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Americans have become huge. Between the 1960s and the 2000s, Americans grew, on the average, an inch taller and  24 pounds heavier. The average American man today weights 194 pounds and the average woman 165 pounds. The growing girth has led to the creation of special-sized ambulances, operating tables and coffins as well as bigger seats on planes and trains. Almost a third of American children and teens are overweight, but  84 percent of parents believe their children are at a healthy weight in one study. Why? The adults are probably overweight too.

Still there are scientific reasons why Americans are blimping up and they aren’t limited to eating too much and exercising too little. Here are a few areas under suspicion.

1. Antibiotics in food and as medicine. A recent article in the New York Times confirms suspicions that the antibiotics routinely given to livestock to make them fat do the same thing to people. Antibiotics are thought to fatten by changing gut bacteria to make absorption of nutrients more efficient. In  1974, an experiment was done on several hundred Navy recruits to see if they would gain weight on antibiotics and, after only seven weeks, they did. An experiment was also done, unethically it sounds, on “mentally deficient spastic” children in Guatemala in the 1950s, reports the Times. The children gained an extra five pounds over a year compared with children who were not given antibiotics. Denmark  researchers found babies given antibiotics within six months of birth were more likely to be overweight by age seven.

Most researchers blame over-prescription of antibiotics for excessive human exposure; US children get as many as  20 antibiotic treatments while they are growing up, says Martin Blaser, a leading antibiotic researcher at New York University Langone Medical Center. But studies show there are  antibiotic residues in US food too, especially in meat and milk, and the  government tests for them. That means even if you avoid unnecessary antibiotics from the doctor, you could be getting them from the grocery store.

2. Other livestock fatteners. If antibiotics used to make livestock fat could make us fat, is there any reason to think other weight-producing drugs for livestock wouldn’t do the same?  Ractopamine, marketed as Paylean for pigs, Optaflexx for cattle and Topmax for turkeys is widely used in the US and banned in many other countries. It is given to 60 to 80 percent of US pigs, 30 percent of ration-fed cattle and an undisclosed number of turkeys. There is no withdrawal period for ractopamine before slaughter but Big Ag says the drug is not in the meat because it exits the animal as manure. Okay, but what happens to the manure?

Also banned in European countries are the hormones US cattle growers rely upon, such as oestradiol-17, trenbolone acetate, zeranol and melengestrol. Zeranol may have more actions than just making mammals fat. It is a “powerful estrogenic chemical, as demonstrated by its ability to stimulate growth and proliferation of human breast tumor cells in vitro at potencies similar to those of the natural hormone estradiol and the known carcinogen diethylstilbestrol,” says the Breast Cancer Fund. Translation: it may be linked to US breast cancer rates, too. No wonder Europe doesn’t want our beef.