Over hundreds of thousands of years, humans have evolved to eat animal products. As a matter of fact, the fat from animals is what led to the development of our brain, the most advanced of any species on earth. Humans have indeed never evolved to survive, let alone thrive, on a vegan diet, and vegans would have actually died on diet with no animal products until very recently in modern history (see: No Vegan Viking).
Interestingly, proponent of veganism tend to justify their life choices by two primary claims: First that a vegan diet is healthier, and second that a vegan diet is better for the environment. Here's a reality check:
Veganism is unhealthy
A vegan diet is based on the majority of caloric intake being derived from carbohydrates. Carbohydrates, aka sugar, has been linked to obesity, and incidentally, cardiovascular disease as well as cancer. (The research on the matter is so extensive that it would be impossible to even start listing sources, but you can start by looking at "Starches, Sugar and Obesity", from the Nutrim School for Nutrition, Toxicology and Metabolism, at Maastricht University Medical Center).
A vegan diet is also highly inadequate to support a functional muscle mass on males without requiring the consumption of an unduly high caloric diet that will inherently increase body fat beyond healthy levels. Moreover, a vegan diet lacks essential nutrients, requiring constant supplementation with foods not readily available locally, which brings, of course, the issue of sustainability.
Veganism has an extremely high Carbon Foot Print
Sourcing and eating food that are not available locally, an essential aspect of veganism, has a particularly adverse effect on the environment, carbon emissions, and thus the very global warming situation vegans tend to be so concerned and vocal about. In fact, the shipping of an avocado has a staggering carbon foot print of 423.18g of CO2 (Carbon Footprint Ltd). Eating an avocado a day for a year therefore translate to the equivalent of driving a Subaru Forrester for nearly 1,000km!
One cannot live on one avocado a day however. On the basis of the average vegan requiring 2,000 Calories a day, and inherently having to source at least half of these Calories from products not readily available locally, we are looking at 2,644.27g of CO2 emissions... a day! This is equivalent of a drive between Chicago and Anchorage Alaska!
This, of course, does not include the carbon foot print of actually producing the avocado, or driving to the grocery store to purchase the avocado (the use of any method of transportation by any vegan outside of their own leg involves carbon emissions).
Interestingly, even environmentalist organizations that promote veganism have to admit that the carbon foot print of milk is lower than that of regular vegetables produced locally, let alone avocados and other exotic products.
Veganism Destroys the Natural Environment
Growing vegetables, essential to any vegan, requires land, and a lot of it. In the United States only, it is over 1.1 billion acres of agricultural land... As in 1.1 billion acres of natural environments, primarily trees and forests, having been wiped out to accommodate the the growing of veggies! This is half of the entire European continent with trees gone, and with it the ability to absorb greenhouse gas, just to feed the vegans!
In contrast, having livestock on ranch land will not lead to any destruction of the natural environment, and will actually naturally regenerate the land.
Veganism Results in Heavy Pollution
Growing vegetables comes at a terrible pollution cost. The fertilizers, pesticides, manure, herbicides and other agrochemicals required to grow the veggies and legumes that are to feed vegans have a devastating effect on water. These also deplete oxygen in water, killing fish and other aquatic life. Pesticides, fertilizers, ammonia, and heavy metals also kill wildlife, and cause a wide array of problems on humans as well.
Again, a balanced diet approach involving raising livestock and hunting, with growing some vegetables naturally on one's land does not lead to such pollution.
Veganism Kills Wildlife
Last but not least is the fact that veganism also kills the very wildlife it is supposedly meant to protect. Every time a forest is logged, a marsh is filled, or a land is otherwise modified to accommodate agricultural needs, wildlife habitat is essentially destroyed, leaving wildlife with nowhere to live and no source of food, and ultimately leading to a long agonizing death.
At least, hunting for food protects key wildlife, and a clean shot is certainly more humane than leaving an animal with nowhere to go slowly starve to death.
Overall, veganism is completely unsustainable from a health perspective, but also an environmental point of view. Vegans don't save the planet: They kill it.