Most people, not just Vegan’s, are usually short on Vitamin D and B12.
Human’s get the majority of their Vitamin D from sunshine, not their diet. So if you live in a sunshine-deprived area, you will probably be short on Vitamin D.
As for B12, humans originally obtained b12 from the dirt and soil, when picking out things like lettuce or vegetables to eat. It doesn’t come from animals naturally; animals are injected with b12, so animal meat is fortified with it.
Human’s no doubt started to lose contact with soil and therefore natural sources of b12, when the Industrial Revolution began and animal agriculture became an industry. For some reason, this revolution began an obsession with hygiene and cleanliness, clearing out all the natural b12, and spraying foods with artificial chemicals, so food would look more uniform and appear fresher for longer.
One of the main arguments against Veganism is because people think animal meat is a natural source of B12 - it’s not, it’s just been fortified with it for so many generations, that people began to rely on the animal meat industry for their b12.
A lot of people, even if they’re not Vegan, recognise that the animal meat industry is unethical, and so many choose organic meat only. But every animal is sent to the same slaughterhouse - they all are shot in the head and their throat cut, whether or not the ‘organically bred’ animals saw an additional few hours of sunlight in their short life span or not.
In the UK, there is such a thing as ‘RSPCA assured’ meat. A lot of consumer’s buy into this marketing ploy, not realising that the RSPCA (the Royal Society for Prevention of Cruelty to Animals), are in cahoots with the animal meat industry for profit. The RSPCA assures meat from pigs, even though 90% of the pigs are murdered by CO2 gas in a gas chamber. This method of murder is not because it is the most humane method, it is because it is the most cost-efficient way to kill en masse (the concentration camps in Germany would have agreed).
To conclude, the animal agriculture and meat industry is a very profitable business, and therefore deliberately sell marketing ploys and lies to the public, in order to maximise their profit and ensure their longevity. When you give your money to this industry, not only have you been hypnotised into believing it is a healthy choice, you are contributing to the supply and demand for mass murder and legalised animal cruelty.