I see a large proportion of vegetarian patients in my practice, and the diet itself is not the problem — a well-planned vegetarian or vegan diet can be entirely nutritionally adequate. The gaps I actually see clinically tend to cluster around a specific, predictable set of nutrients, worth naming directly rather than vaguely.
Vitamin B12 is the one that concerns me most
B12 is found almost exclusively in animal products, and plant sources do not reliably provide meaningful amounts despite some marketing claims to the contrary. This is the single most consistent deficiency I see in long-term vegetarians and especially vegans, and it matters considerably — B12 deficiency can cause irreversible nerve damage if left uncorrected for long enough, alongside anemia and fatigue. This is not a nutrient to leave to chance; regular testing and, for most strict vegetarians and all vegans, supplementation is genuinely necessary, not optional.
Iron absorption, not just iron intake
Plant-based iron, non-heme iron, is absorbed considerably less efficiently than the iron in meat. This does not mean vegetarian diets cannot meet iron needs — many plant foods are genuinely iron-rich — but it means the absorption side of the equation deserves attention: pairing iron-rich foods with vitamin C sources measurably improves absorption, while tea and coffee taken close to meals meaningfully reduce it. As discussed elsewhere on this site, ferritin specifically, not just hemoglobin, is worth checking periodically in this group.
Omega-3 fatty acids, specifically the long-chain forms
Plant sources like flaxseed and walnuts provide ALA, a precursor omega-3, but the conversion to the long-chain forms your body actually uses efficiently — EPA and DHA, found directly in fish — is quite limited, generally under ten percent. For vegetarians and especially vegans, an algae-based EPA/DHA supplement is a genuinely reasonable way to close this specific gap, since it is essentially the only non-fish source of the forms your body uses directly.
Protein quality and completeness, not just quantity
Plant proteins are frequently lower in one or more essential amino acids individually, though combining different plant protein sources across the day — legumes with grains being the classic example — provides a genuinely complete amino acid profile without needing to combine them in the same meal, as older advice used to insist. The larger practical issue I see is simply insufficient total protein intake, addressed in more detail elsewhere on this site, rather than incompleteness specifically.
Vitamin D and calcium, though not unique to vegetarians
These deserve mention because dairy is a significant source for many people, and strict vegans in particular need to actively plan for calcium from other sources — fortified plant milks, tofu prepared with calcium sulfate, and leafy greens — alongside the same sun-exposure and supplementation considerations that apply to everyone, covered elsewhere on this site.
Zinc, the quieter gap
Plant-based zinc sources exist but are less bioavailable, partly due to phytates in grains and legumes that bind zinc and reduce absorption. This rarely causes dramatic symptoms but can contribute to subtle issues with immune function and wound healing over time if intake is consistently low.
What I actually recommend to vegetarian and vegan patients
Regular monitoring of B12 and ferritin specifically, since these are the two most likely to become genuinely deficient without producing obvious symptoms early on. A B12 supplement as a default for strict vegetarians and a firm recommendation, not a suggestion, for vegans. Attention to iron pairing and total protein intake as described above. This is not a case against plant-based eating — it is a case for eating that way with specific, informed attention rather than assuming completeness by default.
This article is general education and not a personalized nutrition plan. If you follow a vegetarian or vegan diet, discuss specific supplementation and monitoring with your doctor.