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The relationship between human daily requirements of CLA, the potential enrichment of milk through cow's nutrition and daily human consumption
Interest in functional foods has increased in recent years, being the enrichment of milk with conjugated linoleic acid (CLA) one of the targeted products. The objectives of this research were: a) to identify the source of human daily recommendations (literature search); b) to determine the effect of feeding strategies on CLA concentration in milk (meta-analysis); and c) to determine current average human intake of CLA and the expected improvement if milk and milk products were consumed in a CLA enriched form (literature search). The most commonly reported intake recommendations for human are 0.8 g/d (from 0.6 to 3.0 g/d). All recommendations have been extrapolated from animal models and the few human studies reported contradictory results. We selected published papers (n=69) where dairy cows were fed different fats and the milk fat content and fatty acid (FA) profile were reported. Treatments were categorized by source (vegetable oils, fish oils or the combination) and method of processing (raw, processed or extruded seeds, and oils). Data were analyzed using meta-analysis techniques. The combination of fish and vegetable oils resulted in the greatest increase (0.61 vs. 1.34 g CLA /100 g FA; P=0.01), but milk fat content decreased (3.61 vs. 3.12%; P=0.01). Linseed increased CLA proportion (0.61 vs. 0.90 g CLA/100 g FA; P=0.01) without affecting milk fat content. The best processing methods to enriched milk with CLA were extruded seeds (0.57 vs. 1.11 g CLA/100 g FA; P=0.01) and oils (0.57 vs. 1.10 g CLA/100 g FA; P=0.01), but extruded seeds decreased milk yield (30.4 vs. 28.9 kg/d; P=0.01) and oils decreased milk fat content (3.61 vs. 3.31%; P=0.01). Considering the changes in CLA and milk fat content, supplementation with fish oils together with vegetable oils would be the best strategy (118% increase). The estimated current average human consumption in Europe, US and Canada is 0.21 g/d, ranging from 0.06 g/d in Portugal to 0.40 g/d in Germany, well below the requirements. If we assume an increase content of 118% in CLA in milk and milk products, average human consumption would increase from 0.21 to 0.46 g/day. Although there is sufficient data on feeding strategies to increase CLA content in milk, human requirements have not been well established and, based on current recommendations, they are unattainable even if all milk and milk products were consumed as CLA enriched products.
Keywords: Conjugated linoleic acid (CLA), dairy products, human requirements