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How Farms Work: How do we know Bt is safe for humans? Science tells us so


Methylxanthines are stimulants that inhibit the activity of the enzyme phosphodiesterase. This enzyme is responsible for breaking down the substance cyclic adenosine monophosphate, which regulates a variety of metabolic processes. At low levels, ingestion of products containing these substances may cause vomiting. At high dosages, seizures, irregular heart rhythms, and even death can occur.

An estimated 41 million children took to the streets in 2015 looking for these deadly chemicals to feed their addictions. Some parents even admit to allowing their children to satisfy their cravings after helping clean the house.

Sound familiar yet?

If you thought of chocolate, you’re correct! You may have also realized that in the first paragraph I was referring to the reaction that dogs have to chocolate! Theobromine and caffeine, which are methylxanthines, are found in chocolate. While not dangerous to humans, dogs have a much different metabolic reaction to these chemicals than humans — but why?

Metabolic pathway

A metabolic pathway is a linked series of chemical reactions occurring within a cell. The components of these reactions are known as metabolites, which are modified by a sequence of chemical reactions that are catalyzed by enzymes (think of an enzyme as the gas in your vehicle and the chemical reactions as the engine). Different species have different metabolic pathways, meaning what affects one, may not have the same effect on another.

Think back to why your dog can’t eat chocolate like you can. Humans have the correct enzymes to process methylxanthines such as theobromine and remove them from our bodies quickly. Dogs however, do not possess the necessary enzymes to efficiently break down and remove these substances from the blood, which means the half-life (time it takes to reach half the original concentration in the blood) of theobromine in dogs is roughly 17 hours. Theobromine causes the release of epinephrine (adrenaline), and since dogs can’t regulate theobromine and caffeine, they tend to have cardiac episodes. In large enough doses, these products can be harmful to any animal, however humans require a dose about three times higher than dogs before reaching toxicity. What can be deadly for dogs, is a staple for humans. The same logic can be applied to why one chemical is deadly for pests, but not for humans.

Bacillus Thuringiensis

Since the 1960s, farmers in the U.S. have been commercially using a bacterium in pesticides known as Bacillus Thuringiensis (Bt). Bt can be found in soil almost everywhere in the world, and it produces proteins that aggregate to form a crystal. These are very toxic to insects yet are harmless to humans and non-larval insects. There are thousands of strains that produce over 200 different proteins that are active against a wide range of insects.

How is Bt harmless to humans?

Bt action is very specific, and it has to be eaten to cause mortality. The Bt toxins dissolve in the high pH of the insect gut and become active. The protein crystals bind to receptors in the gut of the insect, which causes it to stop eating. These receptors are similar to a lock and key, where only certain proteins will bind to them. The spores then replicate and punch through the lining of the gut of the insect and will continue to spread until the insect dies. Humans aren’t harmed by Bt due to the lack of Bt toxin receptors in the epithelial cells in the intestines. This means that the Bt crystals simply pass through the gut in humans.


While Bt is an acceptable use pesticide for certified organic crops, it was hotly criticized when transgenic crops were introduced that produced their own Bt proteins. Opponents of Bt crops argue that the Bt crystals are different as compared to the traditionally applied pesticides, though they both still have the same effects.

Human studies

Several studies have been done on humans to test the toxicity of Bt. In the first study, eighteen humans ate 1 gram of commercial Bt preparation daily for five days, on alternate days. Some inhaled an additional 100 milligrams of Bt powder in addition to the dosage and no complaints were made. Eight men were exposed for seven months to waste products of commercial Bt production with no complaints. This has shown there are no recognized acute nor chronic effects of Bt to be found at levels much higher than anyone would be exposed to under normal circumstances.

Bt has been used for over 70 years with few to no known negative side effects, and because we are simply making plants capable of producing their own Bt crystals to reduce pesticide applications should not keep us from making scientific progress. I believe we should continue to make advancements with chemicals known to be safe and believe increasing a crop’s resistance to pests will reduce the use of other known harmful chemicals on crops.


Ryan Kuster, the force behind the informational and insightful YouTube channel How Farms Work, is a beef and crop farmer based in Wisconsin. He created his channel in 2012 to help show non-rural people how farming is done in the Midwest.

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