According to Gram Research analysis, high levels of zinc oxide in pig feed—a common growth-promoting additive—causes the largest increases in antibiotic-resistant bacteria in the gut, with a 2026 study finding that high-zinc diets induced significant increases in genes conferring resistance to multiple antibiotics. Surprisingly, iron levels had minimal effects on antibiotic resistance, though low iron actually increased dangerous bacterial virulence genes. These findings suggest that excessive zinc in animal feed may be contributing to antibiotic resistance that could eventually affect human health.
Researchers studied how different levels of iron, zinc, and copper in pig feed affect harmful bacteria in their guts. They found that high levels of zinc oxide—a common additive used to help pigs grow faster—caused the biggest changes in antibiotic-resistant bacteria. Interestingly, iron had less impact than expected. The study suggests that using too much zinc in animal feed might be creating bacteria that are harder to treat with antibiotics, which could eventually affect human health. This research helps farmers and scientists understand which feed additives are safest to use.
Key Statistics
A 2026 research article analyzing 24 pigs found that high-zinc diets induced the largest shifts in gut antibiotic-resistant bacteria, including increases in ant(9)-la and adeF genes that help bacteria survive multiple antibiotics.
According to the study, researchers identified 172 different antibiotic resistance genes across all pig samples, with glycopeptide and tetracycline resistance being the most dominant types found.
The research showed that fecal copper levels correlated with pcoC (a plasmid-mediated copper resistance gene) at ρ = 0.66, demonstrating a direct link between dietary copper and bacterial resistance genes.
In the high-zinc diet group, carbohydrate metabolism pathways were significantly enriched compared to control diets, suggesting these resistant bacteria were better equipped to survive and thrive in the gut environment.
The Quick Take
- What they studied: How different amounts of iron, zinc, and copper in pig feed affect the types of bacteria in their stomachs and whether those bacteria can resist antibiotics.
- Who participated: Fifty young pigs (just weaned from their mothers) were divided into five groups and fed different diets for 24 days. Researchers analyzed bacteria samples from 24 of these pigs.
- Key finding: High-zinc diets caused the largest changes in antibiotic-resistant bacteria, while iron had surprisingly little effect. The high-zinc group showed increases in genes that help bacteria survive multiple antibiotics.
- What it means for you: The metals added to animal feed to make them grow faster might be creating bacteria that are harder to kill with antibiotics. This could eventually make infections harder to treat in both animals and humans. However, this is early research in pigs, and more studies are needed before changing farming practices.
The Research Details
Scientists divided 50 young pigs into five groups with different diets. One group ate normal feed (the control), one ate feed with less iron, one with extra iron, one with extra copper, and one with very high zinc oxide—a common additive used to promote growth. All the pigs were exposed to a harmful bacteria called E. coli on days 13-16 to simulate a real infection. On day 24, researchers collected poop samples and used advanced DNA technology to identify all the bacteria present and which genes made them resistant to antibiotics.
The researchers used two main techniques: shotgun metagenomics (which sequences all the DNA in a sample to identify bacteria and resistance genes) and whole-genome sequencing (which completely maps the genetic code of individual E. coli bacteria). This allowed them to see both the overall bacterial community and specific bacteria in detail.
This approach is like taking a snapshot of the entire bacterial neighborhood and then zooming in on individual residents to understand their characteristics.
Understanding how feed additives affect antibiotic resistance in farm animals is crucial because antibiotic-resistant bacteria can spread to humans through food, water, and the environment. By identifying which additives cause the most resistance, farmers can make safer choices. This research helps bridge the gap between animal agriculture and human health.
This was an exploratory study with a relatively small sample size (24 pigs analyzed), so results should be considered preliminary. The researchers used well-established scientific methods (metagenomics and whole-genome sequencing) and compared their findings to established databases of resistance genes. The study was controlled and randomized, which strengthens the findings. However, results in pigs may not directly apply to other animals or humans, and the short 24-day study period may not capture long-term effects.
What the Results Show
The high-zinc diet caused the most dramatic changes in the types of resistance genes found in pig feces. Specifically, the high-zinc group showed increases in genes called ant(9)-la (which helps bacteria resist aminoglycoside antibiotics) and adeF (which acts like a pump to push multiple antibiotics out of bacterial cells). These genes were found on mobile genetic elements—essentially pieces of DNA that can jump between bacteria, spreading resistance like a contagious disease.
Interestingly, dietary iron had minimal effects on antibiotic resistance overall. However, when researchers looked at individual E. coli bacteria, low-iron diets actually increased virulence genes (genes that make bacteria more dangerous and able to cause disease). This suggests that iron restriction makes bacteria more aggressive, even if it doesn’t increase antibiotic resistance.
The high-zinc diet also enriched carbohydrate metabolism pathways, meaning the bacteria in high-zinc groups were better equipped to break down and use different types of sugars. This metabolic shift may help these bacteria survive better in the gut environment.
Copper and zinc levels in the feces correlated with specific resistance genes: fecal copper correlated with a plasmid-mediated copper resistance gene (pcoC), and fecal iron correlated with an iron-containing enzyme (sodB).
