Kinetics of incorporation of N-3 trans fatty acid in rat retina

The retina is more susceptible than the brain and the liver to the incorporation oftransisomers of DHA in rats consumingtransisomers of alpha-linolenic acid

Trans polyunsaturated fatty acids are formed during heat treatments of vegetable oils from polyunsaturated fatty acids containing cis double bonds. After dietary intake, they are distributed in the body and are incorporated into nervous tissues including the retina. Since nervous tissues are known to be rich in n-3 fatty acids such as docosahexaenoic acid (DHA), we studied the ability of the retina and the brain to incorporate trans isomers of DHA formed in vivo from the dietary precursor trans alpha-linolenic acid. Wistar rats were fed with trans isomers of alpha-linolenic acid for 21 months. A linear incorporation of trans DHA and a decrease in cis DHA was observed in the retina, whereas no major changes were observed in the brain. In parallel to the modifications in retinal cis and trans DHA levels, the retinal functionality evaluated by the electroretinogram showed defects in animals that consumed trans alpha-linolenic acid. These results suggest that the mechanisms leading to the incorporation of cis and trans fatty acids are quite different in the retina when compared to the brain and the liver, the retina being more susceptible to changes in the dietary lipid contribution.

Long-term intake of trans (n-3) polyunsaturated fatty acids reduces the b-wave amplitude of electroretinograms in rats

Weanling rats were fed three diets differing in their concentrations of the cis- and trans-isomers of alpha-linolenic acid [18:3(n-3)] for 12 mo to study the long-term effects of these fatty acids on the electroretinogram (ERG). The diets contained 18:3(n-3) in its natural form at 2.0 g/100 g total fatty acids (C group), partially isomerized 18:3(n-3) [1.3 g/100 g cis 18:3(n-3) + 0.7 g/100 g trans 18:3(n-3); cT group] and the control level of cis 18:3(n-3) with trans 18:3(n-3) [2.0 g/100 g cis 18:3(n-3) + 0.7 g/100 g trans 18:3(n-3); CT group]. The ERG and the levels of trans-isomers of the polyunsaturated fatty acids (PUFA) of retinal and hepatic phospholipids were determined after 3, 6, 9 and 12 mo of feeding the experimental diets. Dietary trans alpha-linolenic acid altered the fatty acid composition of retinal and hepatic phospholipids by significantly increasing the Delta19trans-isomer of docosahexaenoic acid. Moreover, dietary trans-isomers of alpha-linolenic acid significantly decreased the b-wave amplitude of the ERG by 9 mo of feeding. We conclude that long-term intake of small amounts of trans-isomers of alpha-linolenic acid could disturb visual function. However, further studies are required to determine the mechanisms responsible for this phenomenon.

Essential fatty acids and their trans geometrical isomers in powdered and liquid infant formulas sold in Canada

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.