Antibiotics have been commonly fed to livestock in the United States for about 50 years. A portion is excreted unchanged in the animals' waste. When manure from antibiotic-fed livestock is used as a fertilizer, some of the antibiotic content enters the soil and is taken up by crops. One experiment by University of Minnesota researchers showed that the antibiotic chlortetracycline was taken up by cabbage, green onions and corn grown in manure treated soil. In a second experiment using liquid hog manure, the antibiotic sulfamethazine ended up in corn, lettuce and potatoes.
Tubers or root vegetables will likely accumulate the highest amount of antibiotic, since they are in closer contact with the soil.
The studies were on crops grown in greenhouses for six weeks. There may be differences in crops that are grown in the field for an entire growing season. In the studies, the amount of antibiotic accumulated by the vegetables was low, less than 0.1% of the amount present in the manure. But it was definitely there.
Foods like corn usually undergo extensive processing before they reach the consumer. This is likely to eliminate any antibiotic content, unless you grow your own food or buy it directly from the grower. Lettuce and spinach undergo little processing and are more likely to retain antibiotics. Tubers or root vegetables will likely accumulate the highest amount of antibiotic, since they are in closer contact with the soil. These include potatoes, carrots and beets.
What are the implications of this? Health officials worry on two accounts. Their primary concern is that consistent exposure of bacteria to low levels of antibiotics in the environment promotes the development of antibiotic resistance in them; this resistance can be transmitted from one species of bacteria to another. There is also the issue of consumption of these antibiotics by humans: one study indicates that ingestion of antibiotics by healthy infants increases their risk of developing allergies and asthma.
On the other hand, those who raise livestock claim that eliminating antibiotics from the animals' feed would lead to an increase in livestock disease, promoting an entirely different set of food safety and human health problems. Antibiotics also cause livestock to grow faster, an economic reason for their continued use.
The European Union banned the use of antibiotics as a food additive for livestock in 2006, citing health concerns. The U.S. FDA does not currently share this viewpoint.
It's particularly troubling that antibiotics can find their way into food labeled as organic. Organic farmers and consumers of their produce have made a choice; they do not want additives that they consider unnatural in their food. Yet food grown using antibiotic-containing manure can still receive the USDA organic label.
Fortunately, there may be a partial solution to this, both for organic and non-organically grown food: composting of manure. Most antibiotics are decomposed by heat. The heat generated in the composting process was shown in one study to reduce the concentration of some antibiotics in manure by up to 99%. While this will not affect the amount of antibiotics released into soil and water by general livestock waste, it may prevent the antibiotics from manure used as fertilizer from entering food crops and being consumed by humans.
An article on all these studies was published in the January 6, 2009 online edition of Scientific American.