Here are some sources on why the carnivore diet is completely unhealthy:

While there is debate in the scientific community over just how much meat belongs in a human diet, it is impossible for all or even most humans to eat primarily meat. Beef production at the scale required to feed billions of humans even at current levels of consumption is environmentally unsustainable. It is not even healthy from a theoretical evolutionary viewpoint, the microbiome expert Gilbert explained to me. Carnivores need to eat meat or else they die; humans do not. “The carnivore gastrointestinal tract is completely different from the human gastrointestinal tract, which is made up of a system designed to consume large quantities of complex fibers.”

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Christopher Gardner, a professor of medicine at Stanford University, is less convinced by the evidence.

“Are these T rex? African lions? Or humans? Assuming [you are referring to] humans, this sounds disastrous on multiple levels,” he said.

The lack of dietary fibre in an all-meat diet is likely to wreak havoc on the bacteria in our colons, known as the microbiome, he said. “Growing evidence suggests that in the absence of adequate fibre, the bacteria in the colon consume and thin the protective mucus lining, which then leads to impaired immune function and inflammation.”

Eating more meat also contributes to a rise in a substance called trimethylamine N-oxide in the blood which, according to research by the Cleveland Heart Lab, may be as bad for heart disease as saturated fat and elevated cholesterol.

Factory farming of animals is also linked to antibiotic resistance in humans and is a huge contributor to the greenhouse gases that contribute to global warming.

“In summary, I think a ‘carnivore diet’ is inappropriate for human health, bad for the health of our planet, abusive of the human labour force that handles the preparation of meat, abusive of animal rights and welfare,” he said.

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By skipping fruits and vegetables, people likely won’t get enough fiber in their diets, which can affect gut health. They also will miss out on carotenoids and polyphenols, substances with antioxidant properties that have been linked to lower risk of chronic diseases such as Type 2 diabetes and some types of cancer. Animal products also contain high amounts of saturated fat and cholesterol.

In addition to the potentially adverse health effects of the diet, Willett, professor of epidemiology and nutrition, noted that industrial production of animal-based foods is harmful to the planet. “There’s also the issue of justice that basically the Global North—Europe [and] the United States—cause most of the problems with climate change that we have today, and this sort of perpetuates that.”

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The burden of evidence is really on them, the world of nutrition science is very mature in terms of evidence based dietary recommendations and that mass of science is going to suddenly go away because these people cry of conspiracies and epidemiology while sharing meat-industry funded papers (including epidemiology related ones). Much like other pseudoscience fandoms, they exist because of being privileged and loud, not because of being correct.