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Fats in Powdered Milk Replacers


Dietary considerations


Fats provide essential energy that are needed to grow, develop and function property. The body needs fats for brain and neurological development, blood clotting, and controlling inflammation. The body cannot make fats by itself nor work without them. Different types of fats work in varied ways.


While there has been extensive research on the importance of milk fat needed for growth and development, far less is known about their structures and exactly how fat is digested. Fat digestion involves the body making the fats more soluble so they can be absorbed into the intestine and transferred into the circulatory system. Digestive enzymes, called lipases, break down dietary fats so they can be absorbed in the small intestine. While bile plays a role in breaking the fats down further into an emulsion, newborns and infants have reduced bile production. Research shows that milk fats are unique in that they enter the digestive tract as an emulsion and “self-assemble” into tiny nanometer-sized structures with a high surface area, rather than large globules as seen in other fats. Those smaller milk fat droplets are more rapidly digested by the lipase enzyme, some believe this is an adaption of the digestive chemistry of the infant gut to facilitate fat digestion. Vegetable and animal fats have different structures, and thus are digested differently.


As mentioned elsewhere, the components and chemistry of milk is worthy of extensive research. While this section briefly describes a few topics relevant to feeding wild mammals a substitute formula in captivity, the subject is complex and absolutely deserving of significantly more study.


Milk fats


Milk fat is one of the major components of mammal milk. It is one of most complex natural fat and includes about 400 different fatty acids in trace quantities. The major function of milk fat is to provide high energy to infants during their early development. The essential fatty acid components vary amongst mammal species milks and are based on their requirements and metabolic features. In addition to the nutrient values, milk fat plays a major role in growth and brain development, gastrointestinal development and health, protection against infection, and delivery of nutrients.


The proportions of fats in milks also vary widely among species. For example, rabbit mothers feed their young infrequently, so the fat content of rabbit milk is exceptionally high. Such a high fat content may be necessary to sustain young rabbits in between feeds. Other species may need the exceptionally high fat values in milk to build thick layers of subcutaneous fat to help cope with extreme cold, such as seals and whales. Other species that are able to suckle more frequently and face less severe environments may have lower fat levels in the mothers' milks. Research continues on factors that influence the proportions of fats in the species milks, including the following: during different stages of lactation, diets consumed by the mothers, times of year, and more.


Vegetable fats


Various plants are highly refined and processed into fat products, including corn, cottonseed, canola, soy, palm, sunflower, and rice. The vegetable oils used in animal foods, including milk replacer powders, are unlikely to be from higher priced or quality vegetables, seeds or nuts. AAFCO defines vegetable oil as "...the product of vegetable origin obtained by extracting the oil from seeds or fruits which are processed for edible purposes." This all-encompassing definition of 'vegetable oil' allows for manufacturers to reformulate their source or type of oil without any updated disclosures or printing of new labels. Therefore, it is impossible to determine the source of the oil from the product label. Some vegetable oils have fewer nutrients than others. Also, vegetable oils have substantially less variety of the important fatty acids than cow’s milk. The processing, uses, digestibility and subsequent results of consumption of vegetable oils are being researched.


Processing vegetable oils involves the use of heat that can exceed their heat tolerance, putting them at risk of rancidity. Vegetable oils can oxidize and degrade, especially when exposed to heat during storage. Some manufacturers add preservatives to the milk powders to reduce the chance of spoilage, but some may still become rancid. This suggests that there are benefits to storing milk replacer powders in a cooler environment and that the formula be monitored by the consumer for signs of an 'off’ odor, which can indicate rancidity (described as an odor similar to crayons, soap, or metals).



Animal fats


There are several different categories of animal fats, including rendered fats, fish oils and milk fats. They all have different compositions, calories and energy. For a number of years, people assumed that the animal fats in high-fat milk powders were dairy-based milk fats, such as cream. That assumption was revealed to be wrong, as the animal fat in in some milk replacers is lard (e.g., Multimilk®).


Lard stays in solid form until it melts at 109 degrees F. People feeding formula prepared with one of the milk replacers containing lard have reported that some issues.  The young animals have been more reluctant to eat it unless it is much warmer than the normal temperature that would be similar to the mother’s natural milk. In addition, there have been concerns expressed regarding: the lard taste, digestibility, and the concept of feeding it to herbivores. Like with vegetable oils, oxidation and degradation can occur, especially when these fats are exposed to heat.  Storing these products in a cooler environment and monitoring for signs of rancidity (as described above) would be suggested as well.



Milk replacer analysis


Since almost all of the milk fat (cream) is removed in the processing of whole milk, manufacturers of animal milk replacer products combine varieties of the wet or dry forms of fat (discussed above) with other ingredients to create a replacer product. These are sold, either in wet or dry form, to feed target species, mostly in the commercial livestock and companion animal industries. The chart below indicates the various sources of fats used in formulation of some of the various powdered milk replacer products available from Fox Valley Animal Nutrition and PetAg® that are used by wildlife rehabilitators:

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Since much of the natural milk fat has been removed from the milk powders, some manufacturers use vegetable oil as the primary ingredient to restore target fat levels in the final product. However, as shown in the table below, even the low-fat dried milk products still contain some small amount of the original milk fat. So some of the key fatty acids found in milk have been retained and are still available in the product, especially in the dried whey ingredients. Additionally, as shown above, dried cream is also an "add-back" ingredient, which can range in the 40-75% concentration of cream, with the rest comprised of some of the original milk ingredients such as proteins, carbs and minerals.

