Comparison of AIN-76A and AIN-93G diets: a 13-week study in rats.

Either purified or cereal-based diets may be used for toxicity testing in rats. Purified diets have advantages in terms of flexibility of formulation to meet specific study objectives and also assurance of relatively low levels of contaminants (e.g. heavy metals and pesticides). The American Institute of Nutrition recommended that the widely used purified diet AIN-76A be replaced by two newer diets, AIN-93G (for use during rapid growth, pregnancy and lactation) and AIN-93M (maintenance diet). The present study compared AIN-76A and AIN-93G by feeding these diets for 13 weeks to male and female rats. A cereal-based diet was also included for reference purposes. The groups fed purified diets had higher serum cholesterol and triglyceride levels than the chow-fed group. An increased incidence and severity of renal tubular mineralization in the purified diet groups was not observed in this study (in contrast to other published studies where rats were fed AIN-76A). Several histopathologic observations, including eosinophilic gastritis and mucification of gastric glands of the glandular stomach, occurred at higher rates in the AIN-76A group than the other dietary treatments. Hepatocellular fatty changes occurred in the purified diet groups at a significantly higher rate than in the chow diet group. In conclusion, AIN-93G is an appropriate diet for use in rat safety evaluation studies.

Interaction of n-3 long-chain polyunsaturated fatty acids with n-6 fatty acids in suckled rat pups.

The addition of long-chain polyunsaturated fatty acids (LCP: C20, and C22) to infant formula may permit fatty acid accretion rates similar to breast-fed infants, and may have long-term outcome benefits, such as improved visual acuity and cognitive development. Although fish oil may provide a source of n-3 LCP, sources of n-6 LCP have been more difficult to identify. The present study evaluates the effects of n-3 and n-6 LCP derived from single-cell oils on liver, plasma, and brain fatty acid levels in a neonatal animal model. Newborn rat pups were suckled for 14 d by dams receiving diets containing n-3 LCP alone or combinations of n-3 LCP and increasing doses of linoleic acid (18:2n-6) or arachidonic acid (20:4n-6). Dietary groups received 2% n-3 LCP and 1, 2, or 5% of either 18:2n-6 or 20:4n-6. The 20:4n-6 source also contained modest levels of 18:2n-6. At the termination of the study, liver, plasma, and brain were obtained from the rat pups and the phospholipid fatty acid profiles determined. The results indicate complex interactions of n-3 and n-6 fatty acids. Groups receiving dietary 20:4n-6 incorporated higher levels of n-6 LCP into tissues than did the groups receiving 18:2n-6. The brain was relatively resistant to changes in fatty acid composition compared with the liver and plasma. As expected, tissue n-3 LCP levels were reciprocally related to n-6 levels. The present results document that single-cell LCP oils are bioavailable in a neonatal animal model. The use of 20:4n-6 is a more effective means of supporting n-6 status than the use of 18:2n-6. These results may have implications for the addition of LCP to infant formula.

The effect of triglyceride positional distribution on fatty acid absorption in rats.

BACKGROUND: Human milk contains palmitic acid predominantly in the triglyceride sn-2 position, and differs from the palmitic acid positional distribution found in most infant formulas (predominantly positions sn-1 and sn-3). Following lipolysis by pancreatic lipase, 2-monoglycerides and free fatty acids are produced. All 2-monoglycerides are well absorbed, including 2-monopalmitin, thus providing one reason for the efficient absorption of palmitic acid in breast-fed infants. If infants are fed fat blends with palmitic acid located in the sn-1 and sn-3 positions, the resulting free fatty acids may form poorly absorbed calcium soaps. Therefore, many infant formulas contain only modest levels of palmitic acid. METHODS: Fat absorption studies were conducted in rats with preparations containing various amounts of palmitic acid in the triglyceride sn-2 position. Determining total fat absorption, specific fatty acid absorption, and the presence of calcium-fatty acid soaps. RESULTS: Betapol, a new triacylglycerol, similar to human milk in its palmitic acid content and positional distribution, demonstrated excellent absorption characteristics compared to fat blends derived from either palm olein or oleo (similar in fatty acid profile to Betapol, but with most palmitic acid in the sn-1 and sn-3 positions). A five-point dose response was used to further evaluate the relationship of positional distribution and fat loss. Palmitic acid excretion and fecal fatty acid soaps were negatively correlated to the presence of palmitic acid in the sn-2 position. CONCLUSION: These studies provide evidence that palmitic acid can be efficiently absorbed, avoiding fatty soap formation of it is present in the sn-2 position.

