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Background: Animal and human studies have suggested that trans fatty acids might alter some physiological functions and adversely affect the growth and essential fatty acid balance of infants. In this context it is important to know the fatty acid composition, including the levels of trans isomers of oleic, linoleic and alpha-linolenic acids in infant formulas., Methods: Ten liquid and fourteen powdered formulas for term infants were purchased from retail stores in Canada. The fatty acid composition of each formula was determined by capillary gas-liquid chromatography., Results: All the formulas met the minimum content of 500 mg of linoleic acid/100 kcal formula (equivalent to 4.5% of energy) specified under current Canadian regulations. The formulas all met the minimum energy levels of 3% as linoleic acid and 0.7% as alpha-linolenic acid recommended recently by an ad hoc committee of Health Canada. However, in nine formulas, the proportion of linoleic acid was more than 20% of total fatty acids, and consequently, in five of them, the ratio of linoleic acid to alpha-linolenic acid exceeded the maximum ratio of 16:1 recommended by the ad hoc committee. Trans fatty acids were present in all the samples, and generally the liquid formulas displayed a higher total trans content (mean 1.9%, range 0.9-3.1%) than powdered formulas (mean 1.4%, range 0.6-2.5%). The amounts of trans isomers of linoleic and alpha-linolenic acids and the degree of isomerization of these fatty acids were also higher in liquid formulas than in powdered formulas., Conclusions: A few of the Canadian infant formulas would provide one-third of alpha-linolenic acid as trans geometric isomers.