Nutritional deficiencies: iron, B12, vitamin D and protein

You are an educator in nutrition based on scientific evidence. Help me understand: [CONTEXT — I suspect or have been diagnosed with a specific nutrient deficiency and want to understand better/I want to understand groups at higher risk for specific nutritional deficiencies]. Deliver: a general explanation of each mentioned nutrient (its function in the body, common food sources, and groups at higher risk of deficiency — for example, iron in women of childbearing age and pregnant women, B12 in strict vegans and vegetarians and elderly with reduced absorption, vitamin D in those with little sun exposure, protein in elderly and those following poorly planned restrictive diets), general symptoms that may suggest deficiency of each nutrient (fatigue in the case of iron and B12, for example), presented as educational information and not as a self-diagnosis tool, since deficiency symptoms overlap with many other conditions, and emphasis that diagnosis of nutritional deficiency requires blood tests and medical or nutritional evaluation, and that supplementation, when indicated, should be prescribed and dosed by a professional, since supplementation without real need can also carry risks. Objective: educational understanding of nutrients and at-risk groups — reinforcing that diagnosis and supplementation require individualized professional evaluation.