This justification for eating animals and their byproducts is based on misinformation and ignorance. Necessities are things that we cannot live without; the existence of humans subsisting on entirely plant based should be enough to disprove this argument entirely.
If this is not enough however, The National Health Service and the American Dietetic Association, both world health authorities, tell us that vegan diets are nutritionally adequate and healthy. A growing body of research also suggests that a vegan diet appears to be useful for increasing the intake of protective nutrients and phytochemicals and for minimizing the intake of dietary factors implicated in several chronic diseases. Vegans also have lower serum cholesterol and blood pressure, have reduced rates of cardiovascular disease and a substantially lower risk of cancer. If eating animals is necessary, it is curious indeed that we gain quite so many benefits when we stop doing it.
However, despite this growing body of evidence, as vegans we do not need to be able to prove that a vegan diet is healthier than one which includes animal products. If the premise is that eating animals is necessary, in order to disprove this, the only thing we need to be able to conclusively demonstrate is that humans can be healthy consuming an entirely vegan diet, not that doing so is any healthier. With the existence of dozens of vegan athletes at the top of their respective fields, including world record holding body builder Patrik Baboumian, it cannot be doubted that even those with the very highest vitamin and nutrient requirements can gain them from an entirely plant based diet.
Of course, there will always be anecdotal evidence and well publicised cases of vegans becoming deficient or even dying on a vegan diet. We should not seek to deny that this happens, as it happens to people on omnivorous diets. It is possible to live in an unhealthy way as a vegan, however, these cases are the result of a diet being poorly planned or a plant based being attempted when it was not medically appropriate. That these cases do occur is not an argument against all vegan diets, it is an argument against that specific vegan diet. However, as vegans we do not need to demonstrate that vegan diets are so superior that they automatically make you healthy, we just need to be able to show that the vast majority of people can be perfectly healthy without consuming animal products, which clearly, we can.
The qualifier of “vast majority” is important here, as we must acknowledge that some people genuinely do not have the option of a fully plant based diet, at least at this current time in their life. This may be due to mental health, extreme food sensitivities, a restrictive living situation, belonging to an indigenous population dependent on animal consumption or those living in a food desert without appropriate plant based options. However, unless you are one of those few who genuinely cannot go vegan, claiming that eating animals is necessary is demonstrably false.
This argument is usually either completely unthinking, or is the last resort for someone who knows that the practices of animal agriculture cannot be reasonably justified, and so is attempting to argue that eating animals falls outside the sphere of ethical consideration as we do not have a choice. As the evidence demonstrates, for the vast majority of people eating animals absolutely is a choice, and like any freely made decision, it is open to moral criticism.