plant oils which are high in unsaturated fatty acids (Griinari and Bauman, 1999). Dietary increase of linoleic acid (C18:2) and linolenic acid (C18:3) is one of the feeding strategies for increasing the CLA concentration in milk which is the main precursor of CLA. The main sources of linoleic acid for feeding animals are cereals, oil seeds, oils and so on.

Sources of CLA
CLA occurs in many foods, however, the principal dietary sources are dairy products and other foods derived from ruminants. The CLA content of some common foods is shown in (Table 1).

Table 1: CLA content of common foods

Foodstuff	Total CLA (mg/g of fat)	Foodstuff	Total CLA (mg/g of fat)
Dairy products		Meats
Homogenised milk	4.5	Ground beef	4.3
Condensed milk	7.0	Lamb	5.6
Butter milk	6.1	Pork	0.6
Mozzarella cheese	4.9	Chicken	0.9
Plain yogurt	4.8	Salmon	0.3
Ice cream	3.6	Ground turkey	2.5

Biosynthesis of CLA
CLA found in milk and meat of ruminants originates from two sources (Griinari and Bauman, 1999). CLA is formed during ruminal biohydrogenation of linoleic acid and the second source is CLA synthesised by animal tissues from trans-11 C18:1, another intermediate in the biohydrogenation of unsaturated fatty acid. Hence, the uniqueness of CLA in ruminant edible products relates to incomplete biohydrogenation of dietary unsaturated fatty acids in the rumen.

Feeding strategies to enhance the CLA content in milk
Milk fat is the richest natural dietary source of CLA. Milk contains an average 4.5 mg CLA/g of fat (range 3-6 mg/g). The level of CLA in milk reflects the quantity, which is available for intestinal absorption (Loor and Herbein 1997). Therefore, there is a need to manipulate the feed in such a way to have higher CLA output in the reticulo-rumen for its increased absorption from the intestinal tract and eventually its secretion in the milk.

Dietary factors that affect CLA content have been grouped into four categories related to the potential mechanisms through which they act (Bauman et al., 2001).

1. The first category includes dietary factors that provide PUFA substrates for rumen production of CLA and trans-11 18:1. This typically corresponds to increasing the dietary supply of plant and/or fish oils. Kathirvelan and Tyagi (2007) reported that addition of mustard oil 2 per cent in the concentrate mixture resulted in three fold increase in milk CLA content in buffaloes.
1. The second group consists of dietary factors that affect rumen bacteria involved in biohydrogenation, either directly or via changes in rumen environment. For example, modifying the forage: concentrate ratio of the diet, inclusion of ionophores and fish oil typically alters the biohydrogenation of PUFA.
2. The third category includes dietary factors that involve a combination of lipid substrates and modification of rumen biohydrogenation. For example, several investigations have demonstrated that feeding fresh grass to dairy cows doubles the CLA content of milk fat (Lock and Garnsworthy, 2002) and this cannot be fully explained in terms of PUFA supply to the rumen. Other factors or components of grass must promote the production of CLA in the dairy cows.
3. The fourth category is dietary supplements of CLA or trans-11 18:1 fatty acids. These must be protected from rumen biohydrogenation, typically with calcium soaps or formaldehyde.

Potential health benefits of CLA
The biological properties of dietary CLA are currently attracting considerable interest because of its diverse physiological outcomes in animal studies. CLA is not only a powerful anticarcinogen, but it also has antiatherogenic, immunomodulating, growth promoting, lean body mass enhancing and antidiabetic properties. Experiments using several animal models indicate that dietary CLA is a potent nutrient partitioning agent that favours lean tissue deposition over body accretion. Additionally CLA is effective in preventing catabolic effects of immune stimulation and possess a potent immunostimulatory effect in mammalian species.

CLA and cancer
CLA was repeatedly shown to have anticarcinogenic effects in animal models for stomach neoplasia (Ha et al. 1990), mammary tumours (Ip et al., 1999), and skin papillomas (Belury et al. 1997). As low as 0.05% level of CLA are enough to significantly decrease the induced mammary tumours in rodents (Ip et al. 1999). CLA is effective in reducing the size and metastasis of transplanted human breast cancer cells and prostate cancer cells in severely compromised immunodeficient mice. Kathirvelan and Tyagi (2009) reported that feeding of CLA enriched ghee at 20% in diet resulted in 37% reduction of chemically induced mammary cancer incidence in female Wistar rats. CLA-enriched butterfat was reported to alter mammary gland morphogenesis and to reduce the risk of cancer in rats.

CLA and atherosclerosis
While considerable research has focussed on a potential anticarcinogenic effect of CLA, there are few studies indicating that CLA may also reduce the risk of cardiovascular diseases in animal models. Recently, a study conducted in hamsters showed that a mixture of CLA influenced body weight gain and plasma lipids (Lee et al., 1994). In this study, the three experimental diets fed to the hamsters consisted of the mild atherogenic diet plus CLA mixture at 10g/kg diet (CLA group), c-9, t-11 CLA at 2g/kg diet (c-9, t-11 group) or linoleic acid at 2g/kg diet (LA group). The CLA mixture was reported to decrease the levels of plasma triacylglycerol, total cholesterol and non-HDL cholesterol significantly after 2 weeks or 6 weeks of feeding the treatments compared to the LA group, but not the c-9, t-11 CLA group (Noone et al., 2002). Various studies suggest that some of the cardio-protective effects of CLA shown in animal studies were relevant to humans as well.

CLA and lipid metabolism
A major effect of CLA in this respect is the reduction in lipid uptake by adipocytes (Pariza et al., 2003) which leads to the reduction in body fat gain (Kim et al., 2002). The specific mechanism by which dietary CLA reduces body fat content is likely to vary from one animal species to another. The results of animal studies are also not conclusive. The mechanism by which CLA alters lipid metabolism and body composition in animals is not fully elucidated. It may be tissue and species-specific. In rodents, CLA-induced alteration of lipid metabolism appears to involve increases in rates of lipolysis and fatty acid oxidation. Support for this mechanism comes from observation of increased hormone sensitive lipase activity and enhanced catecholamine induced lipolysis in adipocytes isolated from rats fed CLA (Pariza et al., 2003).

CLA and immune system
Cook et al (2003) showed that CLA not only enhances immune response, but also protects tissues from collateral damage. Sugano et al (1999) proposed that the immune enhancing effect of CLA was by modulating eicosanoid and immunoglobin production. Whigham et al. (2000) concluded that t-10, c-12 isomar competitively inhibited the conversion of archidonic acid to prostaglandin E2. CLA also diminished lipopolysaccharide- induced inflammatory events in macrophages through reduced mRNA and protein expression of nitric oxide synthatase and cyclooxigenase-2 as well as subsequent production of nitric oxide and prostaglandin E2 (Cheng et al., 2004).

Conclusion
Milk fat is the richest natural source of CLA and it is possible that CLA concentration could be increased by manipulation of the nutritional regimes of the animals. Diverse biological roles of CLA in inhibiting cancer, diabetes, lipid metabolism, atherosclerosis, immune function, bone modelling and so on. As a result, natural enrichment of milk food products through increasing the CLA content of milk to obtain the designer milk and milk products for positive health benefits in human beings.

Reference
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(Kathirvelan is assistant professor, department of animal nutrition, Veterinary College and Research Institute, Namakkal. Banupriya and Tyagi are from Veterinary College and Research Institute, Namakkal. They can be contacted at kadhirc@gmail.com)