Red blood cell and tissue phospholipid fatty acid profiles of weanling rats fed infant formula fat blends containing soy and/or corn oil.

Soy oil or corn oil may be employed to provide essential fatty acids in infant formulas. Both of these sources are high in linoleic acid; soy oil contains modest levels of alpha-linolenic acid, while corn oil contains very low levels of this essential omega 3 fatty acid. We examined the omega 3 long-chain polyunsaturated fatty acid (LCP) accretion in red blood cells, liver, and brain phospholipids of rats on diets containing infant formula fat blends with essential fatty acids provided from soy and/or corn oil. Although modest alterations occurred in the red blood cell omega 3 LCP fatty acid status, substantially larger changes were noted in liver LCP profiles. Due to the relatively mature nature of the rats employed in this experiment, no alterations were noted in brain fatty acid profiles. In conclusion, we have observed substantial tissue differences in animals fed soy or corn oil containing diets. It appears that corn oil is inappropriate for use in infant formulas.

Effect of maternal dietary arachidonic or linoleic acid on rat pup fatty acid profiles.

Rapidly growing neonatal mammals accrete relatively large quantities of long chain (> or = C20) polyunsaturated fatty acids (LCP) in membrane phospholipids. We have examined accumulation of omega 6 LCP in suckling neonatal rat pups during the first 14 d of life when their dams received essential fatty acids in the form of triglycerides containing linoleic acid or arachidonic acid. Dietary levels of these fatty acids were either 1 or 5% of total dietary fatty acids. The fatty acid profile of pup stomach contents (composed solely of the dams' milk) and plasma lipids, as well as liver and brain phospholipids, were determined. Stomach linoleic and arachidonic acid levels reflected the diet of the dams. Pup plasma and liver arachidonic acid levels increased progressively from the group receiving 1% linoleic acid to 5% linoleic acid and from 1% arachidonic acid to 5% arachidonic acid. Interestingly, brain phosphatidylethanolamine and phosphatidylcholine arachidonic acid levels were more stable than plasma or liver levels. These results suggest that the brain may be capable of either selective transport of omega 6 LCP or chain elongation/desaturation of linoleic acid. These data indicate that care must be exercised when adding LCP to infant formula since widely divergent accretion rates of arachidonic acid may occur in various tissues.

Corandomization of fats improves absorption in rats.

Human milk fat is well absorbed despite its large concentration of long-chain saturated fatty acids (LCSFA), particularly palmitic acid. The superior absorption has been ascribed in part to the uncommonly high proportion of the palmitic acid in the sn-2 position of the triglycerides, 70% in human milk triglycerides compared with 10-15% in the fats and oils commonly used in infant formula blends. Colipase-dependent pancreatic lipase selectively hydrolyzes the fatty acids at the sn-1 and 3 positions, yielding free fatty acids and the 2-monoglyceride, which are absorbable. Free palmitic acid, but not monopalmitin, can be lost as calcium soaps in the feces. The present study demonstrated that mixtures of coconut oil and palm olein are better absorbed by rats if the proportion of LCSFA in the sn-2 position is increased by the process of chemical randomization, in which the fatty acids of the native oils are redistributed equally to all three positions in the rearranged triglycerides. The fecal excretion of total fatty acids, essentially LCSFA, from the mixtures in which the oils were randomized together (corandomized) was 30 to 60% less than from the mixtures of native oils.