What Do We Feed to Food-Production Animals? A Review of Animal Feed Ingredients and Their Potential Impacts on Human Health

Animal-based food products derived from cattle, swine, sheep, poultry, and farmed fish constitute a significant portion of the current U.S. diet. In 2003, the U.S. per capita consumption of total meats (including beef, pork, veal, lamb, poultry, fish, and shellfish) was 90.5 kg/year [U.S. Department of Agriculture (USDA) 2005a]. Data from animal-production researchers demonstrate that the quality of these products is directly related to animal feeding practices (Capucille et al. 2004; Gatlin et al. 2003; Zaghini et al. 2005). Therefore, given the high consumption of animal-based food products in the United States, the ingredients used in animal feed are fundamentally important in terms of both the quality of the resulting food products and the potential human health impacts associated with the animal-based food-production chain. In the early 1900s, animals produced for food in the United States were raised on small family farms where cows predominantly grazed on pasture and young chickens were fed primarily a corn-based diet (Erf 1907). However, in the past 60 years, farms and animal feed formulations have undergone significant changes. Small family-owned and -operated farms have been replaced almost entirely by a system of large-scale operations where individual farmers contract with vertically integrated corporations. High rates of food production have been achieved through these systems in which the scale of operations requires the high throughput generation of animals for processing. Animals are raised in confinement and fed defined feeds that are formulated to increase growth rates and feed-conversion efficiencies. These present day animal feeds contain mixtures of plant-based products, as well as other ingredients ranging from rendered animals and animal waste to antibiotics and organoarsenicals. The inclusion of these ingredients in animal feeds can result in the presence of a range of biological, chemical, and other etiologic agents in feed that can affect the quality and safety of animal-based food products and pose potential risks to human health. Since December 2003, when the first U.S. case of bovine spongiform encephalopathy (BSE) was identified in a dairy cow in Washington State, there has been increased attention from veterinary and public health professionals regarding the quality and safety of U.S. animal feed, as well as the safety of subsequent animal-based food products. Yet, the focus of such attention is often limited to one particular facet of animal feed and its associated animal or human health effect (i.e., the impact of rendered animals in feed formulations on the risk of BSE, the impact of bacterial contamination of animal feed on human bacterial illnesses). However, to begin to understand the broad range of potential human health impacts associated with current animal feeding practices, it is necessary to examine the full spectrum of feeding practices and assess their potential human health implications collectively. In this article we review U.S. animal feed–production practices; animal feed ingredients; and biological, chemical, and other etiologic agents that have been detected in animal feed. In addition, we evaluate evidence that current feeding practices may be associated with adverse human health impacts, and address the data gaps that prevent comprehensive assessments of human health risks associated with animal feed.