All of the milk replacer products have a Guaranteed Analysis (GA) on the package label that discloses a minimum value for fat (as a percent of total weight) that is contained in the product. Many of the products have incorporated this minimum value in the actual product name (e.g., Fox Valley 20/50 or Zoologic® 32/40), with the second of the two numbers indicating the minimum guaranteed percent value for fat. The first chart below shows both the GA value for each product (the black dash), as well as the range of lab test values, with the yellow-center dot indicating average values for the lots tested. Each dot is also accompanied by a range that shows high and low values that were reported from the lab tests. (The ranges shown in pink are for Fox Valley products while the blue shaded ranges are PetAg® products.).

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The next chart below shows the variance from the GA for fat in percentage terms over the period of years that tests were conducted. As shown, most lots tested at or just above the GA for fat through 2016. More recent lots have tended to test in the range of 5-20% below the GA for fat content.

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Since averages can at times be misleading, a closer look at a few of the products where multiple test values are available between time periods, can reveal how the fat concentration values may have changed over time. Then if those changes are significant, either increase or decrease, the reader may want to focus on the most recent profile of the product. As shown, the fat concentration values have declined, on average, for all products shown, some more than 10%. (Note: These lower fat values are not inconsistent with a recent Tufts University (et al) study of 15 puppy milk replacer products where half of the products tested had lower fat values than the level of fat found in lactating dog's milk.)

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References and further reading (not intended as an exhaustive list)


Dils, R.R. 1986. Comparative Aspects of Milk Fat Synthesis. Journal of Dairy Science, V. 69 (3), pp. 904-910.


Hageman, Jeske, et al. Comparison of bovine milk fat and vegetable fat for infant formula: implications for infant health. International Dairy Journal. V. 92, May, pp. 37-49.


Hamosh, M. et al. 1999. Protective function of human milk: the milk fat globule. Seminars in Perinatology. V. 23(3), pp. 242-9.


Hernell, Olle, et al. 2016. Clinical Benefits of Milk Fat Globule Membranes for Infants and Children. Journal of Pediatrics. V. 173, pp. S60-S65.


Lee, Hanna, et al. 2018. Compositional Dynamics of the Milk Fat Globule and its role in infant Development. Frontiers in Pediatrics. 


Martin, Camilla, et al. 2016. Review of Infant Feeding: Key Features of Breast Milk and Infant Formula. Nutrients. 8(5), pps. 


Miner, John and Richard Allison. 1999. The Role of Dietary Fat in Child Nutrition and Development. The Journal of Nutrition. V. 12 (11), pp. 2094-2105.


Rudloff, S and B Lonnerdal. 1992. Solubility and digestibility of milk proteins in infant formulas exposed to different heat treatments. Journal of Pediatric Gastroenterology and Nutrition. V. 15(1), pp. 25-33.


Saleninig, S. et al. 203. Formation of highly organized nanostructures during the digestion of milk. ACS nano. V. 7 pp. 10904-10911.


Skibiel, Amy, et al. 2013. The Evolution of Nutrient Composition of Mammalian Milks. Journal of Animal Ecology. V.82, pp. 1254-1264.

Williams, P et al. 1998. Effect of long-chain polyunsaturated fatty acids in infant formula on problem solving at 10 months of age. The Lancet. Issue 9129, 29 August 1998, pages 688-691.

The information included on this website for the Proximate Analysis components (Proteins, fats, carbohydrates, fiber and moisture content) is extremely narrow in its scope and nature. It is limited to certain charts and graphs displaying content values (% of total) of various powdered milk replacers as tested by an independent chemical lab. Extremely brief overview information is provided as to the primary nutritional and medical benefits of each component, as well as, in some cases, a limited discussion of issues that may arise from concentration levels in the body that may be considered deficit or toxic. Entire textbooks on these primary dietary components are written for the medical and veterinary professions, in addition to the internet providing ready access to both scholarly and popular literature. Some of those references are included above.


The data values presented above only represent the test values for the presence and concentration of the component conducted according to standard chemical testing methods in a controlled laboratory setting. Any point test value is accompanied by a measurement uncertainty range as indicated in the charts. The concentration values are in no measure an indication of how much of the component may be provided to an animal in reconstituted formula or its bioavailability (its degree of digestibility, absorption, or ultimate utilization). Additionally, no testing was performed as to the source of the component in the product (such as inorganic salts) or the grade of any added supplements containing the component.


What the data can do is inform the reader as to (1) concentration levels in a product as most recently tested (2) changes over time and between lots, and (3) comparisons of relative concentration levels between products. It is merely data that may serve as a starting point when deciding on a milk replacer product(s) and a recipe, or information to consider if certain medical symptoms appear that could be a result of absence or excess of a specific component in the formula. The reader is encouraged to consult veterinary or nutritional professionals prior to providing additional supplementation of any component.

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