Is it possible for vegetarians who eat plenty of dairy food to have iron or protein deficiency?
Everyone who consumes too little iron can develop iron-deficiency anemia, but this is not necessary more common in vegetarians than in omnivores.
An appropriately planned well-balanced vegetarian diet is compatible
with an adequate iron status. Although the iron stores of vegetarians
may be reduced, the incidence of iron-deficiency anemia in vegetarians
is not significantly different from that in omnivores (PubMed,
It is a blood test that can reveal the presence and severity of anemia. It may be unreliable to estimate anemia from symptoms.
Dairy is not a good source of iron; an egg contains 0.88 mg, 50 g of most cheeses less than 0.30 mg, and a cup of milk (237 mL) only 0.15 mg (USDA.gov). Calcium and phosphate from milk can significantly reduce the absorption of iron (AJCN). There are plant foods that contain much more iron, up to 6 mg per serving (Chart 2, NutrientsReview).
Protein deficiency in vegetarians who regularly eat protein-containing foods is very unlikely (PubMed, 2018).
There is a widespread myth that we have to be careful about what we
eat so that we do not cause protein deficiency. We know today that it
is virtually impossible to design a calorie-sufficient diet, whether
it is based on meat, fish, eggs, various vegetarian diets or even
unprocessed whole natural plant foods, which is lacking in protein and
any of the amino acids.
Are there demonstrable health benefits from eating small amounts of shellfish? Or is there evidence it can be bad for health in the way red meat can be?
Certain shellfish contain 2-8 mg of iron per serving (USDA.gov). They also contain quite some protein, vitamin B12 and omega-3 fatty acids (USDA.gov). It seems they contain less mercury than fish (FDA.gov). Rarely, shellfish poisoning can occur; this mainly depend on the location of their origin (Canadian Food Inspection Agency). Shellfish are often considered a healthier food choice than red meat (Dietary Guidelines for Americans, 2015, p.24); no food is healthy as such, though.
Is it still the case that there is little or no evidence that taking dietary supplements is an effective way of remedying iron or vitamin deficiency?
Supplements are an effective way to treat iron or other mineral or vitamin deficiencies. In a diagnosed deficiency, prescribed rather than commercially available supplements may be needed. Linus Pauling Institute has a detailed article about iron, including iron supplements, anemia, foods high in iron, etc. Individuals who are not at risk of iron deficiency should not take iron supplements without an appropriate medical evaluation to avoid iron overload.
Healthy people without nutrient deficiencies will unlikely benefit from dietary supplements (PubMed, 2012).