Fat, oxidation and the swine diet Scott Olson/Getty Images
Oxidized fat in the summer swine diet appears to have adverse impacts on herd performance.

Fat, oxidation and the swine diet

Adding fats to feed in the summer is a common practice to boost caloric intake but if it is oxidized due to heat or handling, is there a concern?

Drs. Jerry Shurson, Department of Animal Science, University of Minnesota, St. Paul; Brian Kerr, USDA-Agricultural Research Service-National Laboratory for Agriculture and the Environment, Ames, Iowa; Chi Chen, and Pedro Urriola, Departments of Food Science and Nutrition, University of Minnesota, St. Paul

Supplemental fats and oils are commonly added to swine diets, especially in the summer, to increase caloric density and maintain energy consumption during hot weather. To capture these benefits, pork producers need to know how to evaluate and preserve the energy value of these ingredients.

Sources of animal fats and vegetable oils, such as rendered animal fats, restaurant grease, highly unsaturated vegetable oils (i.e. corn oil and soybean oil), and animal-vegetable blends, vary greatly in their fatty acid composition and oxidative status, both of which affect their caloric value in feeding programs.

In fact, a U.S. feed industry survey conducted from 2000 to 2005 by Novus International indicated that 40 to 50% of the fats and oils (i.e. poultry fat, tallow, choice white grease, canola oil, corn oil, and soybean oil) used in animal feeds were substantially oxidized, especially in summer months. Results from this survey showed that samples contained peroxide values (PV) as high as 180 meq/kg lipid during the months of April through June, and numerous samples had a PV of > 10 meq/kg lipid, which were considered to be significantly oxidized.

Much of this can be attributed to the extensive use of heated restaurant oils and other thermally processed lipids in animal feeds, as well as lack of cleaning of fat storage tanks in feed mills over extended periods of time. Therefore, assessing the extent of oxidative damage in supplemental fats and oils should be a priority in swine feeding programs.

How do we determine the quality of fats and oils?

Many methods are used to assess the quality of fats and oils in the feed ingredient market. Unfortunately, the most common quality measures (e.g. color; titer - solidification point of saponified lipids); moisture, insolubles, and unsaponifiable content - MIU; and free fatty acid content) do not accurately characterize the extent of oxidative damage.

When unsaturated fatty acids are exposed to high temperatures, oxygen, and transition metals (e.g. copper, iron, zinc) during processing and storage, the primary peroxidation and secondary degradation processes are accelerated. This degradation process forms lipid oxidation products (LOP) at different stages of the processes.

When LOP are present in diets in significant quantities, they can cause adverse effects on growth performance and may potentially be toxic to animals. Many types of LOP have been identified in oxidized oils and fats, including peroxides, cyclic and hydroxylated fatty acids, reactive aldehydes, and various polymers. Therefore, a single chemical analysis is inadequate to determine the presence and concentrations of these LOP, and their relationship to pig growth performance and health.

Of the several oxidation indicator assays available, PV, which measures peroxides, and thiobarbituric acid reactive substances (TBARS), which mainly measures malondialdehyde, have been the most commonly used. However, these methods only measure selective LOP and do not provide a comprehensive assessment of all of the LOP in a fat or oil.

Most recently, we showed that 4-hydroxynonenal (4-HNE) and a defined aldehyde ratio are more accurate than some traditional methods for determining the oxidative status of vegetable oils. Unfortunately, the measurement of 4-HNE and the aldehyde ratio requires using a high-resolution liquid chromatography-mass spectrometry system.

In fact, establishing the accurate and comprehensive analysis of lipid oxidation, as well as understanding the mechanisms underlying the adverse effects of LOP on pig growth performance and health, is an active area of our collaborative research, and should be an area of interest to the pork industry.

Does feeding oxidized lipids impact pigs?

Feeding diets containing oxidized lipids can reduce growth performance, induce oxidative stress, compromise the antioxidant defense system, as well as impair intestinal integrity and function. A summary of 8 published studies involving 23 comparisons that evaluated the effects of feeding oxidized lipids to pigs on growth performance responses is shown in Table 1 below. Although responses were inconsistent, and likely due to the extent of oxidation and the amount of lipid added to the diet, 78% of the observations reported in these studies showed a reduction in average daily gain, 74% showed reduced average daily feed intake, and 61% reported reduced gain efficiency (gain:feed).

Overall, the average reduction in ADG, ADFI, and G:F was 9%, 6%, and 3%, respectively, but the range in the percentage change in ADG reported in these studies was + 5% to - 35%, and was + 6% to - 16% for gain efficiency. Much of this variation in growth performance responses is likely related to the source of lipid, as well as temperature and time of thermal exposure.

Furthermore, serum vitamin E concentrations were consistently reduced by feeding oxidized lipids with an average reduction of 46% compared with pigs fed diets with nonoxidized lipids. This implies that feeding oxidized lipids can significantly reduce the vitamin E status of pigs, and compromise their ability to cope with oxidative stress.

Are there other adverse effects?

There is limited published information on the nutritional and metabolic effects and mechanisms of responses from feeding oxidized lipids to pigs. Most of these responses, shown in Table 2, have not been consistently shown in all studies.

However, there is some evidence that feeding oxidized lipids reduces the antioxidant status of pigs, which can lead to oxidative stress. Furthermore, lipid oxidation has been shown to reduce energy content and digestibility of lipids, alters lipid metabolism, and may reduce the nutrient absorptive capacity of the small intestine.

Feeding oxidized lipids may impair immune function and increase mortality. Finally, exposure of fat soluble vitamins (e.g. vitamins A, D, and E) to oxidizing conditions reduces their concentration and activity in premixes and diets.

How can we prevent lipid oxidation?

Consider using commercially available synthetic antioxidants to prevent additional lipid oxidation (even though they do not reverse or remove any formed LOPs). Common antioxidants that are added to feed fats, oils and other high-lipid feed ingredients include ethoxyquin, TBHQ, propyl gallate, BHA, and BHT.

Fat storage tanks at feed mills should be cleaned, at least on a quarterly basis, to remove both oxidized and oxidizing sediment that accumulates at the bottom of the tanks.

Prevent prolonged exposure of high lipid ingredients and diets to high temperatures, air, and moisture by minimizing storage time and inventory.

Conclusions

The quality of fats and oils used in swine diets should be a major consideration in commercial pork production systems to optimize caloric and nutritional efficiency, and minimize adverse effects of oxidative stress, immune function, and overall pig health.

Currently, our understanding of the best analytical measures to use, “safe” concentrations of LOP in swine feeds, and our ability to predict potential growth performance and health reductions from feeding oxidized lipids to pigs is not well-defined. Collaborative efforts between researchers, feed industry nutritionists, and pork producers are needed to find practical solutions for managing lipid quality and avoiding potential negative consequences in pork production.

References

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