Good nutrition is aimed at preventing nutrient deficiencies (among other things), which means that, no matter what type of diet you eat, there are circumstances where supplements may play a role. Regular vitamin B12 supplements are absolutely essential for all vegans who don’t eat B12-fortified foods every day. Depending on sun exposure, vegans may also require vitamin D (although this isn’t a vegan issue; it’s true of most omnivores, too.) And depending on individual requirements and diet, some vegans (and again, some omnivores) might require other supplements to ensure adequate intake.
An alternative to supplementing is to monitor blood levels of particular nutrients, delaying supplements until there is medical evidence that they are needed. In an effort to portray a vegan diet as a naturally optimal way of eating, some vegans will go to great lengths to avoid supplements unless they have physical proof that their diet is falling short. But that’s a bad idea. Occasional monitoring of blood levels of certain nutrients can be helpful, but it’s not a substitute for good preventative nutrition.
For one thing, being deficient—even marginally so—is not harmless. An extended period of sub-optimal B12 status—when you may feel absolutely fine because you have no acute B12 deficiency symptoms—can raise risk for heart disease. And borderline inadequacies in vitamin D status might be associated with a whole host of chronic illnesses. For some nutrients like calcium, you can go for years without feeling any physical symptoms of inadequate intake. (And you can’t assess calcium status by monitoring blood levels.)
Also, nutrient deficiencies aren’t immediately reversible. It takes time to bring blood levels back up to normal—sometimes many weeks or even months—and during that time you could be damaging your health.
Nutrient deficiencies are not really something you want to take a chance on. It’s a whole lot better to eat—and supplement—appropriately to prevent them than it is to allow a marginal intake to catch up with you. In most cases, you can prevent deficiencies with a good balanced diet based on whole plant foods. But that just isn’t true for all nutrients. If you don’t get reliable sun exposure, take a regular vitamin D supplement. If you don’t eat any animal products, make sure your diet provides adequate vitamin B12 from fortified foods or supplements. If you have reason to believe that your diet falls short on other nutrients, modify your eating habits accordingly, or—if you just can’t seem to get enough from food intake—use supplements to take up the slack.
None of this should blemish the image of veganism. A vegan diet is indeed our optimal way of eating since it represents a choice that can easily be planned to meet nutrient needs while respecting the lives of animals. Taking appropriate supplements to ensure that your vegan diet meets all nutrient needs doesn’t change that.