Why would someone be vegan? The real reasons people switch

Most people assume the answer is animals. The best prospective data says something else, at least about where people start. In a six-month longitudinal study that followed new vegans and vegetarians from the week they switched, health was the most commonly chosen strongest reason. Animal protection was the strongest reason for around 20%, concern for the environment for 18%.

In the month before they switched, more than half of those participants had learned about all three cases: the animals, the environment and health. Animal ethics, land use and human health are separate subjects, resting on separate evidence, studied by people who do not read each other's journals, and each one has been, on its own, somebody's whole reason. They are not equally strong. The one most people name first is the most modest of the three.

The animals

Most people already believe it would be wrong to kill a dog for a product they could get another way. Ask what separates a dog from a pig and the answers run out quickly: not intelligence, not the capacity to suffer, not social bonding. What is left is familiarity.

The practices below are not exceptions or abuses, and none of them is illegal. They are how the ordinary versions of pork, eggs and milk get made.

Pigs. 90% of pigs in the UK are killed using high-concentration carbon dioxide, according to the RSPCA, which has campaigned against the method for years. In October 2025 the government's Animal Welfare Committee agreed the practice can cause avoidable pain, distress and suffering, and recommended the whole industry move to something else within five years.

CO2 is still permitted under the RSPCA's own higher-welfare standards for pigs, and the RSPCA says so plainly: there is no legal, commercially viable humane alternative available. Britain's largest animal welfare charity campaigns against a method it cannot yet prohibit in its own assurance scheme. Buying the higher-welfare label does not route the animal to a different building.

Eggs. Male chicks cannot lay and are the wrong breed to raise for meat, so they are killed on the day they hatch. A country-by-country estimate built from FAOSTAT data across 176 countries puts the global figure at 7.08 billion in 2019, rising to 7.83 billion in 2024. Older numbers in circulation, the 6 billion and 6.5 billion figures, trace back to sources that never published a transparent method.

Campaigners usually attack the method. The RSPCA's own position is that maceration kills instantly and counts as humane when the equipment is correctly maintained, and that UK hatcheries use argon gas rather than blades. The fact that survives that defence: roughly half of every hatch is surplus at birth, for being the wrong sex.

Dairy. A cow produces milk because she has been made pregnant. The calf is what the milk was for. Reviewing the USDA's Dairy 2014 national survey for an audience of cattle veterinarians, Cornell's Sabine Mann reports that 24.2% of US producers separated the newborn calf from their mother within one hour of birth and another 57.5% between one and 14 hours. Cows and calves both respond with increased vocalisation and restlessness for 24 to 48 hours afterwards. There is no version of commercial dairy that skips this step.

The land and the emissions

Poore and Nemecek's meta-analysis in Science covers roughly 38,000 farms across 119 countries and 40 products. Their finding: a global shift to plant-based eating would cut agricultural land use from 4.1 billion hectares to 1 billion, a reduction of 75%. Around 80% of all agricultural land currently goes to meat and dairy.

The mechanism is conversion loss. Feed a cow 100 calories and roughly two come back as beef. Only 48% of the world's cereals are eaten by people; 41% goes to animal feed and 11% to biofuels. Growing crops to feed animals to feed humans discards most of the calories in the middle.

Two complications work against that number. Around 65% of grazing land cannot grow crops, so a portion of pasture competes with nothing. Smallholder and pastoral systems in lower-income countries raise a real food-security question.

The health evidence

This is the reason people cite most often, and the one that carries the least weight.

The anchor is the 2016 position of the Academy of Nutrition and Dietetics, the largest organisation of food and nutrition professionals in the world. Built on 117 references through the Academy's Evidence Analysis Center using systematic-review methods, it states that appropriately planned vegetarian and vegan diets are nutritionally adequate and appropriate for all stages of the life cycle: pregnancy, lactation, infancy, childhood, adolescence, older adulthood, and for athletes. A 2025 update covering nonpregnant, nonlactating adults reaffirmed the conclusion, adding that these patterns "can offer long-term health benefits such as improving several health outcomes associated with cardiometabolic diseases." It runs to the end of 2032.

For magnitude, the largest pooled estimate is a meta-analysis of 96 observational studies: vegetarians and vegans had lower body mass index, total and LDL cholesterol and blood glucose than omnivores, and across the prospective cohorts vegetarians had 25% lower incidence of or mortality from ischaemic heart disease and 8% lower cancer incidence. Vegans alone showed 15% lower cancer incidence, from a smaller number of studies. The same pooling found no significant difference in all-cause mortality or in mortality from cancer. Observational evidence, with everything that implies about confounding.

"Appropriately planned" is doing real work in both position papers. A plant-exclusive diet built carelessly fails in predictable, documented ways. B12 is not optional, and calcium needs deliberate attention rather than good intentions.

None of this says you must eliminate animal products. The evidence supports adequacy and direction: a well-planned plant-exclusive diet works, and moving toward one improves some cardiometabolic markers. A reader whose only reason is health has good grounds to shift most of the plate and no evidence-based obligation to reach zero. Anyone claiming the nutrition literature demands elimination is reading past it.

Does the reason you start with matter?

Less than most advocacy assumes. In the same prospective study that followed new vegans and vegetarians for six months, stated motivation did not predict who maintained the diet and who abandoned it. What did: watching a documentary, seeing footage of farmed animals, and receiving information from an advocacy group were each associated with sticking closer to the goal diet. Receiving information from a celebrity or influencer was associated with drifting away from it.

What happens to the reason afterwards

In a 1997 study in Psychological Science, Paul Rozin and colleagues compared vegetarians who had given up meat for moral reasons with those who had done it for health. The moral group found meat more disgusting, gave more reasons for avoiding it - including reasons unrelated to either morality or health - and avoided a wider range of animal foods. Rozin's name for the process is moralization: a preference turning into a value. The two behave differently. Values invoke stronger emotion, are internalized more, and are passed on in families more effectively than preferences are.

One of their findings ran against their own prediction, and they report it: moral vegetarians said they liked the taste of meat about as much as health vegetarians did.

Each of the three reasons has been, for someone, the whole reason, and any of them is enough to start. The word people arrive at is more recent than most of the arguments for it: "vegan" was coined in 1944.