The study identified 172 different antibiotic resistance genes across all samples, with glycopeptide and tetracycline resistance being the most common. The high-zinc diet produced the most distinct changes in E. coli virulence profiles on day 24, while the low-iron diet showed the most distinct virulence changes on day 12. These timing differences suggest that different dietary factors affect bacterial behavior at different stages of infection.
Previous research has shown that zinc oxide is widely used in pig farming as a growth promoter because it works well and is cheaper than antibiotics. However, concerns have been growing about whether this practice contributes to antibiotic resistance. This study provides direct evidence that high zinc levels do indeed alter the resistance profile of gut bacteria. The finding that iron has minimal effects on antibiotic resistance is somewhat surprising and contradicts some earlier assumptions, suggesting that iron’s role in gut health may be more complex than previously thought.
The sample size was small (24 pigs analyzed), which limits how confidently we can apply these findings. The study lasted only 24 days, so we don’t know if these changes persist long-term or if bacteria adapt over time. The research was conducted in pigs, and results may not directly transfer to other animals or humans. The study exposed all pigs to a harmful E. coli strain, which is more severe than typical farm conditions. Additionally, the researchers couldn’t measure whether these resistance genes actually made bacteria harder to treat with real antibiotics in practice—they only identified the genes present.
The Bottom Line
Based on this research, farmers should consider reducing zinc oxide levels in pig feed, as high levels appear to promote antibiotic-resistant bacteria without providing proportional growth benefits. Iron levels in current feeds appear safe and don’t need adjustment based on this study. However, these are preliminary findings from a single study, and farmers should consult with veterinarians before making major feed changes. The evidence is moderate—strong enough to warrant caution but not yet strong enough to mandate immediate changes across the industry.
Pig farmers and veterinarians should pay attention to these findings when deciding on feed additives. Food safety officials and public health experts should consider how farm practices affect antibiotic resistance in human populations. Consumers concerned about antibiotic resistance in their food supply may want to seek out pork from farms using lower zinc levels. This research is less directly relevant to people who don’t work in agriculture, though antibiotic resistance affects everyone’s health eventually.
If farmers reduced zinc oxide in feed today, it would likely take several weeks to months to see measurable decreases in antibiotic-resistant bacteria in their herds. The benefits would accumulate over time as resistant bacteria are naturally replaced by less resistant strains. However, any bacteria already carrying resistance genes would persist in the environment for years.
Frequently Asked Questions
Does zinc in animal feed create antibiotic-resistant bacteria?
Research shows that high levels of zinc oxide in pig feed do increase antibiotic-resistant bacteria in the gut. A 2026 study found that high-zinc diets induced the largest increases in resistance genes, particularly those helping bacteria survive multiple antibiotics simultaneously.
Is iron in pig feed safe or does it cause antibiotic resistance?
Dietary iron had minimal effects on antibiotic resistance according to this research. However, low-iron diets increased dangerous bacterial virulence genes, suggesting iron restriction makes bacteria more aggressive rather than more antibiotic-resistant.
Can antibiotic-resistant bacteria from farm animals affect human health?
Yes, resistant bacteria from farm animals can spread to humans through food, water, and the environment. This is why researchers study how farm practices affect bacterial resistance—to prevent resistance from becoming a widespread human health problem.
Should farmers stop using zinc oxide in pig feed?
This single study suggests caution with high zinc levels, but farmers should consult veterinarians before making changes. The research is preliminary and doesn’t yet provide enough evidence to mandate industry-wide changes, though it warrants consideration when choosing feed additives.
How long does it take to see benefits from reducing zinc in animal feed?
If zinc levels were reduced, measurable decreases in antibiotic-resistant bacteria would likely take several weeks to months as resistant bacteria are naturally replaced by less resistant strains. Long-term benefits would accumulate over time.
Want to Apply This Research?
- If you raise pigs or manage a farm, track the zinc oxide levels in your feed batches and monitor antibiotic use in your herd. Record any disease outbreaks and which antibiotics were effective, noting whether effectiveness changes over time as you adjust feed composition.
- Farmers using the app could set reminders to review feed ingredient lists quarterly and compare zinc oxide levels across suppliers. Users could log feed changes and track herd health metrics to see if reducing zinc impacts growth rates and disease incidence.
- Establish a baseline of current zinc levels and antibiotic effectiveness in your herd. After making feed adjustments, monitor growth rates, disease frequency, and antibiotic response over 3-6 months. Use the app to track trends and share data with your veterinarian to make informed decisions about future feed formulations.
This research was conducted in pigs and represents preliminary findings from a single exploratory study with a small sample size. Results may not directly apply to other animals or humans. This information is for educational purposes and should not replace professional veterinary or medical advice. Farmers considering changes to feed formulations should consult with their veterinarians. Individuals with concerns about antibiotic resistance should speak with their healthcare providers. The study identifies associations between dietary metals and bacterial resistance genes but does not prove that these genes cause treatment failures in real-world conditions.
This research translation is published by Gram Research, the science division of Gram, an AI-powered nutrition tracking app.