Animals eating their natural diet don't get diabetes, cancer or heart disease
They do. Zoo and wild animals get cancer, arthritis, heart disease and diabetes, and a large study of zoo mammals found cancer deaths highest among the meat-eaters. Wild animals also seldom live long enough, or get examined closely enough, for chronic disease to show up.
Animals in the wild and in well-run zoos eat the diet their bodies evolved for, and we rarely see them suffer from diabetes, autoimmune disease or heart disease the way modern humans do. That suggests these conditions are diseases of a mismatched diet. If so, the answer for humans is to return to the diet we evolved on, which included meat, rather than to move further from it.
The premise is not true. A 2022 study in Nature looked at cancer deaths in more than 110,000 zoo animals across 191 mammal species, all fed diets meant to match their species. Carnivores had the highest cancer mortality, especially those eating other mammals. Herbivores, primates and rodents had lower rates. Arthritis is common in zoo animal skeletons, including marine mammals. Heart disease is a leading cause of death in captive great apes. Diabetes is recorded in zoo primates and in domestic cats and dogs.
In the wild these diseases are harder to see, not absent. Most wild animals die young from predation, starvation, injury or infection, well before the age at which cancer and heart disease usually appear. Few are ever examined after death. Even so, cancer is documented in wild populations, from Tasmanian devils to beluga whales. Low recorded rates mostly reflect short lives and little observation, not a protective diet.
The comparison also wouldn't tell us much about people. What counts as a "species-appropriate" diet for humans is exactly the point in dispute, and other species are a poor guide to it. The direct evidence comes from human studies. Across large cohorts run by different researchers in different countries, replacing red meat with plant protein is linked to lower death rates and lower cardiovascular risk. That is the evidence the question has to answer to.
Treating 'natural' as evidence of 'good', or 'unnatural' as evidence of 'bad'. Whether something occurs in nature tells you nothing about whether it is beneficial or right: hemlock, malaria and infant mortality are all perfectly natural.
How it applies here: The argument assumes that a diet is healthy because it is the animal's natural one, and that eating 'naturally' protects against chronic disease. It then treats that assumption as settling what humans should eat, instead of looking at evidence about human health.
Treating the absence of proof against a claim as proof of it, or the absence of proof for a claim as proof against it.
How it applies here: The argument reads the absence of recorded chronic disease in wild animals as proof that the disease is absent. Wild animals mostly die young and are rarely examined, so the lack of diagnoses reflects missing data, not good health.